ANTENNZE OF DIPTERA. 33 I 



almost invariably a male character. There is evidently a sexual 

 use for this greater development of the eyes in the male that has 

 preserved the character with but little tendency to transmission 

 to the female. It has, however, been transmitted to the female 

 in some instances, and there are a very few forms in which the 

 female has acquired the character in advance of the male. Ex- 

 amples of the former may be found among the Cyrtidae, but a 

 better one is that of Systropits of the Bombyliidae, with contig- 

 uous eyes in both sexes, while the very nearly related Dolichomyia 

 has the female eyes separated by the front. It was for the con- 

 tiguity of the eyes in the male that Osten Sacken twenty-five 

 years ago proposed the convenient term " holoptic," the antithesis 

 of which, " dichoptic," was suggested by me a little later. But 

 Osten Sacken's meaning of the term has been somewhat misun- 

 derstood. He gives as a definition of his Nemocera vera the 

 nonholopticism of the eyes, while it is well known that some 

 forms in this group do have contiguous eyes. But Osten Sacken 

 really meant sexual holopticism, not simply contiguity of the 

 eyes on the front. It remains to be proven that sexual holopti- 

 cism does not really occur among these families of flies. If so, 

 however, the occurrence must be extremely rare. 



The primitive dipteron must have had eight fully developed 

 longitudinal veins (including the auxiliary vein), with the second, 

 third, fourth and fifth furcate; and a complete discal cell. The 

 head was rather small, with the compound eyes separated equally 

 by the front in both sexes. The ocelli were functional, and the 

 maxillary palpi had four freely articulated joints ; the labial palpi 

 had probably already disappeared, though Wesche thinks differ- 

 ently. There were at least thirty-nine antennal joints in the male. 

 The prothorax, mesothorax and metathorax were imperfectly 

 fused, and the metanotum was visible from above. The abdomen 

 had nine functional segments ; the body was without differentiated 

 bristles ; and the tarsi had membranous pulvilli and empodia. 

 The primitive flies were of moderate or small size, and probably 

 crepuscular in habit, or at least denizens of shady forests. 



Of modern diptera the Tipulidae approach most nearly this 

 hypothetical ancestor, but they have become specialized by a 

 general increase in size, by the almost complete loss of the 



