D Thiesson

A Jougnot

Maineult

A Tabbagh

F Rejiba

C Finco

C Schamper

B Souffaché

C Camerlynck

J Thiesson

D Jougnot

A Maineult

The case for considering polarization in the interpretation of electrical and electromagnetic measurements in the 3 kHz to 3 MHz frequency range

come

The case for considering polarization in the interpretation of electrical and electromagnetic measurements in the 3 kHz to 3 MHz frequency range

in an effort to promote polarisation measurements at intermediate frequencies, and to emphasize the importance of measuring this phenomenon.

Key words: electrical polarisation, permittivity, [3 kHz -3 MHz] frequency range

1.-Introduction

For more than a century, electrical resistivity/conductivity has played a crucial role in near-surface geophysics [START_REF] Jakosky | Exploration geophysics[END_REF], for both large surface mapping and localised tomographic/sounding studies based on the measurement of this property. Reference books dealing with the electrical and electromagnetic methods themselves [START_REF] Keller | Electrical methods in geophysical prospecting[END_REF], Nabighian ed. 1988and 1991) and describing different fields of application have contributed to the development of their physical and theoretical foundations and provided numerous practical examples (Scollar et al. 1990[START_REF] Rubin | Hydrogeophysics[END_REF][START_REF] Sharma | Environmental and engineering geophysics[END_REF][START_REF] Dentith | Geophysics for the Mineral Exploration Geoscientist[END_REF]. Since the seminal work of [START_REF] Archie | The electrical resistivity log as an aid in determining some reservoir characteristics[END_REF], considerable research has been devoted to the analysis of several specific parameters -porosity, clay content, water saturation, water mineralization, etc… -affecting the electrical resistivity (e.g. [START_REF] Glover | 11.04-Geophysical Properties of the Near Surface Earth: Electrical Properties[END_REF].

Various applications, ranging from mining exploration to precision farming can benefit from airborne and ground surveys of this property, over large surface areas. Extensive review papers can be found in the literature, as for example by [START_REF] Pellerin | Applications of electrical and electromagnetic methods for environmental and geotechnical investigations[END_REF] or [START_REF] Doolittle | The use of electromagnetic induction techniques in soils studies[END_REF] in the case of near-surface applications. However, as the use of induction electromagnetic methods is commonly limited to electrical resistivity alone, they do not provide a full description of the electrical response of near surface rocks and soils. The determination of their polarisation properties can also be considered, as one cannot a priori exclude unknown but possibly valuable information. Polarisation phenomena were observed very early, in both surface and logging measurements, and [START_REF] Schlumberger | Etude sur la prospection électrique du sous-sol: Paris, Gauthier-Villars Scollar[END_REF] coined the term 'polarisation provoquée' ("provoked polarisation" in English) which was later (with a possible confusion with the electromagnetic induction) changed in English to 'induced polarisation' (IP). During the 50's the IP method was applied in time domain measurements [START_REF] Bleil | Induced Polarisation: a method of geophysical prospecting[END_REF], and encountered considerable success in ground prospection campaigns applied to mining, as it was able to reveal disseminated mineralization (mainly semiconductor sulphides, e.g. [START_REF] Pelton | Mineral discrimination and removal of inductive coupling with multifrequency IP[END_REF]). However, even in the absence of minerals, researchers became aware of the role played by clay in IP, including mixtures of clay and coarser particles [START_REF] Vaquier | Prospecting for groundwater by induced polarization[END_REF][START_REF] Okay | Spectral induced polarization of clay-sand mixtures: Experiments and modeling[END_REF]. Over the last thirty years, significant developments were achieved on the role of the clay and of the fluid characteristics on the IP signal in the context of groundwater and environmental studies (e.g. [START_REF] Luo | Theory and application of spectral induced polarization[END_REF][START_REF] Kemna | An overview of the spectral induced polarization method for near-surface applications[END_REF][START_REF] Binley | The emergence of Hydrogeophysics improved understanding of subsurface processes over multiple scales[END_REF].

Various laboratory studies and field exploration campaigns have been reported, and theoretical models have been proposed. The latter, in particular, established links between electrical polarisation and hydraulic properties [START_REF] Börner | Evaluation of transport and storage properties in the soil and groundwater zone from induced polarization measurements[END_REF][START_REF] Revil | Determination of permeability from spectral induced polarisation data in granular media[END_REF][START_REF] Fiandaca | Permeability estimation directly from logging-while-drilling Induced Polarization data[END_REF]. Field measurements were performed mostly in the time domain, by measuring the voltage decay after cut-off, over a period ranging from one second to several minutes. However, frequency domain measurements made it possible to gain a better understanding of the frequency dependence of IP (the spectral IP, SIP). Regular workshops allowed the IP community to report on-going research, to publish in special issues, of which the most recent were edited by Camerlynck, Chauris, Maineult and[START_REF] Camerlynck | Special issue on new trends in induced polarization-Part 1[END_REF][START_REF] Simon | GEM-2 HCP L=1.66m[END_REF], [START_REF] Fiandaca | Special issue on induced polarization -Narrowing the gap between theory and observations[END_REF] and by [START_REF] Ntarlagiannis | Special issue: Recent Developments in Induced polarization[END_REF].

IP requires significantly long measurement periods: most frequency domain laboratory studies are carried out over the [1 mHz -1 kHz] frequency range, field measurements last for several minutes per point if such low frequencies are included. This requirement limits complete field studies of IP to fixed point measurements, compatible with 1D soundings.

Current electrical resistivity tomography (ERT) acquisition devices used for IP limit the practical frequency range above 1 Hz and below 100 Hz, , but as the number of periods necessary for a good staking controls in the field the progression speed of towed instruments, such frequencies do not match with rapid motion. In order to overcome this limitation, shorter measurement times and higher frequency ranges are needed. Adding higher frequencies to classical IP field investigations will also give access to potential new information (identification of relaxation phenomena with shorter time constant for example) and to a better characterisation of known IP phenomena.

There also exists specific in-situ difficulties associated with galvanic IP measurements: ground dependent EM coupling between cables or electrode polarisation [START_REF] Bhattacharyya | Some theoretical aspects of electrode polarization in rocks[END_REF][START_REF] Pelton | Mineral discrimination and removal of inductive coupling with multifrequency IP[END_REF][START_REF] Kelter | Field evaluation of broadband spectral electrical imaging for soil and aquifer characterization[END_REF]). In the [3 kHz -3 Hz] frequency range these specific difficulties can be overcome by using other measurement principles: magnetic sensors (coils) [START_REF] Simon | GEM-2 HCP L=1.66m[END_REF]) and capacitive poles [START_REF] Grard | A mobile four electrode array and its application to the electrical survey of planetary grounds at shallow depths[END_REF]. If one uses metallic electrodes (or liquid electrodes) inserted in the ground one has both a capacitance and a resistance thus a phase shift and significant risk of phase rotation if one of the two characteristics changes, while if one only uses capacitances there is no phase shift and the only difficulty is linked with the relative magnitude of the capacitance at the poles and at the feeding wire (but the leaks are proportional to the capacitances and a model can be performed), poles are only required to present significantly higher capacities than feeding wires

The range of permittivity and conductivity values relevant to Ground Penetrating Radars (GPR) and Time Domain Reflectometry (TDR) applications, have been well documented at frequencies above 30 MHz, and electrical polarisation is usually interpreted in terms of the orientation of polar water molecules. However, little is known between SIP and GPR frequency ranges. At frequencies in the [3 kHz -3 MHz] interval, which includes VLF (Very Low Frequency), LF (Low Frequency) and MF (Medium Frequency) measurements, a small number of field observations and tests have been carried out, using three different techniques: Time-Domain Electromagnetics (TDEM), Frequency-Domain Electromagnetic induction (FDEM), and the electrostatic method (Capacitive Coupled Resistivity, CCR).

