312 



UNITED STATES NATIONAL MUSEUM BULLETIN 224 



PART 1 



terminalia minute and short, without spines. No males 



seen. 



Distribution : Neotropical : Acronyches fenestratulus 

 Hermann (1921) ; imitator Hermann (1921) ; willistoni 

 Hermann (1921). 



Megonyx, new genus 



FIGUEE 204 



Type of genus : Megonyx giganteus, new species. 

 Extremely large flies, related to Acronyches 

 Hermann, and distinguished by the comparatively wide 

 vertex, the greatly reduced height of the face, widely 

 covered with slender bristles and bristly pile, and the 

 conspicuous, bristly, shaggy fringes above and below on 

 the outer half of the strongly swollen hind femur. 

 Length 29 mm. including antenna, and wing spread of 

 over 40 mm. 



Head, lateral aspect : The head is comparatively long, 

 with the face of greatly reduced height but moderately 

 prominent. The height of the face is at most only a 

 fourth the distance from the antenna to the bottom of 

 the subepistoma above the proboscis, and while the face 

 is plane in profile it is produced forward more exten- 

 sively below, so that at the margin of the epistoma it 

 presents a moderately sharp angle with the epistoma. 

 The whole of the subepistoma is prominently pro- 

 duced in profile, and entirely covered with the same 

 appressed, greyish white micropubescence, which is 

 found over the whole surface of the face, cheeks, and 

 front. Occiput only moderately prominent over the 

 middle part of the head, recessive and reduced at the 

 bottom of the head and also at the vertex. Only at the 

 lower medial corner is there a little pile at the bottom 

 of the occiput. Adjacent to it are moderately numer- 

 ous, slender, black, bristly hairs, which continue in 2 or 

 3 irregular rows upward on the occiput, changing to a 

 single row near the vertex. A few of the dorsal ele- 

 ments are comparatively stout. Proboscis robust, 

 bluntly rounded, thrust obliquely forward and with a 

 distinct, medial ridge above, and with an oblique, lat- 

 eral furrow or crease some distance from the apex; at 

 the bottom of which furrow there is a pair of slender, 

 moderately long, stout, black bristles. At the apex 

 are only a few, short, stiff hairs; pile otherwise absent. 

 Palpus large, apparently with a well developed, basal 

 segment, the ultimate portion cylindrical, with large, 

 oblique, apical, apilose opening, and with several 

 slender bristles and bristly hairs. The antenna promi- 

 nent and elongate, the first two segments are short ; the 

 first is slightly longer than the second and it bears 5 

 slender, long, bristly setae below. The second segment 

 bears 4 or 5 similar setae at the apex above and below, 

 and all setae are directed straight forward. Third 

 segment, with microsegment included, is nearly 4 times 

 the combined length of the first two. It is strongly 

 compressed laterally, spindle-shaped in profile, dis- 

 tinctly attenuate from the middle in both directions, 



and the middle width is twice the basal width. The 

 style is not as long as the second antennal segment but 

 is sharp at the apex. 



Head, anterior aspect: The head is approximately 

 iy 2 times wider than high. The vertex, while narrow, 

 is considerably (at least 3 times) wider than in Acro- 

 nyches Hermann, and the front widens below, though 

 only moderately. Beginning below the antenna the 

 face rapidly widens below and the extremely high, 

 prominent subepistoma continues to expand and di- 

 verge at the same rate, so that at the bottom of the eyes 

 the separation is quite extensive and the eyes extend 

 below the base of the proboscis. The whole face is 

 loosely covered with slender, black bristles of moderate 

 length, except for a narrow space along the eye margin 

 and in the middle on the lower part of the face are 2 

 close-set rows, one above the other containing long, 

 comparatively stout, black bristles. The long, black 

 bristles number 10, the short, slender bristles are 

 about 12 in number on each side, among which are 1 

 or 2 white hairs. Front with fine, grey white pollen or 

 micropubescence and no pile, except for 2 or 3 minute, 

 very fine hairs in front of the ocelli. Ocellarium quite 

 low, without pile or bristles and the vertex very little 

 excavated. 



Thorax: The mesonotiun only moderately high, 

 rather long and opaque pollinose, in the type of genus 

 the medial fourth is opaque black, the remaining lateral 

 portion light, opaque grey. Pile of mesonotum very 

 scanty, fine, erect and setate, with a single row of black, 

 acrostical setae widely separated from adjacent pile. 

 Bristles of mesonotum few but long and stout. There 

 are 2 on the notopleuron, 1 or 2 on the postalar callosity ; 

 scutellum flat, grey pollinose, the thick angulate mar- 

 gin with some 10 pairs of fine, short, black setae. Pro- 

 sternum small and strongly dissociated, postmeta- 

 coxal area of the metasternum wide and membranous. 

 Legs: The hind femur and tibia quite elongate, the 

 femur strongly swollen on the outer half and with 

 dense pile on all surfaces beginning at or just before 

 the middle. This dense pile dorsally and ventrally is 

 long, shaggy, tufted and appressed, and is also long 

 and conspicuous medially but quite short laterally. 

 The hind tibia gradually dilated from the base to the 

 apex, the pile is fine and scanty except on the medial 

 and ventral sufaces where it is dense and semierect. 

 Bristles absent on the hind tibia except at the apex, and 

 are confined to 4 ventral, stout, black bristles. Middle 

 tibia with 9 quite stout, rather short, strongly appressed, 

 posteroventral bristles and moderately abundant setae 

 both ventrally and anteriorly, fewer above and behind. 

 Anterior legs missing. The claws are quite stout and 

 long, only slightly curved and slightly more curved at 

 apex and apposed. Empodium absent, also pulvilli. 



Wings: The wings are very dark brownish black 

 with a strong purplish reflection, a little paler on the 

 posterior margin and in the type of genus with 1 or 2 

 small, hyaline spots. The medial crossvein of about the 

 same length as the upper part of the anterior inter- 

 calary vein. Both make the same angle with the second 



