1911] Study of Muscoid Flies 151 



A considerable number of minor sclerites occur in various 

 forms, but all seem to be derived from the above seven main 

 ones, except those of the oral region which belong to the pseudo- 

 cephalon and are probably developed from it. I have an ex- 

 tensive series of drawings of a very large number of types of 

 first-stage skeleton, which will be published in due time in 

 connection with a comparative study of all the sclerites that go 

 to make up the skeleton in the various forms. 



Under the Sarcophagine series in these addenda is mentioned 

 TD 354 from Massachusetts, which most clearly shows all the 

 above main sclerites except the dorsopharyngeal. The infra- 

 pharyngeal is so clearly exhibited in this form as to prove beyond 

 doubt, I think, its distinctness as one of the main sclerites. 

 In most forms the infrapharyngeal, while more or less present, 

 is so intimately connected or welded with the pharyngeal as to 

 appear a part of the latter. This has misled Nielsen and others. 



It is worth while mentioning in a speculative way that the 

 above seven main pairs of sclerites may represent the seven 

 embryonic segments absent in the muscoid maggot, which have 

 been inverted to form the skeleton proper of the maggot mouth 

 and pharynx. The head in the most primitive insects is believed 

 to be composed of seven segments and the abdomen of ten, the 

 three thoracic segments bringing the total number to twenty. 

 There are twelve segments in the muscoid maggot besides the 

 pseudocephalon. Dr. C. Gordon Hewitt's admirable mono- 

 graph of Musca may be studied with much profit in this con- 

 connection. 



What I have heretofore called the clypeus in the fly 

 (Taxonomy, pp. 22-24) seems to be a part of the everted dorsal 

 wall of the pharynx chitinized. The true clypeus is apparently 

 the lower portion of what I have called the facial plate. The 

 labrum seems to have projected itself forward, carrying with it 

 the epipharnyx to form the dorsal part of the haustellum, and 

 in this way becoming widely separated from the clypeus. 

 Into the space thus left there appears to have been everted the 

 chitinous portion of the pharynx to form the rostrum of the 

 proboscis, which attaches basally in front to the epistoma or 

 anterior edge of the clypeus, The morphological sequence of 

 the parts is thus lost when the proboscis is extended, but when 

 the latter is retracted the rostrum is inverted to a semblance of 

 its original morphological position. 