However, with all these techniques the measured responses are usually dominated by the electrical conductivity and the polarisation part is most often neglected. Nevertheless a larger number of both laboratory and field experiments are needed to evaluate the validity of this choice. An accurate evaluation the influence of electrical polarisation would permit a better understanding of its magnitude in various field contexts.

At the macroscopic scale, electrical polarisation corresponds to a situation in which the barycentre of the positive charges does not coincide with the barycentre of the negative charges. At the microscopic pore/grain scales, different processes with different relaxation geometric scales and time constants limit the free displacement of electric charges. Although their detailed classification [START_REF] Kemna | An overview of the spectral induced polarization method for near-surface applications[END_REF]) could be questioned, one can cite: electrode polarisation, membrane polarisation [START_REF] Marshall | Induced polarization, a study of its causes[END_REF], electrochemical polarisation, interfacial polarisation/diffuse layer polarisation [START_REF] Revil | Effective conductivity and permittivity of unsaturated porous materials in the frequency range 1 mHz -1GHz[END_REF], and Maxwell-Wagner polarisation [START_REF] Chen | Effects of Maxwell-Wagner polarization on soil complex dielectric permittivity under variable temperature and electrical conductivity[END_REF]. Depending on the frequency and temperature ranges involved, all of these processes can contribute to the magnitude of the polarisation and can add together so that it could be difficult to single one out and a regular decrease with frequency can be often observed. It must also be underlined that anisotropy in the shape of the polarized volume plays a major part in the magnitude of their moment [START_REF] Tabbagh | Modelling Maxwell-Wagner induced polarization amplitude for clayed material[END_REF].

In a first step towards the assessment of polarisation in the [3 kHz -3 MHz] frequency range and in view of a better interpretation of EMI or CCR surveys, the present paper provides an overview of published experimental results, at both field and laboratory scales, and in both time and frequency domains.

Part of these experimental results were collected during the course of laboratory SIP experiments, at frequencies of 10 kHz or higher. The considered published results serve as a basis to illustrate the global magnitude of polarisation and conductivity values without being able to identify the different polarisation processes. In an effort to focus this study on common rocks and soils, we do not consider measurements carried out on media containing electronic conductor/semiconductor minerals (oxides, sulphides, metallic particles or graphite).

2.-How should the electrical polarisation be expressed?

In the Maxwell-Ampère equation, the electrical polarisation is represented by the displacement currents. In the frequency domain this can be written:

(1), where E is the electric field, H is the magnetic field, is the direct current electrical conductivity, ω is the angular frequency (ω=2πf),

1   i
, and ε is the dielectric permittivity. Although this last property describes the dielectric behaviour, in order to take the dielectric losses into account it must be considered complex quantity:

. Here, the imaginary term has the same phase as the motion of free carriers (direct current conductivity, σ 0 ), from which it cannot be experimentally distinguished. The measured conductivity thus corresponds to: σ'=σ 0 +ωε", which leads to the Maxwell-Ampère equation being written as:

(2)

These properties can be also expressed in the form of a complex conductivity (σ'+ iσ'') or a complex resistivity (ρ'-iρ''), leading to the following correspondences:

and .

In several publications authors usually refer to a formula derived from the [START_REF] Cole | Dispersion and absorption in dielectrics, I. alternating current characteristics[END_REF] model to describe polarisation phenomena (e.g. [START_REF] Pelton | Mineral discrimination and removal of inductive coupling with multifrequency IP[END_REF]:

                  c i m ) ( 1 1 1 1 ) ( 0     (3)
where ρ 0 is the direct current resistivity, m is the chargeability and c expresses the enlargement of the relaxation constant (τ) distribution. Note that Eq. ( 3) is an empirical representation of the polarization mechanisms occurring in geological media, but other empirical models exist, among which multiple or generalized Cole-Cole (see [START_REF] Ghorbani | Non-invasive monitoring of water content and textural changes in clay-rocks using spectral induced polarization: A laboratory investigation[END_REF], or constant phase angle model (see for instance [START_REF] Dias | Developments in a model to describe low-frequency electrical polarization of rocks[END_REF] for an extended model collection).

In the following text, in an effort to homogenise and clarify the representation of polarisation and in agreement with other researcher approaches [START_REF] Knight | An introduction to rock physics principles for nearsurface geophysics[END_REF], [START_REF] Revil | Effective conductivity and permittivity of unsaturated porous materials in the frequency range 1 mHz -1GHz[END_REF], [START_REF] Loewer | Ultrabroad-band electrical spectroscopy of soils and sediments-a combined permittivity and conductivity model[END_REF]), we regroup all the different polarization mechanisms that can contribute and we express the electrical polarisation in terms of a real effective parameter, the relative permittivity ε r , defined by ε'=ε 0 ε r (ε 0 being the permittivity in vacuum, ε 0 =8.854 x 10 -12 Fm -1 ). Consequently, when considering the published results expressed by the imaginary part of the conductivity we introduce . In the following text this relative effective permittivity is for sake of simplicity denominated permittivity, we also use the term imaginary part of the permittivity for ε"/ε 0 .

Looking first at simple experimental cases

As introductive illustrations of both the magnitude and the frequency dependence of the permittivity we present in Figure 1a The soil is a clay loam coming from the A horizon of the 'Jardin du Roy' in Paris.

This material is indeed complex, where beside mineral grains exist a lot of biological active and residual elements but the presence of a soil layer is usual in geophysical surveys. It has a high porosity, n=0.481 (bulk density 1.34 g.cm -3 ). When dried, at 105°C for 24 hours, the real measured when the sample is dry, θ=0.017, and the conductivity is too low to be measurable.

In addition to the high frequency permittivity behaviour, one can observe a unique relaxation process which fits the following expression (thin lines in Figure 1b),

(5),

where Δε =28, the real part of the high frequency permittivity ε h ' = 4.5, its imaginary part equals ε h " = 0.5, c =0.65 and τ =1.75 µs.

In this second sample, the behaviour of the permittivity is very different in amplitude and in frequency dependence from that of the moist soil, but even this dry rock sample has a real part of the permittivity staying above 30 between 100 Hz and 10 kHz, significantly higher than 4.5 at high frequencies.

4.-Published effective permittivity values in laboratory

Experiments covering the [3 kHz -3 MHz] frequency range

In order to understand in-field observations made at ground surface or by means of well logging, and to assess the different hypotheses that can be proposed to interpret the data, various laboratory studies have been carried out on both natural and synthetic samples.

Unfortunately, there are more published results derived from laboratory studies than from field campaigns. Several studies have covered a wide frequency range, and the results of some are given in Table 1. It can be observed that clayey formations have permittivity values above hundred or thousand between 10 and 100 kHz.

Values determined at 10 kHz by SIP experiments

SIP gave rise to a large number of laboratory studies, designed to assess the roles played by minerals, their grain size distribution, as well as their fluid nature and content.

Several different models have also been tested in the context of these studies, some of which were carried out at frequencies of several tenths of a kilohertz, allowing the permittivity to be determined at 10 kHz (Table 2). Again values of permittivity above 1000 are reported for clayey formations.

5.-Published in-Situ data

Electrostatic measurements (CCR)

This method can be seen as a generalization of the DC resistivity, where the injection electrodes are replaced by an open capacitor and the voltage electrodes by another open capacitor [START_REF] Grard | A mobile four electrode array and its application to the electrical survey of planetary grounds at shallow depths[END_REF][START_REF] Tabbagh | Determination of electrical properties of the ground at shallow depth with an electrostatic quadrupole: field trials on archaeological sites[END_REF][START_REF] Kuras | Fundamentals of capacitive resistivity technique[END_REF]). To reduce the pole impedances, the transmitted frequency(ies) must in practice be larger than or equal to 10 kHz. By adopting 100 kHz as the upper frequency limit imposed by the induction effect [START_REF] Benderitter | Application of the electrostatic quadripole to sounding in the hectometric depth range[END_REF][START_REF] Tabbagh | 1D complete calculation for electrostatic soundings Interpretation[END_REF], an adequate depth of investigation is achieved for most engineering and environmental applications.

