Tachyempis Melander, 1928

Shamshev, I. V. & Grootaert, P .., 2024, Revision of the Nearctic species of the genus Tachyempis Melander, 1928 (Diptera: Hybotidae), Russian Entomological Journal 33 (1), pp. 124-135 : 125-126

Genus Tachyempis Melander, 1928

Tachyempis Melander, 1928: 288 . Type species: Tachydromia agens Melander, 1910 (by original designation).

DIAGNOSIS. Tachyempis is distinguished from other Nearctic genera of Tachydromiini by a combination of the following characters: face entirely obliterated by eyes; eyes with enlarged ommatidia below antennae; frons with more or less divergent margins above, near anterior ocellus at least 1.5 times broader than near antennae; occiput with scattered, fine setae on lower part; vertical setae lateroclinate; thorax with mesopleuron tomentose; wing membrane hyaline to more or less infuscate, cell br shorter than cell bm at apex, cell cua and vein CuA absent.

DESCRIPTION. Very small (about 1.5 mm) to small (2– 2.5 mm) brown to brownish black and densely greyish pruinose flies. Frons near anterior ocellus at least 1.5 times broader than near antennae (broadly to narrowly V-shaped or sometimes nearly Y-shaped), with sides gently bowed outwardly to straight. Eyes without ommatrichia, convex and with enlarged ommatidia below antennae. Face entirely obliterated by eyes. Vertical setae prominent, 1 pair, lateroclinate, short to very long, broadly to narrowly spaced. Ocellar triangle usually with 2 pairs of setae; anterior pair (when present) always minute, slightly inclinate; posterior pair distinct, lateroclinate; usually 2 minute, slightly lateroclinate postocellars. Antenna with postpedicel short, nearly as long as broad, usually slightly asymmetrical, drop-like or nearly so (sometimes onion-shaped); stylus arising dorsoapically, moderately long to long, usually distinctly pubescent. Gena not prominent. Proboscis moderately long, straight. Palpus subrectangular to elongate oval (moderately broad to narrow, lanceolate); clothed in dense pubescence giving to palpus silvery or purple glisten in some view; subapical seta present or absent, sometimes 3–4 short submarginal setae present.

Thorax with mesoscutum tomentose or shiny; mesopleuron tomentose. Precoxal bridge somewhat narrower than in other genera of Tachydromiini . Proepisternum large, subtriangular, separated by suture from katepisternum of mesopleuron, bare. Postpronotal lobe distinct, not separated by suture from proepisternum, separated by suture from mesoscutum, subglobular to slightly elongate oval; postpronotal seta present or absent. Mesoscutum gently, slightly convex; slightly longer than broad (usually 1.2–1.3 times, rarely about 1.5 times); acrostichal setae present or absent, biserial when present, minute; dorsocentral setae uniserial, similar to acrostichals, prescutellar pair short to very long; usually 1 notopleural (sometimes 2), 1 postalar and 4 scutellars (sometimes 2; when 4 setae, then lateral pair fine and very short); mesopleuron bare (although, Collin [1933: 26] notes “microscopic hairs below notopleural suture” in his T. adunca from Chile).

Legs short; fore femur slightly swollen, broader than mid and hind femora; mid femur slender, sometimes with complete rows of spine-like setae ventrally; mid leg lacking secondary sexual characters in male.

Wing normally developed, usually broad and blunt-tipped (sometimes narrow) [an undescribed species of Tachyempis with reduced wings noted in illustrated key of Sinclair et al. [2023] actually belongs to Tachydromia ]; membrane nearly hyaline to more or less infuscate, very rarely banded. Very short basal costal seta usually present. Rs moderately long, nearly as long as or slightly longer than basal portion of vein R 4+5, originating proximal to middle of cell bm or near middle of R 1. R 1 meeting costa before or near middle of wing. R 2+3 long, meeting costa beyond mid-point of wing, evenly arched towards costa. Vein R 4+5 running parallel with vein M 1+2. Vein CuA+CuP (= anal vein by authors) absent. Crossvein CuA (= CuA 2) absent; crossvein bm-m (= bm-cu) transverse or slightly oblique. Cell br shorter than cell bm at apex; cell bm nearly as broad as or broader than cell br.

Abdomen with tergites 1–7 unmodified; segment 8 asymmetrical, tergite 8 broadly, deeply concave posteriorly, fused with sternite 8 on left side, bearing large anteromarginal apodeme closer to left side. Male hypopygium rotated 90°, moderately large, more or less elongate oval. Epandrium rhomboid viewed dorsally, not divided (epandrial bridge present); fused (or articulated) with hypandrium at a narrow point. Right epandrial lamella elongate oval, somewhat produced mid-ventrally, bearing scattered, simple setae. Right surstylus differentiated, single, nearly digitiform, long, overlapping terminalia posteriorly, with or without spine-like setae. Left epandrial lamella more or less shifted anteriad relative to right epandrial lamella (dorsal view); with large basal apodeme. Left surstylus barely differentiated from epandrial lamella, single, with or without internal projection; no rod-shaped process beneath left surstylus. Hypandrium narrow, bare. Cerci separated; left cercus shifted anteriad relative to right cercus; shape and setation specifically variable. Hypoproct unmodified, membranous to weakly sclerotised. Subepandrial sclerite bare or setose.Two rod-shaped apodemes of subequal lengths. Phallus short, membraneous.

