ROBBER FLIES OF THE WORLD 



513 



Genus Cerdistus Loew 



FiQTJKES 328, 329, 370, 690, 711, 713, 715, 1513, 1522, 2287, 2322 



Cerdistus Loew, Linnaea Entomologica, vol. 4, p. 74, 1849. Type 

 of genus : Asilus erythrurus Meigen, 1820. Designated by 

 Engel, 1927. 



Paritatnus Verrall, British flies, vol. 5, p. 679, 1909. Type of 

 subgenus: Asiliis geniculatus Meigen, 1820. Designated by 

 Engel, 1927. 



Rhahdoitamus White, Papers Proc. Roy. Soc. Tasmania, 1916, 

 p. 95, 1917. Type of genus: Rhahdoitamus vittipes Mac- 

 quart, 1847, as Neoitamus Irunneus White, 1914. Desig- 

 nated by G. H. Hardy, 1920. 



Cerdistus has 1 subgenus, Neocerdistus Hardy. 



Small, rather diverse flies, which have a microseginent 

 attached to the third antennal segment and have meta- 

 notal pile and lateral tergal bristles. They are dis- 

 tinguished primarily by the rather low, distinctly short, 

 facial gibbosity, which is restricted to the lower half 

 of the face; the upper half is generally bare, though 

 occasionally jjilose as in Cerdistus pallidus. This short 

 face separates them from the Machimus Loew group 

 of genera. The absence of strong bristles on the ab- 

 dominal sternites and a more simple type of male 

 terminalia separates them from their near ally, 

 Neomoctheinis Osten Sacken. Length 10 to 20 mm. 



Head, lateral aspect : The face has a low, rather in- 

 conspicuous, rounded gibbosity which is confined to the 

 lower half of the face; the upper portion is nearly 

 plane with the eye. The eye is strongly convex an- 

 teriorly, nearly plane in the middle behuid and intero- 

 ventrally recessive on less than the lower fifth of the 

 eye. The occiput is rather short but present through- 

 out the full height of the eye ; dorsally it slopes gradu- 

 ally down the eye margin. Pile of occiput fine and 

 scanty in the middle, a little longer and considerably 

 more abimdant ventrally. There are weak bristles in 

 the middle of the occiput and 5 to 8 pairs of rather 

 stouter bristles dorsally. The proboscis is moderately 

 stout; the base is a little swollen either in lateral or 

 dorsal view and the apex is bluntly rounded, with a few, 

 stiff hairs ; the base ventrally has a few to many, long, 

 fine hairs. Palpus cylindrical and short with fine, stiff 

 hairs apically and on all sides except medially. The 

 antenna is attached at the upper third of the head and 

 comparatively short with the third segment 21/^ times 

 as long as its maximum height, but occasionally much 

 more slender. The first segment is as long or slightly 

 longer than the second; the third segment is not quite 

 twice as long as the second segment except in Cerdistus 

 anonyjnus Williston. The third segment carries a 

 short, distinct microsegment ; the style may be only 

 half as long as the third segment or of equal length. 

 The pile of the first and second segments consists 

 usually of 4 or 5 moderately long, stout, appressed, 

 bristly hairs ventrally and fewer hairs dorsally; oc- 

 casionally there are more numerous and somewhat 

 longer stiffer hairs on the ventral surface of the fu-st 

 segment. 



Head, anterior aspect : The face below the antenna is 

 a sixth the head width and divergent below. Subepis- 



tomal area moderately large, oblique, nearly plane and 

 bare, sometimes pubescent, or concave in Cerdistus 

 geniculatus Meigen. Face pubescent and usually with- 

 out pile but with dense, matted, coarse, bristly pile aris- 

 ing on the upper part below the base of the antenna in 

 Cerdistus pallidus. The low facial gibbosity carries 

 rather numerous bristles which are often stout centrally, 

 pale, black, or intermixed, with as few as 8 pairs or 

 more than twice this niunber and with bristles carried 

 down laterally along the sides of the subepistoma. The 

 front is rather short, pubescent, with 7 or 8 pairs of 

 stiff bristles laterally or with only 5 or 6 stiff hairs. 

 Sides of front very slightly divergent, convergent at 

 the vertex; the vertex is comparatively narrow and 

 deeply excavated with vertical sides in Cerdistus 

 xanthocerus or only moderately excavated in the type 

 of genus and with sloping sides. The ocellarimn is 

 comparatively large, of moderate height, bearing a 

 pair of small bristles across the middle and a pair be- 

 hind the ocelli and occasionally with 1 or 2 additional 

 pairs of fine hairs, the length rather variable. Anterior 

 eye facets enlarged. 



Thorax: The thorax is everywhere pubescent; the 

 mesonotum is comparatively high m contrast to its 

 length. Pile of mesonotmn composed of numerous, 

 fine, backwardly curved bristly hairs on the antero- 

 medial portion; acrostical elements are not differenti- 

 ated; dorsocentral elements are differentiated only 

 weakly before the suture, where there may be 1 or 2 

 pairs and there are 3 or 4 pairs of long, strong dorso- 

 central bristles behind the suture. Humerus with fine, 

 long pile. The following complement of lateral, long, 

 stout bristles present; 2 notopleural with a shorter, 

 more slender posthumeral element, 1 supraalar, 1 post- 

 supraalar, 1 or 2 postalar and usually a single pair of 

 scutellar bristles but sometimes with 2 pairs. Scutel- 

 lum only moderately thickened, convex with distinct 

 impressed rim; the disc is pubescent with a few fuie 

 hairs or in Cerdistus pallidus it bears numerous, long, 

 stiff, bristly hairs curved forward. Propleuron pos- 

 teriorly and ventrally with abundant, long pile. Pro- 

 notum with 3 or 4 pairs of fine bristles. Mesopleuron 

 and the sternopleuron, except anteriorly, without pile. 

 There are a few short hairs along the dorsal edge of 

 the mesopleuron, 2 or 3 short hairs on the pteropleuron 

 and 2 or 3 on the middle of the dorsal portion of the 

 sternopleuron. On the posthypopleuron there are 1 to 

 4 long, slender bristles and usually at least 1 rather 

 stout bristle and usually with additional long hairs; in 

 Cerdistus xanthocerus bristles are absent and there are 

 only 1 or 2 fine, short hairs. Metapleuron in the type 

 of genus with 4 long, slender bristles, in the others with 

 10 or more; in Cerdistus xanthocerus with only 2 

 bristles, together with some longer hairs ; in Cerdistus 

 pallidus with 8 and also with a conspicuous tuft of long, 

 stiff pile on the pteropleuron and stei-nopleuron. Meta- 

 notal slopes with a tuft of long, stiff hairs; lateral 

 metastemum with pile, ventral metasternmn long 

 pilose, the postmetacoxal area pubescent and membra- 