Table 3 summarises the ranges of resistivity and permittivity obtained following the acquisition of profile or map data series.

Electromagnetic frequency domain measurements

Is it possible to measure IP using an inductive system, which would pave the way to direct and large ground and airborne applications? This old question (Hohmann et al. 1970) led to a series of studies that concluded negatively, although these were carried out using simulations and experiments limited to 1.5 kHz for transmitter coil -receiver coil instrument configurations. More recently, some in situ experiments have been performed at higher frequencies, using both helicopter-borne and ground-based devices (Table 4) and the coil-coil configuration gave reliable results even at medium frequencies [START_REF] Kessouri | Medium-frequency electromagnetic device to measure electric conductivity and dielectric permittivity of soils[END_REF]). For FDEM measurements using uniform primary fields, radio-magnetotellurics (RMT) or wavetilt measurements, the influence of the electrical polarisation has been considered in theory

TDEM measurements

In principle, TDEM measurements [START_REF] Nabighian | Time domain electromagnetic prospection method[END_REF] correspond to an extended frequency range, and, as the cut-off time can be as short as 4 µs [START_REF] Thiesson | TDEM magnetic viscosity prospecting using a Slingram coil configuration[END_REF], Auken et al. 2019), the frequency range considered in the present review corresponds to the early time-domain samples but IP effects can also be observed at late times. The presence of electrical polarisation is observed as an unexpected cancellation of monotonicity in the decrease of the step and impulse responses by reference to what would be expected with the same transmitter/receiver configuration when only conductivity and magnetic viscosity are considered. This phenomenon has been observed since the 70's [START_REF] Lee | Sign reversals in the transient method of electrical prospecting (one loop version)[END_REF], and can only be explained by electrical polarisation [START_REF] Weidelt | Response characteristics of coincident loop transient electromagnetic systems[END_REF]. In some cases, the authors were able to check these observations by direct current IP measurements and/or by Cole-Cole modelling.

The corresponding permittivity values, listed in Table 5 below, are estimated from the following equation:

) ) ( 1 Im( 1 0      r .

6-General appraisal: amplitude of the polarisation response and relation with conductivity

An overview of the permittivity and resistivity variations is synthetized in Figure 2.

Although these results do not give an exhaustive list of the experimental studies reported in the literature, they are sufficient to establish what the electrical polarisation looks like in the [3 kHz -3 MHz] frequency range.

(1)

The magnitude of the electrical polarisation expressed in ε r can exceed 10 000. It is certainly higher than that observed in the range of frequencies used for GPR/TDR, as summarized by the [START_REF] Topp | Electromagnetic determination of soil water content: measurements in coaxial transmission lines[END_REF] cubic formula;

(2) It can be seen that there is consistency between laboratory (Tables 1 and2)

and field measurements (Tables 3, 4 and5).

(3) Below approximately 300 Ω.m (above 3.3 S.m -1 ), the permittivity looks inversely correlated with resistivity. It exhibits a high variability at least as strong as that of the resistivity. Figure 2 illustrates this anti-correlation.

However points are not grouped along a straight or curved line, but scattered. Consequently a significant part of information existing in permittivity is independent on that existing in resistivity.

(4) As expected, the existence of several relaxation mechanisms with gradually increasing time constants corresponds to a decrease of the polarisation with frequency.

(5) In all tabulated results, clay content and ionic strength contribute dominantly to polarisation.

Figure 3 plots the product ε 0 ε r ω as a function of σ, derived from laboratory samples. It can be seen that the samples can be divided into two distinct groups, depending on the presence (higher conductivity) or absence (lower conductivity) of clay. In the first group, most of the data points lay between the lines indicating a contribution of 0.1 and 0.01 of the permittivity relatively to the conductivity. This shows that although conductivity dominates the response, the permittivity responses should also be taken into account. In the second group, the permittivity responses clearly dominate the results. It is thus always necessary to consider both responses.

Tables 1 to 5 reveal that, when the measured resistivity is high, its decrease as a function of frequency due to the presence of dielectric losses, ωε'', is more easily observable.

In cases where the dielectric losses contribution is significant, it would be useful to compare these frequency-dependent measurements with the direct current resistivity, in the field as well as in the laboratory.

Implications for field measurements

It is clear that any processing of field data should consider the polarisation processes independently from the usual conduction processes in the [3 kHz -3 MHz] frequency range where short measurement durations open the way to high density surveys. In this range the different electrical methods can be split in in three groups: the coilcoil EMI methods, the EMI methods using distant sources, and the electrostatic method (CCR). In the first group, for lower frequencies, measurement interpretation may be even more complicated by the influence of the magnetic properties. Thus, two situations can be considered: in the first, the method is not sensitive to magnetic properties, which may intervene in the second situation.

In absence of magnetic property influences

In the first situation, given that the Maxwell-Ampère equation states that the conduction the displacement currents are 90 degree's out-of-phase from the conduction ones, the measurement of both in-phase and out-of-phase components (by reference to the primary source signal) is necessary and sufficient to determine the apparent resistivity and the apparent permittivity. Recent electronics devices can easily perform the separation between in-phase and out-of-phase components. The calculation of apparent properties can be done by inversion [START_REF] Kessouri | Medium-frequency electromagnetic device to measure electric conductivity and dielectric permittivity of soils[END_REF], by lookup tables [START_REF] Huang | Dielectric permittivity and resistivity mapping using highfrequency, helicopter-borne EM data[END_REF] or master curves.

Using an inversion procedure is of common use with the electrostatic method [START_REF] Souffaché | Electrostatic multipole for electrical resistivity measurements at the decimetric scale[END_REF][START_REF] Przyklenk | Capacitively Coupled Resistivity measurements to determine frequency-dependent electrical parameters in periglacial environment-theoretical considerations and first field tests[END_REF]) and with coil -coil FDEM instruments where the frequency is sufficiently high and/or the coil separation sufficiently large [START_REF] Simon | GEM-2 HCP L=1.66m[END_REF]). However, in Magnetotellurics (or Radio MT in the considered range) the influence of the permittivity modifies both the amplitude of the wave impedance and the phase lag between the electric and magnetic fields. Consequently both amplitude and phase sounding curves are distorted and a precise interpretation would be impossible in absence of a phase reference locked on the source of the primary field.

In presence of magnetic property influences

For small FDEM or TDEM instruments setup, the magnetic properties may have a strong influence. For FDEM, to the best of our knowledge for ground measurements if the magnetic susceptibility do not exceed 10 -3 SI, the rule of thumb criterion (f L)>10 6 Hz.m is relevant (L being the separation between coils) for neglecting the magnetic susceptibility response. If not, several possibilities exist:

(1) in FDEM determine the magnetic susceptibility by moving the frequency at the lower possible values (for example for a metric inter-coil spacing instrument use a frequency around 10 kHz), where the rule of thumb criterion (f L)<3.10 4 Hz.m is relevant to neglect the permittivity response, and in TDEM determine the magnetic viscosity by the late time response.

