a Genus of Dipterous Insects. 289 



(inento?) brevi, cylindrico, apice vel capitulo carnoso, compresso, bilo- 

 bato, corrugate, tenuit^r pubescent!. Truncus elongatus, subovatus, antic^ 

 attenuatus, mesothorace inteidihm, scutello metathoraceque semper utrin- 

 que unispinosis, hoc distincto, subquadrato. Halteres audi. Alee ut iu 

 Calobatd reticulatse, nervo angulari basali interno nullo. Pedes elongati, 

 antici raptorii coxis longis, femoribus pliis minflsve incrassatis, et subtus 

 serie duplici denticulationum parvarum instructis, tibiis subarcuatis. Fe- 

 mora 4 postica gracilia, ad apicem interdum unispinosa. Tibice postic* 

 inermes, rectse. Tarsi 5-articulati, articulo Inio longissimo. PulviUi 

 magni. Abdomen elongatum, angustum, pli\s minilsve clavatum, ad basin 

 attenuatum, supr^ convexum, subtus tamen concavum, segmentis 4 anticis 

 arctfe conjunctis, baud articulatis, ad basin supra subcanaliculatuin. 

 The differentia; sexuales in this genus have not hitherto been clearly ascer- 

 tained. Dalman says, "Abdomen maris lineare, feminse pone medium in- 

 crassatum, subclavatum ;" adding, " Ob formam abdominis in una eademque 

 specie diversam, linearem nempe vel clavatam, illam maris, banc feminse sexum 

 indicare, suspicari liceat." Dalman, however, had observed this variation in 

 one species only, D. signata. From the differences, however, existing in spe- 

 cimens of D.fasciata, D. assimilis, and D. Syhesii, it would seem that the cla- 

 vation of the abdomen is not confined to the female ; whilst it also appears 

 that in some species the males are distinguished by the greater length of the 

 ocular peduncles : that this, however, is not always the case is evident from 

 these organs not being longer in the slender specimens of D. signata, fasciata, 

 and assimilis, than they are in the more robust ones. I observed, moreover 

 in the robust specimens of D. Sykesii, as well as in D.fasciata, a minute 

 exserted style at the extremity of the last (incurved) segment of the abdomen : 

 hence, taking the characters of all the species into consideration, it appears 

 that the females are larger and more robust than the males, their abdomens 

 more distinctly clavate, whilst the ocular peduncles of the males are more 

 slender and often longer than those of the opposite sex*. 



* The celebrated Danish traveller and naturalist M. Lund informed me that the males alone in 

 Diopsis possess the elongated processes of the head ; but it is evident that he referred to the insects 

 which he had collected in Brazil, and wliich Wiedemann has described under the name of Zygothrka 

 dispar. 



