48 ILLINOIS BIOLOGICAL MONOGRAPHS [218 



U-shaped or open ring-like thickenings are present in the pseudo- 

 tracheae of the more specialized Diptera. They do not occur in the 

 simple pseudotracheae of Mycetophila or in some of the highly special- 

 ized forms. The histological structure of a pseudotrachea has been 

 clearly demonstrated by several workers. According to Dimmock, "The 

 pseudotracheae on the inner surfaces of the labellae of Musca are cylin- 

 drical channels, sunk more or less deeply into the surfaces of the labellae 

 according to the amount that that surface is inflated, and they open on 

 the surface in zig-zag slits. These channels are held open by partial 

 rings, more strongly chitinized than the rest of the membrane of the 

 cylinder. As seen from above in Musca, [Fig. 485], the pseudotracheae 

 appear to be supported by partial rings, one end of each of which is 



forked The pseudotracheae of Eristalis are so nearly like 



those of Musca [Calliphora] vomitoria that I have not figured those 

 of the former." All my observations of the histological structure of 

 pseudotracheae agree with those made by Dimmock. Tho no attempt 

 was made to work out the detail of the histological structure in the 

 various genera studied, a number of interesting facts were observed. 

 The chitinized, taenidia-like thickenings (ps. th) in Ochthera (Fig. 445 

 and 483) are large U-shaped structures which are partially embedded 

 in the membrane. The ends of these thickenings project considerably 

 beyond the surface of the membrane and resemble these structures in 

 Bombylius major (Fig. 482), as figured by Dimmock. The pseudo- 

 tracheae of Calobata (Fig. 446) have developed into rows of small chi- 

 tinized teeth (tee). 



The pseudotracheal area of the paraglossae undergoes its greatest 

 specialization in forms in which the paraglossae assume a biting func- 

 tion. This biting type is brought about by the development of distinct 

 chitinized teeth arising between the proximal ends of the pseudotracheae. 

 Eudimentary or well-developed teeth occur in Musca (Fig. 467), Sar- 

 cophaga (Fig. 478), Scatophaga (Fig. 472), Lispa (Fig. 481), and 

 Stomoxys (Fig. 480). In Musca the small, chitinized, so-called pre- 

 stomal teeth (tee) are present between the proximal ends of the pseudo- 

 tracheae. In Scatophaga and Lispa these teeth are large and distinct. 

 Their greatest development occurs in Stomoxys, and so far as observed 

 pseudotracheae are wanting in this form. An extensive discussion of 

 the development and the structure of the chitinized teeth of the para- 

 glossae has been given by Patton and Cragg (1913). 



The glossae (gl) of a typical labium (Fig. 1 and 73) are two small 

 lobes located between the proximal portions of the paraglossae distad 

 of the furrow on the theca and at the distal end of the cephalic groove. 

 Thruout the order the glossae are between the paraglossae and at the 