(2) in FDEM set the height of the instrument in HCP (horizontal coplanar) coil configuration in such way that the magnetic response is negligible as it is known that the magnetic susceptibility response crosses 0 if the instrument altitude equals 0.38L (for example if L=3.66 m the instrument can be held at a height of 1.4 m)

(3) in FDEM use both HCP and VCP (vertical coplanar) configurations because in the HCP the permittivity in-phase response opposes the magnetic susceptibility while in the VCP it adds [START_REF] Benech | Demonstrating the contribution of dielectric permittivity to the in-phase EMI response of soils: example of an archaeological site in Bahrain[END_REF]. In all cases, instruments must be carefully calibrated [START_REF] Thiesson | Calibration of frequency-domain electromagnetic devices used in near-surface surveying[END_REF].

The same ideas of combining coil separation and clearance can be applied in TDEM [START_REF] Finco | Étude de l'impact simultané des propriétés électriques, diélectriques et magnétiques du sous-sol sur la mesure géophysique par méthode électromagnétique inductive dans le domaine temporel (TDEM)[END_REF].

8-Conclusion

We present the results of varied laboratory and field experiments conducted over different material for different objectives. A very brief review of our scope and objectives is in order here.

First, since the magnitude of the permittivity response can be significant (and in some case dominant), exploration geophysicists should not neglect εω in favour of σ, when implementing EM inductive (both frequency and time domain) and electrostatic exploration methods. In other words, (i) even if present practices are limited to conductivity, taking into account the permittivity will allow a better determination of the conductivity itself which is imperative when simultaneously interpreting different type of conductivity measurement techniques and (ii) there is no reason to a priori exclude information.

Second, the measurement and in-field mapping of permittivity may open opportunities for the exploration of a broad range of new types of geophysical information. In the considered frequency range, both conductivity and permittivity depend on the total amount of mobile ions, conductivity corresponding to free motion and permittivity to constrained motions limited by what may oppose to the ion displacements: pore size and throat configuration, disposition, shapes and types of clay platelets, etc… (Notations: a is the side of a square cell, e is the thickness of disk, cylindrical or square cells and d is the diameter of cylindrical or disk cell).

Figure captions

  and b the experimental results obtained in laboratory for one soil sample and one rock sample in the [100 Hz -10 MHz] range using a capacitive cell and a vector multi-meter (PSM 1735, NumetricQ, Ltd). The cell dimensions are, Figure 1c, 19 x 160 x 160 mm 3 .

  part of the permittivity equals 3 over the whole frequency range in agreement with the[START_REF] Topp | Electromagnetic determination of soil water content: measurements in coaxial transmission lines[END_REF] equation. In that soil sample the drying has eliminated all the polarisation mechanisms except the atomic polarisation of solid grains. The presented measurements have been achieved at a high volumetric water content: θ=0.446 (quasi saturation). One can observe (Figure1a) that the imaginary part is linearly decreasing with frequency, the slope corresponding to a DC conductivity of 21.6 mS.m -1 thus to 46 Ω.m (thin line). The real part has very high values in low frequency as it starts from 9 x 10 5 at 100 Hz, and reaches 4 000 at 3 kHz. Its linear decrease fits with a Jonscher's exponent of -1.45[START_REF] Jonscher | The universal dielectric response[END_REF]. Such a regularity in the decrease corresponds to a large distribution of relaxation times. Higher frequency values agrees relatively well with the Topp et al. formula: for θ=0.446, ε r =29. The curves corresponding to the modelling expression, ε r =(9 10 5 (ω/ω 0 ) -1.45 + 29 -i 0.0216/(ωε 0 )) (4), are drawn by a thin continuous lines in the Figure1a, ω 0 being the angular frequency corresponding to 100 Hz.The rock sample is a Lutetian limestone extracted from the Saint Pierre Aigle quarry (Aisne, France). It contains calcite, 85% in weight, sand 10% and a few percent of phyllosilicates(clay). The open porosity n equals 0.18. The results presented in Figure1b are

(

  Grossley 1981) with permittivity values as high as 200[START_REF] Sinha | Influence of altitude and displacement currents on plane wave EM fields[END_REF] but unfortunately the published experiments are limited to very high resistive areas (permafrost and/or crystalline basement) where the permittivity remains below 10 ([START_REF] Kalscheuer | Radiomagnetotelluric twodimensional forward and inverse modelling accounting for displacement currents[END_REF]).

Figure 1 :

 1 Figure 1: Example of frequency variation of the permittivity of (a) a clay loam soil (A

Fig 2 :

 2 Fig 2: Relationship between the relative permittivity and the conductivity (in Sm -1 )/resistivity

Fig 3 :

 3 Fig 3: Laboratory results: comparison between ε 0 ε r ω (in S m -1 ) and σ (in Sm -1 ) values

  Figure 1a

Figure 1c

 1c Figure 1c

Figure

  Figure. 2

Table captions Table 1 :

 captions1 Permittivity values in laboratory experiments covering the [3 kHz -3 MHz] range.

Table 2 :

 2 Permittivity values delivered at 10 kHz by laboratory SIP experiments. (Notations: e is the thickness of disk or cylindrical cells and d is the diameter of cylindrical or disk cell).

Table 3 :

 3 Permittivity values obtained by in-situ electrostatic measurements.

Table 4 :

 4 Permittivity values obtained by in situ frequency domain measurements.

Table 5 :

 5 Permittivity values obtained by in situ TDEM measurements.

Table 2

 2

	Clay loam	1.56 MHz	20 -60 Ω.m	35 -110

Table 5

 5

Reference Array configuration

Soil or rock context

Frequency Electrical resistivity range Relative permittivity range Grard & Tabbagh 1991, J.G.R., 96-B3, 4117-4123. 1m


Valerie Ngô-Muller

Michael S Engel

Romain Garrouste

Jean-Marc Pouillon

André Nel
email: anel@mnhn.fr

The first predatory dance fly of the subfamily Ocydromiinae with specialized, raptorial legs in mid-Cretaceous amber from Myanmar (Diptera: Hybotidae)

Keywords: zoobank Brachycera, Cenomanian, Empidoidea, predatory dance flies, taxonomy

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The family Hybotidae largely includes predaceous flies that typically frequent on fallen logs or semi-aquatic vegetation in forested environments. Although generally predators, a few are known to feed on pollen. Where known for the predatory taxa, many species capture preys while in flight although tachydromiines more readily subdue their prey on plant surfaces [START_REF] Chvála | The Empidoidea (Diptera) of Fennoscandia and Denmark. 2. General part. The families Hybotidae, Atelestidae and Microphoridae[END_REF]. During feeding, hybotids grasp the prey item with their hind legs while manipulating their food and stabilizing themselves with their fore and midlegs [START_REF] Wilder | A revision of the genus Syneches Walker (Diptera: Empididae) for North America and the Antilles[END_REF].

Accordingly, various modifications of the hind legs have arisen among Hybotidae which appear to be associated with capturing and holding their prey, the most elaborate of which belong to groups believed to take their victims while in flight. The fossil record of the family is scanty, with comparatively few records [START_REF] Evenhuis | Catalogue of the fossil flies of the World (Insecta: Diptera)[END_REF]. . Hybotid flies are quite rare in the Mesozoic, with only the Early Cretaceous genus Pseudoacarterus [START_REF] Waters | A new hybotine dipteran from the Cretaceous of Botswana[END_REF], the mid-Cretaceous Electrocyrtoma [START_REF] Cockerell | Insects in Burmese amber[END_REF], and the Late Cretaceous Archiplatypus Kovalev, 1974. Herein we report from the mid-Cretaceous Burmese amber a remarkable new hybotid fly of the subfamily Ocydromiinae with highly specialized male hind legs, implying that the evolution of such features appeared early in the family's history. We provide a description of this new taxon and comparisons with the diversity of modern and other extinct genera.

