LErTOGASTRINAE 



ROBBER FLIES OF THE WORLD 



311 



Distribution: Neotropical: Systellogaster aurian- 

 tiaca Hermann (1926) ; calceolatus Bigot (1878) ; fas- 

 cipennis Schiner (1867) ; breviventris Rondani (1848). 



Genus Acronyches Williston 



Figures 190, 601, 117S, 1186, 2038, 2062, 2067 



Acronyches Williston, Manual . . . North American Diptera, 



ed. 3, p. 388, fig. 163, full figure and name, 190S. Type of 



genus: Acronyches willistoni Hermann, 1921. Descrip- 

 tion and type designation by Hermann, 1921. 



Flies of medium size and readily distinguished by 

 the ventrally wide and triangular face with the strongly 

 convergent and narrowed front and vertex. Palpus 

 with 2 segments. Pulvilli and empodium absent. 

 Length 20 mm. 



Head, lateral aspect: The head is moderately long 

 and strongly convex anteriorly; it is nearly plane 

 through the posterior part of the middle and slightly 

 recessive above. The lower posterior angle of the eye 

 is recessive but broadly rounded. The face is mod- 

 erately produced from the middle to the base of palpus 

 but is gently receding and equally long in the middle 

 and below. The upper half of the face recedes down 

 to the eye margin at the base of the antenna. The 

 occiput in profile is extremely narrow, obliterated be- 

 low; the lower face and cheeks are excavated so that 

 the eye descends below the base of the proboscis as far 

 as the occiput is concerned. The upper occiput has 3 

 or 4 short, pale bristles placed quite submarginally. 

 The lower face and proboscis are on the same level as 

 the bottom of the eye. The proboscis is directed 

 straight forward, cylindrical at the base, somewhat 

 narrowed with a distinct but low, thin dorsomedial 

 ridge. The apex is rounded and short pilose. Palpus 

 short, of two segments, the basal segment quite short, 

 the second longer, club-shaped, pointed at apex and held 

 at a right angle. The second segment has 6 or 7 long 

 hairs, the basal segment with 3 long hairs. Antenna 

 long, slender, the first two segments of equal length; 

 the third 3 times as long as the first two and attenuate 

 apically; it has a short, thick style, and very minute, 

 distinct, apical spine. 



Head, anterior aspect: The face and front together 

 form almost an equilateral triangle similar to Plesiom- 

 ma Macquart. The middle of the face bears a trans- 

 verse row of bristles containing 2 long, adjacent, closely 

 set, pale bristles held horizontally forward, nearly as 

 long as the proboscis, and 2 pairs of other weak bris- 

 tles placed laterally. Sides of face below with a deep, 

 lateral crease. The vertex is extremely narrow, not 

 excavated, the eyes almost touching behind the ocelli. 



Thorax : The mesonotum is strongly sloping behind, 

 everywhere pollinose. It bears a single row of short, 

 fine, acrostical elements and is bare on either side fol- 

 lowed by submedial stripe of short pile. Humerus 

 with 2 or 3 hairs. Thorax with moderately long, 

 strong, pale bristles as follows: 1 notopleural, 1 post- 

 alar, 1 supraalar, but no scutellar. The scutellum 



535914— 62— pt. 1 21 



is flattened, not pilose. Mesopleuron without pile or 

 bristles. Metapleuron with only fine pile which covers 

 an oval patch posterodorsally ; it is very slightly convex. 

 Pleuron otherwise without pile but everywhere golden 

 pollinose. The cervical sclerite is covered with nu- 

 merous, very long, extremely delicate, silky hairs. Up- 

 per and lower propleuron fused. Slopes of the 

 metanotum without pile. Slopes of the metasternum 

 also without pile; postmetacoxal area membranous. 



Legs: The hind femur is elongate and distinctly 

 thickened distally where it is twice as wide as at the 

 base; this femur is densely covered with appressed, 

 short, setate pile except ventrally where there are only 

 a few minute hairs. Laterally there are short, stout, 

 pale bristles, 1 near the apex subdorsally, 1 behind it at 

 the apical fourth, and 1 very short bristle near the base. 

 There is also 1 at the apex dorsomedially, 1 on the me- 

 dial surface mediodorsally, and also a downwardly di- 

 rected patch of rather long, dense, coarse, red pile. 

 Hind tibia slightly longer than the femur with scanty, 

 appressed pile ; the bristles are all short and consist of 3 

 dorsolateral, 4 ventrolateral and at the apex a dorsal 

 pair and 1 medial, 2 ventral, and 1 lateral bristle, all 

 weak and short. Middle femur short, appressed pilose 

 dorsally and anteriorly; its tibia with 3 dorsal, 7 long, 

 appressed postventral bristles and the subapex bears 

 2 moderately strong ventral and posteroventral and 1 

 anteroventral bristle. The anterior femur is similar to 

 the middle femur. Anterior tibia with 7 dorsal bristles ; 

 posteroventral margin with 10 rather short bristles; 

 apex with 1 ventral, 1 anterior, and 1 posterior bristle, 

 all long and strong. No apical spine present. Poste- 

 rior basitarsus slender, longer than the next three seg- 

 ments, with the ventral surface covered with only short, 

 stout, spinous bristles, few in number and with a very 

 little pile is confined to a narrow row on the medial 

 margin. Middle and anterior tarsi similar and with 

 still less pile. Claws quite long, only slightly curved 

 toward the outer third, sharp, convergent, without 

 pulvilli and without empodium. 



Wings : The marginal cell is widely open, the fourth 

 posterior cell narrowed to half the width or less. An- 

 terior margin of wing often dark or smoky. The anal 

 cell is closed in the margin. The two outer end veins 

 of the discal cell are of equal length. The wing is 

 peculiar in that the second basal cell is exceptionally 

 long; the two end veins of the second basal cell fuse 

 briefly in the middle of the posterior margin of the 

 discal cell. Ambient vein complete but alula reduced 

 to a minute linear strip. 



Abdomen: The abdomen is distinctly petiolate or 

 clavate and as long as the wings. The second tergite 

 is elongate and subcylindrical ; the abdomen expands 

 from the base of the third segment. The fifth tergite 

 is more than twice as wide as the second. First tergite 

 extremely short, bearing a vertical row of 3 weak, red- 

 dish bristles. Sides of second tergite with long, deli- 

 cate, erect pile. Remaining tergites with short, scanty, 

 appressed pile. Eight tergites are present. Female 