Female similar to male.Secondary sexual characters are rarely present in male (e.g., hind basitarsus with 1 strong, long, black, dorsal subapical seta in species of the T. longispina complex).

REMARKS. Tachyempis is represented in the Nearctic by very small, relatively uniform species ( Figs 1–3 ). However, in the Neotropical region the genus shows curious morphological diversity. The above-given diagnosis was somewhat expanded to cover some undescribed Neotropical species [Shamshev, Grootaert, unpublished data), based on our redefinition of the genus herein. However, the Neotropical species assigned to Tachyempis needs a separate revision. Some of the species described by Melander [1928] may be not congeneric.

Melander [1928: 269–270], and subsequent authors (e.g., recently Cumming & Sinclair [2009]; Sinclair et al. [2023]), distinguished Tachyempis from Tachydromia primarily by shape of the frons (i.e., frons V-shaped in Tachyempis [ Sinclair et al., 2023: 13, Couplet 7, fig. A] versus frons with nearly parallel margins in Tachydromia ). However, this character somewhat varies in the both genera (especially in Tachyempis ) and undistinguishable or hardly distinguishable cases are present (e.g., when Melander indicated that the frons is “narrowly V-shaped”). The length of the mesoscutum is variable in Tachydromia and in many cases Tachyempis and Tachydromia are not distinguishable by this character or differences are uncertain to be readily recognisable. Tachydromia is well known by banded wings. In the species assigned to Tachyempis the wing membrane is usually hyaline to more or less infuscate and very rarely banded (one undescribed species from Bolivia). However, it should be noted that there are many members of Tachydromia with not banded wings, e.g., about 20% of the Nearctic species (Shamshev, Grootaert, unpublished data). In the Neotropical region, all known to us species of Tachydromia have banded wings but the genus is relatively poorly represented [Shamshev, unpublished data]. Though none of these characters are conclusive, sometimes, they are useful for practical purposes. Based on our survey of the World fauna of Tachydromia , Tachyempis could be distinguished from Tachydromia primarily by entirely tomentose mesopleuron (versus extensively shiny) and by lateroclinate vertical setae (versus inclinate), as keyed below. There are two curious Palearctic species of Tachydromia with extensively tomentose mesonotum ( T. catalonica (Strobl, 1906) and T. sabulosa Meigen, 1830 ). However, they possess shiny anepisternum and katepisternum of the mesopleuron.

We did not include in Tachyempis two species described by Bezzi [1908] from South Africa, as Smith [1969] did. Our conclusion is based on an examination of Bezzi’s type material and additional material noted by Smith. The discussion of systematic position of these species would be beyond the scope of our paper.

DISTRIBUTION. Currently, Tachyempis comprises 20 species described from the New World only. The genus is poorly represented in the Nearctic (four described and one undescribed species) and relatively diverse in the Neotropical region (17 species). Most of the Neotropical species were described from Costa Rica. However, there are many undescribed species from other areas of the region. In addition, one undescribed species of Tachyempis is present in South Africa and one species in Japan.

KEY TO THE NEARCTIC GENERA OF THE TRIBE TACHYDROMIINI

1. Eyes with tiny ommatrichia. Postpronotal lobe usually indistinct (except Austrodromia Collin and Chaetodromia Chillcott ) .... ................................................................................... Drapetidini

– Eyes bare. Postpronotal lobe distinctly differentiated ( Tachydromiini ) ........................................................................ 2

2. Wing cell cua (= anal cell) more or less formed, with vein CuA distinct and vein CuA+CuP present, although sometimes faint. Mid leg usually raptorial, with femur thickened and armed with rows of spine-like setae ventrally, tibia slightly arched and often ending in sharp spur ............................. Platypalpus Macquart

– Wing cell cua absent (vein CuA+CuP absent); vein CuA present or absent. Mid leg simple, at most with some modifications in the male (subbasal clusters of spine-like setae, excavations, tubercles, etc.) ........................................................................... 3

3. Ocellar triangle placed some distance in front of upper (vertical) angle of eyes. Eyes more or less approximated behind ocellar triangle,wheretheyusuallyareclosertogetherthanimmediately in front of ocellar triangle. Occiput with numerous, whitish, slightly flattened setae on lower part.Vein CuA usually present (sometimes absent) ..................................... Tachypeza Meigen

– Ocellar triangle level with upper angle of eyes.Eye margins more or less divergent on vertex.Occiput with scattered, fine setae on lower part. Vein CuA always absent ....................................... 4

4. Thorax with mesopleuron extensively shiny. Vertical setae inclinate (sometimes also slightly proclinate). Frons usually with nearly parallel sides. Wing membrane often with crossbands or maculae ........................... Tachydromia Meigen

– Thorax with mesopleuron tomentose. Vertical setae lateroclinate. Frons often with sides broadly divergent above, V-shaped to nearly Y-shaped. Wing membrane hyaline to more or less infuscate, very rarely banded ................ Tachyempis Melander

Figures

Figs 1–3. Tachyempis Melander, habitus: 1 — T. agens (Melander), ♀, paratype, antero-lateral view; 2 — T. calva (Melander), ♂, lateral view; 3 — T. universalis (Melander), ♂, lateral view. Рис. 1–3. Tachyempis Melander, габитус: 1 — T. agens (Melander), ♀, паратип, вид спереди и сбоку; 2 — T. calva (Melander), ♂, вид сбоку; 3 — T. universalis (Melander), ♂, вид сбоку.