Material and methods

The specimen comes from the earliest Cenomanian amber (98.79 ± 0.62 Ma, based on U-Pb zircon dating of the volcanoclastic matrix; [START_REF] Shi | Age constraint on Burmese amber based on U-Pb dating of zircons[END_REF]; location map of amber-bearing locality in [START_REF] Yin | A potentially diverse fauna of springtail-hunting scydmaenines during the late Mesozoic (Coleoptera, Staphylinidae, Scydmaeninae)[END_REF] fig. 1A). It is preserved in a piece of clear, yellow amber. The amber piece was cut, shaped, and polished using a diamond disk under water and diatomite, and was then mounted between two coverslips with sugar medium dissolved at saturation in water, before being examined and photographed. The inclusion was examined and measured under incident light with an Olympus SZX9 and Leitz Wetzlar binocular microscopes. The holotype is deposited in the amber collection of the Musée d'Histoire Naturelle et d'Ethnographie de Colmar (MHNEC), France under collection number MHNE.2020.7.1. We follow the terminology of [START_REF] Cumming | Chapter 2. Adult morphology and terminology[END_REF] and the empidoid classification of [START_REF] Sinclair | The morphology, higher-level phylogeny and classification of the Empidoidea (Diptera)[END_REF]. Description. Male, body 2.03 mm long (Fig. 1); head 0.22 mm long, 0.30 mm wide; compound eyes apparently setose, with few visible very short setae, large, meeting at one point above (a gas bubble occludes views of the upper part of the head); ptilinal fissure absent; occiput and frons bare; apex of antenna with lengthened, arista-like stylus (Fig. 3A), possibly dimerous as a very short structure is present between it and postpedicel, apical on postpedicel; postpedicel pear-shaped, 0.06 mm long, quite shorter than stylus (0.66 mm long) and shorter than pedicel; pedicel with an apical crown of strong setae, longer than scape; proboscis short, oriented ventrad.

Systematic palaeontology

Thorax 0.64 mm long, 0.45 mm wide, with bare laterotergite; two elongate setae on scutellum and three near base of wing.

Wing hyaline (Fig. 1B), longer than abdomen, apex rounded, 2.22 mm long, 0.45 mm wide, covered with microtrichia, without darkened pterostigma; Sc ending on C, 1.08 mm from wing base; C extending to wing apex, terminating at apex of M1; Rs originating well distad level of humeral vein (h), 0.39 mm from it; branches of Rs not strongly thickened and crowded anterobasally; R2+3 elongate and straight, 0.08 mm long; R4+5 unbranched, 1.29 mm long, nearly straight and parallel to M1; cell dm present, 0.69 mm long, 0.13 mm wide, with three veins distally emerging from it: M1 (0.6 mm long), M2 (present as an exceedingly faint nebulous vein disappearing well before wing margin, distinctly shorter than m-m crossvein), and M4 (0.19 mm long); CuA forming a distinct angle with basal part of CuP; cell cua scarcely shorter than cell bm (bm-cm and CuA nearly confluent), with outer angle obtuse; CuA+CuP vein not reaching wing margin; narrow anal lobe, 0.13 mm wide; lunule absent; a series of long setae on anal margin; haltere elongate, 0.41 mm long, 0.08 mm wide. Figs 2,3A). Pretarsal claws simple, empodia setiform; protibial gland present; profemur 0.29 mm long, 0.06 mm wide, curved, with a series of at least five long setae along inner surface; protibia 0.29 mm long, 0.04 mm wide, with three rows of stout setae, one apical spur; probasitarsus elongate, 0.18 mm long, with two series of long setae; protarsomere II 0.11 mm long; protarsomere III 0.08 mm long; protarsomere IV 0.04 mm long; protarsomere V 0.06 mm long; mesofemur crassate, approximately 0.42 mm long, 0.06 mm wide, with four stout, thickened, erect, spine-like setae on ventral surface; mesotibia 0.53 mm long, 0.04 mm wide, basally geniculate, with two series of rather short, oblique setae, a single elongate apical spur; mesotarsus ca. 0.53 mm long; metacoxa with a single, elongate, posteroventral seta, 0.29 mm long; metafemur 0.76 mm long, 0.09 mm wide, with a pronounced inner curvature and two stout, thickened, enlarged, subapical spurs (0.16 mm long) and a series of long, stout, erect, spine-like setae along ventral surface, a series of shorter, finer, curved, oblique setae along dorsal surface (Fig. 2); metatibia basally geniculate, strongly curved, with a subapical hooklike lobe (0.1 mm long), apex of lobe narrowly rounded, a row of long, oblique setae on inner surface, a single, short, basally thickened apical spur, outer surface with a row of erect to semierect, short, fine setae; metabasitarsus greatly broadened and somewhat trapezoidal as lateral margins slightly converge apically, 0.09 mm long, 0.04 mm wide, outer edge with a row of short, strongly curved setae, inner edge with a row of longer, obliquely erect, largely straight setae; metarsomere II elongate, 0.16 mm long, 0.01 mm wide, with two rows of fine, long setae; metataresomere III 0.09 mm long, 0.01 mm wide, metatarsomere IV 0.08 mm long, 0.008 mm wide, metatarsomere V 0.08 mm long, 0.008 mm wide.

Legs (

Abdomen elongate (Fig. 2), rather narrow, 1.0 mm long, 0.24 mm wide, with a single, elongate spine-like seta laterally on segments II-V; male terminalia asymmetrical and rotated 90° (Fig. 3B); cercus thin with a small inner extension; hypoproct small near cercal base; phallus large, as long as and broader than cerci; epandrium greatly enlarged, spoon-shaped, apparently with an apical pair of articulated surstyli.

Female unknown.

Discussion

Pouillonhybos gen. nov. can be placed within the Empidoidea owing to the following characters: ptilinal fissure and lunule absent; empodia setiform; pedicel shorter than flagellum; flagellum with three flagellomeres; cell cua small; stylus elongate and thin; wing apex rounded; branches of R not strongly thickened and crowded anterobasally; C extending to wing apex;

CuA+CuP not reaching wing margin. Using those traits outlined by recent authors (e.g., [START_REF] Steyskal | Chapter 47 Empididae[END_REF][START_REF] Sinclair | The morphology, higher-level phylogeny and classification of the Empidoidea (Diptera)[END_REF], 2017), the new fossil genus falls among the Hybotidae based on the following characters: apex of antenna with lengthened, bristle-like stylus (stylus also not setulose); thorax with bare laterotergite; protibial gland present; C ending at wing apex, instead of circumambient (but some empidids also have a costa terminating at the wing apex); Rs originating well distal to level of humeral vein (h); R4+5 unbranched; CuA forming a distinct angle with basal part of CuP. It should be noted that some Empididae have a wing venation quite similar to that of Pouillonhybos (viz. Macrostomus Wiedemann, 1817), especially in the narrow anal lobe and shape of cell cua, and even in the C terminating on M1. Such empidids, however, differ from Pouillonhybos in the setulose scape (not so in the fossil), shape of the more elongate flagellum, and, in the case of Macrostomus, lacking a defined stylus (Smith, 1961: fig. 3;[START_REF] Rafael | The Neotropical genera Macrostomus Wiedemann and Porphyrochroa Melander (Diptera, Empididae, Empidinae)[END_REF]: figs 1, 3). [START_REF] Wahlberg | Molecular phylogenetics reveals novel relationships within Empidoidea (Diptera)[END_REF] presented a molecular phylogenetic analysis of the Empidoidea, in which Hybotidae are subdivided into six subfamilies: Bicellariinae [START_REF] Sinclair | The morphology, higher-level phylogeny and classification of the Empidoidea (Diptera)[END_REF]Ocydromiinae Schiner, 1862;Oedaleinae Chvála, 1983;Hybotinae Meigen, 1820;Tachydromiinae Meigen, 1822;andTrichininae Chvála, 1983. Most recently, Sinclair (2019) added the subfamily Stuckenbergomyiinae [START_REF] Sinclair | Revision of the southern African genus Stuckenbergomyia Smith, 1971 (Diptera, Empidoidea) and proposal of a new subfamily[END_REF]. Affinities between the new fossil and Trichininae (Trichinomyia Tuomikoski, 1959 andTrichina Meigen, 1830) are excluded because of the absence of an alula, the narrow anal lobe, the absence of a pterostigma, and the elongate stylus [START_REF] Tuomikoski | Mitteilungen über die Empididen (Diptera) Finnlands. 6. Trichinomyia gen. n., eine neue Ocydromiinengattung[END_REF]. The Tachydromiinae are excluded because M2 is present, albeit only as a short, faintly nebulous vein, cell dm present, and cell cua elongate [START_REF] Sinclair | The morphology, higher-level phylogeny and classification of the Empidoidea (Diptera)[END_REF], 2017). Similarly, the subfamily Bicellariinae (Bicellaria Macquart, 1823) can be discounted owing to the narrow anal area and presence of cell dm [START_REF] Kato | On the genus Bicellaria Macquart from Japan (Diptera, Empididae)[END_REF]. The Oedaleinae have a branched M1+2 and an anterad projecting proboscis, quite unlike Pouillonhybos, and the Stuckenbergomyiinae differ from the new fossil genus in the truncate cell cua, shortened stylus, and three veins emitted from cell dm.

