NORTH AMERICAN DIPTERA. 323 



be again referred to later. The development in Po/istes, where there 

 is a bladder-like membranous expansion, covered with sensitive hairs 

 (the hypopharynx ?), is also to be noted, with the farther note that 

 it is not paired or jointed, but a mere membranous development or 

 expansion, a true lapping organ. 



The hypopharynx and epipharynx are fleshy, internal mouth 

 structures, supposed to represent the tongue and palate of vertebrates. 

 Now let us see what developments must have taken place if the 

 accepted explanation of the Dipterous mouth parts is correct. The 

 labrum, from an external, head piece, shielding the mouth, becomes 

 a true, functional, internal mouth organ ; the mandibles become part 

 of the maxilla? and migrate within the palpi ; the galea disappears, 

 and the whole maxilla becomes reduced to a single piece ; the lal)ium 

 becomes most enormously developed, and adds two joints ; the labial 

 palpi disappear, the paraglossse are wanting, and the ligula is not 

 mentioned. But, on the other hand, besides the labrura, the epi- 

 pharynx and hypopharynx become functional, chitinous, and, from 

 a sensory or tactile development, become mechanical. 



How this striking change in the functions and location of parts 

 occurred, had never been explained, and that was my task. I as- 

 sumed the correctness of the theory that insects, being develojDed 

 from a common stock, would somewhere have a generalized type of 

 mouth ; or, at least, that there would be such variations of develop- 

 ment, that the points at which the mandibulate and haustellate 

 mouth branched, might be discovered. The natural point at which 

 an explanation might be sought was not among the highly special- 

 ized forms such as Musm, but was i-ather among those forms in which 

 division of the mouth parts reached the extreme, and thence the 

 work would be in the direction of the specialized forms. A short 

 study of the forms allied to Bomhus, Xylocopa and Apis, among the 

 Hymenoptera, showed that the tendency in the labium was nowhere 

 toward a segmentation, but to an elongation; and my first object 

 was to try and identify, among the Diptera, the true labium and the 

 paragloss^. Then those forms in which the proboscis was incom- 

 pletely developed, were sought, and, finally, in a minute midge 

 {Siynulinm sp. ?), which swarmed at Anglesea, N. J., early this Spring, 

 and almost drove me distracted by its painful bite, I found the solu- 

 tion ! 



Dr. Riley kindly sent me some specimens of the Buffalo gnat for 

 study, and I had then the species which showed not only all the parts 