Affinities with the Hybotinae would also be excluded because of the presence of M2 in

Pouillonhybos, but as it is rudimentary in the present fossil we prefer to compare the new genus to extant and fossil hybotine genera. The short proboscis of Pouillonhybos excludes affinities with Syneches [START_REF] Walker | Diptera. Part III. Insecta Saundersiana: or characters of undescribed insects in the collection of William Wilson Saunders[END_REF](incl. Parahybos Kertész, 1899, Harpamerus Bigot, 1859, and Epiceia Walker, 1860), Hybos Meigen, 1803, Smithybos Ale-Rocha, 2000, Syndyas [START_REF] Loew | Bidrag till kännedomen om Afrikas Diptera[END_REF]Lactistomyia Melander, 1902. In the genera Afrohybos [START_REF] Smith | Afrikanische Empididae. (Ergebnisse der Deutschen Zoologischen Ostafrika -Expedition[END_REF], Cerathybos Bezzi, 1909, Euhybus Coquillett, 1895, and Neohybos Ale-Rocha & Carvalho, 2003 cell cua extends far distal to cell bm [START_REF] Melander | A monograph of the North American Empididae. Part 1[END_REF][START_REF] Smith | Afrikanische Empididae. (Ergebnisse der Deutschen Zoologischen Ostafrika -Expedition[END_REF][START_REF] Ale-Rocha | Description of Smithybos equatoriensis, new genus and species of Hybotinae from Ecuador (Diptera, Empididae)[END_REF][START_REF] Ale-Rocha | Revisão dos gênero Euhybus Coquillett (Diptera, Empididae, Hybotinae) da regiao neotropical. Grupo dimidiatus[END_REF], 2008;[START_REF] Ale-Rocha | Neohybos gen. nov. (Diptera, Empidoidea, Hybotinae) from the Neotropical Region[END_REF][START_REF] Grootaert | A new Syndyas Loew, 1857 (Diptera: Hybotidae: Hybotinae) from mangroves in Singapore, with a review of the Oriental and Australasian species[END_REF][START_REF] Sinclair | Revision of the Southern Africa genus Acarterus Loew (Diptera: Empidoidea; Hybotinae), with description of seven new species[END_REF][START_REF] Sinclair | Revision of Fijian Syneches (Diptera: Empidoidea: Hybotidae), with a reassessment of the genus[END_REF][START_REF] Liu | Two new species of Syneches belonging to S. signatus species-group from Vietnam (Diptera: Empidoidea, Hybotinae)[END_REF][START_REF] Liu | Genus Euhybus (Diptera: Empididae) newly found in Shaanxi province with description of a new species[END_REF], quite unlike Pouillonhybos in which these cells are about subequal in length, with cua only scarcely shorter than bm. Acarterus [START_REF] Loew | Bidrag till kännedomen om Afrikas Diptera[END_REF], Lamachella Melander, 1928, and the Cretaceous genus Pseudoacarterus Waters, 1989 have cell cua apically convex (i.e., CuA is convexly arched), while in the new genus CuA is straight and oblique [START_REF] Waters | A new hybotine dipteran from the Cretaceous of Botswana[END_REF][START_REF] Sinclair | Revision of the Southern Africa genus Acarterus Loew (Diptera: Empidoidea; Hybotinae), with description of seven new species[END_REF]. Additionally, in Lamachella cua is distinctly shorter than bm, a pterostigma is usually present, m-cu is more elongate and not in near alignment with CuA, and R2+3 is sinuate, unlike Pouillonhybos (Smith, 1969: figs 137-138;[START_REF] Sinclair | Manual of Afrotropical Diptera. Volume 2. Nematocerous Diptera and Brachycera[END_REF]. Chillcottomyia [START_REF] Saigusa | New genera of Empididae (Diptera) from Eastern Asia[END_REF], Stenoproctus Loew, 1858, and the Miocene genus Syneproctus Solórzano-Kraemer et al., 2020 have cell cua quite shorter than bm, and with apex convex, nearly perpendicular to the two veins CuA and CuP, unlike Pouillonhybos (Smith, 1969: figs 127-128, 130;[START_REF] Yang | A new species of Chillcottomyia from China (Diptera: Empididae)[END_REF][START_REF] Solórzano-Kraemer | New genus and first record of Hybotinae (Diptera: Empidoidea: Hybotidae) in Middle Miocene Dominican amber[END_REF].

The fossil shares many traits with the subfamily Ocydromiinae, in which cell cua is shorter than or about as long as cell bm, with its outer angle obtuse; the postpedicel shorter than the arista-like stylus; a cell dm present; and the proboscis oriented ventrad, all as in Pouillonhybos [START_REF] Sinclair | Revision of the genus Apterodromia Oldroyd (Diptera: Empidoidea), with a redefinition of the tribe Ocydromiini[END_REF][START_REF] Sinclair | The morphology, higher-level phylogeny and classification of the Empidoidea (Diptera)[END_REF][START_REF] Wahlberg | Molecular phylogenetics reveals novel relationships within Empidoidea (Diptera)[END_REF]. In addition, ocydromiines also have an epandrium with an apical pair of articulated surstyli, another trait apparently present in Pouillonhybos, pointing to a placement within the subfamily for this new Cretaceous genus.

Within this subfamily, Leptopezella [START_REF] Sinclair | Leptopezella, a new Southern Hemisphere genus of Ocydromiinae (Diptera: Empidoidea: Hybotidae)[END_REF] has no discal cell, a pterostigma, cell cua clearly shorter than bm, and no metatibial extension, and is therefore quite different from Pouillonhybos [START_REF] Sinclair | Leptopezella, a new Southern Hemisphere genus of Ocydromiinae (Diptera: Empidoidea: Hybotidae)[END_REF]. Abocciputa [START_REF] Plant | A revision of the Ocydromiinae (Diptera: Empidoidea: Hybotidae) of New Zealand with descriptions of new genera and species[END_REF]Chvalaea Papp &[START_REF] Papp | A new genus and three new species of Hybotidae with new records of the Hungarian Empidoidea (Diptera)[END_REF] differ from Pouillonhybos in that cell cua is significantly shorter than cell bm, there is no trace of M2, and R4+5 is not parallel to M1 [START_REF] Plant | A revision of the Ocydromiinae (Diptera: Empidoidea: Hybotidae) of New Zealand with descriptions of new genera and species[END_REF]Papp & Földvári, 2002;[START_REF] Shamshev | New data on the genera Allanthalia Melander, Chvalaea Papp et Földvári and Leptodromiella Tuomikoski (Diptera: Hybotidae) from the Palaearctic[END_REF], and the genera Austropeza [START_REF] Plant | A revision of the Ocydromiinae (Diptera: Empidoidea: Hybotidae) of New Zealand with descriptions of new genera and species[END_REF], Pseudoscelolabes Collin, 1933, and Leptodromiella Tuomikoski, 1936 also have cell bm much longer than cell cua [START_REF] Tuomikoski | Bemerkungen über die Clusiiden (Dipt.) Finnlands[END_REF][START_REF] Collin | Notes on the Empididae (Diptera) with additions and corrections to the British list[END_REF][START_REF] Collin | New Zealand Empididae based on material in the British Museum (Natural History)[END_REF][START_REF] Collin | Empididae. Diptera of Patagonia and South Chile based mainly on material in the British Museum[END_REF][START_REF] Chvála | The Empidoidea (Diptera) of Fennoscandia and Denmark. 2. General part. The families Hybotidae, Atelestidae and Microphoridae[END_REF][START_REF] Plant | A revision of the Ocydromiinae (Diptera: Empidoidea: Hybotidae) of New Zealand with descriptions of new genera and species[END_REF].

Oropezella also has a rather long cell dm and quite short veins emerging from it, unlike Pouillonhybos [START_REF] Chvála | The Empidoidea (Diptera) of Fennoscandia and Denmark. 2. General part. The families Hybotidae, Atelestidae and Microphoridae[END_REF][START_REF] Ale-Rocha | Redescription of Oropezella abdominalis Collin from Chile with description of the male and a key to the Neotropical genera of Ocydromiinae (Diptera, Hybotidae)[END_REF]. In the genus Stylocydromia [START_REF] Saigusa | New genera of Empididae (Diptera) from Eastern Asia[END_REF] there is a long cell dm, while the base of Rs is much closer to h than to r-m, unlike the condition in Pouillonhybos [START_REF] Saigusa | New genera of Empididae (Diptera) from Eastern Asia[END_REF]. The genus Leptopeza Macquart, 1827 has a rudimentary M1 and cell bm much longer than cell cua [START_REF] Chvála | The Empidoidea (Diptera) of Fennoscandia and Denmark. 2. General part. The families Hybotidae, Atelestidae and Microphoridae[END_REF], while in Ocydromia Meigen, 1820, the stylus is supra-apical rather than apical in Pouillonhybos, and moreover cell bm is much longer than cell cua and there is no trace of a third vein emerging from dm [START_REF] Chvála | The Empidoidea (Diptera) of Fennoscandia and Denmark. 2. General part. The families Hybotidae, Atelestidae and Microphoridae[END_REF].

Apterodromia Oldroyd, 1949 shares with Pouillonhybos a cell cua nearly as long as bm, but it has an anal area much more reduced than Pouillonhybos and a well-defined M2 [START_REF] Sinclair | Revision of the genus Apterodromia Oldroyd (Diptera: Empidoidea), with a redefinition of the tribe Ocydromiini[END_REF]. Neotrichina Sinclair & Cumming, 2000 has the stylus scarcely as long as the postpedicel, three veins emitted from dm that reach the wing margin, cell cua much shorter than bm, an anal vein almost complete to the wing margin, CuA closing cua almost straight, all quite distinct from the character states present in Pouillonhybos; also some species of Neotrichina have the metatibia dilated apically but without a lateral expansion, and therefore unlike that in Pouillonhybos [START_REF] Collin | Empididae. Diptera of Patagonia and South Chile based mainly on material in the British Museum[END_REF][START_REF] Sinclair | Revision of the genus Apterodromia Oldroyd (Diptera: Empidoidea), with a redefinition of the tribe Ocydromiini[END_REF]. Hoplopeza [START_REF] Bezzi | Beiträge zur Kenntnis der südamerikanischen Dipterenfauna auf Grund der Sammelergebnisse einer Reise in Chile, Peru und Bolivia, ausgeführt in den Jahren[END_REF] and Scelolabes Philippi, 1865 can be excluded from having any affinity with the new fossil genus because they have Rs short, arising near the apex of cell bm, and cell cua much shorter than bm [START_REF] Collin | Empididae. Diptera of Patagonia and South Chile based mainly on material in the British Museum[END_REF][START_REF] Rafael | Revisão das espécies neotropicais de Empididae (Diptera) descritas por Mario Bezzi. III. Ocydromiinae[END_REF][START_REF] Sinclair | Revision of the genus Apterodromia Oldroyd (Diptera: Empidoidea), with a redefinition of the tribe Ocydromiini[END_REF]. Lastly, Leptodromia Sinclair & Cumming, 2000 has cell cua two-thirds length of cell bm and a short M1 that does not reach the wing margin (Bezzi, 1904;[START_REF] Sinclair | Revision of the genus Apterodromia Oldroyd (Diptera: Empidoidea), with a redefinition of the tribe Ocydromiini[END_REF].

Among known extinct genera, the Eocene Baltic amber genus Palaeoleptopeza [START_REF] Meunier | Monographie des Empidae de l'ambre de la Baltique[END_REF], was considered as near to "Leptopeza but with spinose hind femora" by Melander (1928: 371), and differs from Pouillonhybos in that the postpedicel is as long as the stylus, R2+3 is distinctly arcuate, and M2 is more clearly defined as a fuscous nebulous trace (Meunier, 1908: pl. 7, fig. 15, pl. 8, fig. 4). Ecommocydromia difficilis [START_REF] Schlüter | Zur Systematik und Palökologie harzkonservieter Arthropoda einer Taphozönose aus dem Cenomanium von NW-Frankreich[END_REF] is the unique Cretaceous taxon currently attributed to the Ocydromiinae; it is based on an incomplete specimen in amber with numerous characters of wing venation missing (in particular, the relative positions of r-m, base of Rs, and h are unknown), rendering it impossible to determine if this assignment is valid or whether it might belong to the Dolichopodidae. It has three long veins emerging from an elongate cell dm, which is quite unlike any other Ocydromiinae. The cells bm and cua are not preserved, and we believe that attribution to Ocydromiinae is difficult to justify (Schlüter, 1978: figs 86-89). Ecommocydromia difficilis is perhaps best considered as incertae sedis until more completely preserved specimens are recovered. The Eocene Baltic amber genus Meghyperella [START_REF] Meunier | Monographie des Empidae de l'ambre de la Baltique[END_REF] was considered by Melander (1928: 370) as "apparently related to Leptopeza" (Ocydromiinae), and possibly a "predecessor of Stenoproctus" (Hybotinae). It differs from Pouillonhybos in the exceptionally short stylus [START_REF] Meunier | Monographie des Empidae de l'ambre de la Baltique[END_REF], and needs to be revised. [START_REF] Evenhuis | Catalogue of the fossil flies of the World (Insecta: Diptera)[END_REF] listed it among the fossil 'Empididae'. Note that the alleged Oligocene hybotine genus Eternia [START_REF] Martins-Neto | Dípteros (Insecta, Empidoidea) da Formação Tremembé, Bacia de Taubaté, Oligoceno do Estado de São Paulo. 1. Família Hybotidae[END_REF] is based on a poorly preserved fossil, but it preserves cua as long as bm with an obtuse outer angle, suggesting it belongs to the Ocydromiinae [START_REF] Martins-Neto | Dípteros (Insecta, Empidoidea) da Formação Tremembé, Bacia de Taubaté, Oligoceno do Estado de São Paulo. 1. Família Hybotidae[END_REF]. Lastly, the putative Oligocene hybotine genus Tremembella Martins-Neto et al., 1992 is based on an exceptionally poorly preserved specimen and until more complete material is discovered it remains impossible to compare with the other genera.

The only other Hybotidae from Burmese amber is Electrocyrtoma burmanica [START_REF] Cockerell | Insects in Burmese amber[END_REF], a genus that differs from Pouillonhybos in the absence of cell dm, a much broader cell bm, and a cell cua much shorter than bm (Cockerell, 1917: fig. 5). Melander (1928: 368) indicated that Electrocyrtoma resembled Bicellaria, while Grimaldi & Cumming (1999: 51) considered it as belonging to Tachydromiinae.

Conclusions

Pouillonhybos is the first ocydromiine from Burmese amber and probably the only definitive representative of this subfamily from the Cretaceous. Modern Ocydromiinae are predators [START_REF] Chvála | Swarming, mating and feeding habits in Empididae (Diptera), and their significance in evolution of the family[END_REF], and like other hybotids often hold their prey with the hind legs while feeding. However, no living ocydromiines have a massive prominence on the metatibia like the one present in Pouillonhybos. This structure, together with the stout setae and spines on the metafemur, erect and spine-like setae on ventral surface of mesofemur, and setae on the other femora and tibiae, coxae, and abdomen, strongly suggest that Pouillonhybos was a predator who, like surviving relatives, captured and hold prey with its hind legs. The hybotine genera Hybos, Syndyas, and Syneches have raptorial hind legs [START_REF] Grootaert | Species radiation of predatory empidoids (Insecta, Diptera) in lowland rainforest in northern Papua New Guinea[END_REF], with strong spines on the metafemur but no prominent extension on the metatibia. Some species of Lactistomyia also have raptorial hind legs with a metatibial lateral extension, albeit situated in a distinctly more proximal position than that of Pouillonhybos [START_REF] Ale-Rocha | Revision of the Neotropical genus Lactistomyia Melander (Diptera, Hybotidae, Hybotinae)[END_REF]. Afrohybos has also a highly modified hind leg with a ventral extension of the metafemur and strong spines on the metatibia (Smith, 1969: fig. 96), while the hybotine genus Syneproctus has a metatibia with an apical extension most analogous to that of the fossil, but quite a bit smaller than that of Pouillonhybos.

Many Hybotidae and Dolichopodidae have raptorial fore and/or mid legs with greatly modified femora and/or tibiae, and some dolichopodids have a strong spine on an apical extension of the meso-and/or metatibia and/or structures on the metabasitarsus, resembling the metatibial extension of Pouillonhybos and Syneproctus (Bickel, 1985: figs 7-8;[START_REF] Zhu | A review of the species of Thambemyia Oldroyd (Diptera: Dolichopodidae) from China[END_REF][START_REF] Runyon | New species of long-legged flies (Diptera: Dolichopodidae) from Central Pennsylvania[END_REF][START_REF] Grichanov | Dolichopodidae (long-legged dance flies)[END_REF]. It is clear that not only various leg modifications have evolved convergently among empidoid lineages for subduing and handling prey, but such traits also appeared in the early phases of their diversification as evidence by their presence in at least one Cretaceous ocydromiine. The special shape of the legs of Pouillonhybos suggests that it was catching its preys during flight, as for the extant Ocydromiinae [START_REF] Daugeron | Evolution of feeding and mating behaviors in the Empidoidea (Diptera: Eremoneura)[END_REF].

Many Empidoidea are well-known for their complex mating behavior, with gifts to the female and semaphore behaviors. Nevertheless, after Downes (1969: 290), both sexes of 'the extant Hybotinae and some Ocydromiinae' .... 'capture prey and the feeding habits have no relationship to the flight in the mating swarm'. Even if the extant Ocydromiinae are making swarms without mating [START_REF] Daugeron | Evolution of feeding and mating behaviors in the Empidoidea (Diptera: Eremoneura)[END_REF], in absence of information on the morphology of the female of Pouillonhybos, it is not possible to infer hypotheses about the mating behavior in this ancient Hybotidae. Ngô-Muller, Engel, & Nel, gen. et sp. nov., holotype MHNE.2020.7.1. A, habitus, ventral view. B, wing, arrow Ngô-Muller, Engel, & Nel, gen. et sp. nov., holotype MHNE.2020.7.1. Hind leg, c.s. coxal seta, bt. basitarsus, f. femur, t. tibia, s. spur. Scale bar = 1 mm. Ngô-Muller, Engel, & Nel, gen. et sp. nov., holotype MHNE.2020.7.1. A, head, above. B, male terminalia, lateral view; ce cercus, ep epandrium, ph phallus. Scale bar = 0.5 mm (A), 0.1 mm (B).

FIG. 1. Pouillonhybos venator

FIG. 2. Pouillonhybos venator

FIG. 3. Pouillonhybos venator

  base of vein M2. Scale bars = 1 mm.

  Pouillonhybos venatorNgô-Muller, Engel & Nel, sp. nov. Diagnosis. Proboscis short, oriented ventrally. Antenna with stylus elongate, arista-like, longer than postpedicel. Protibial gland present. Mesofemur crassate, with four stout, elongate, ventral setae; mesotibia elongate, basally slightly geniculate, with two outer rows of short, oblique setae and a single, elongate, apical spur. Hind legs greatly modified; metacoxa with one elongate seta; metafemur with pronounced inner curvature, with two stout, thickened, elongate subapical spines and a series of stout, elongate setae ventrally as well as a row of shorter, curved prolateral (ectal) setae; metatibia strongly arched with a prominent, thick, subapical, hook-like lobe, and a row of long inner setae, and an apical spur; metabasitarsus broadened, with an ectal row of curved setae and an ental row of long, rather straight setae. Wings well developed, much longer than abdomen; pterostigma lacking; C terminating at wing apex, on M1; stem of Rs short;

	Etymology. The specific epithet is taken from the Latin word 'venator', meaning, 'hunter',
	referring to the specialized metatibiae of this species.
	Material. Holotype MHNE.2020.7.1 (a complete specimen in amber, coll. Jean-Marc Pouillon);
	Musée d'Histoire Naturelle et d'Ethnographie de Colmar, France.
	Horizon and locality. Noije Bum Hill, Hukawng Valley, Kachin State, Myanmar; lower
	Cenomanian, mid-Cretaceous.
	Diagnosis. As for genus; only one included species.
	Order Diptera Linnaeus, 1758 Superfamily Empidoidea Latreille, 1809 Family Hybotidae Meigen, 1820 Subfamily Ocydromiinae Schiner, 1862 Genus Pouillonhybos Ngô-Muller, Engel & Nel, gen. nov. urn:lsid:zoobank.org:act:39BCAEAE-6677-496A-A30B-1B9691ADA0FD with faint nebulous trace of M2, trace disappearing after length about 0.5× m-m length; two veins emitted from dm reaching wing margin (M1 and M4); cua present, elongate, about as long as bm, with apex oblique, with outer angle obtuse, without spur vein; vein dm-m strongly oblique; CuA not broadly arching apically to CuA+CuP; CuP and CuA+CuP faint, not reaching wing margin; anal lobe narrow; alula absent. Epandrium with pair of articulated surstyli apically. Etymology. The new generic name is a combination of the surname Pouillon, honoring Jean-Marc Pouillon, who donated the type specimen, and Hybos Meigen, the type genus of the family. The gender of the name is masculine. Pouillonhybos venator Ngô-Muller, Engel & Nel, sp. nov. Figs 1-3 Type species: R2+3 comparatively straight; R4+5 and M1 roughly parallel; dm present, elongate; m-m arched, urn:lsid:zoobank.org:act:AB82FF4F-897E-4784-ADCC-5F3D8D025C1C

Acknowledgments

We are sincerely grateful to two anonymous referees for their useful comments on the first version of the paper.