217] HEAD OF DIPTERA — PETERSON 47 



specialized forms, such as Olfersia (Fig. 488), Couops (Pig. 418), 

 Siphona (Fig. 355), Empis (Fig. 421), and others, are not differen- 

 tiated, since the greater part of the lateral aspects of the paraglossae is 

 chitinized. 



In the typical labium two simple tracliea-like structures, commonly 

 known as pseudotracheae (ps), arise from the proximal part of each 

 glossa and extend onto the mesal membranous aspect of each paraglossa. 

 These trachea-like structures are in reality small chitinized troughs 

 which serve as conduits for the liquid food. Pseudotracheae are unique 

 structures and peculiar to Diptera, so far as known. They are present 

 in only a few generalized forms, but from these genera it is possible 

 to develop the pseudotracheal arrangement and structure of the more 

 specialized Diptera. It is consequently as.sumed that the pseudotracheae 

 have probably arisen only once within the order, and that this happened 

 some time after the group as a whole -was set off as a distinct order. 



The psedotracheae (ps) of Tipula (Fig. 383) resemble those of the 

 typical labium in that the two main pseudotracheae arise from each 

 glossa and extend over the mesal membranous area of the paraglossa, 

 one of the pseudotracheae extending caudad and the other cephalad. 

 These ducts are secondarily branched and resemble a fern. The pseudo- 

 tracheae of Mycetophila (Fig. 11) and Leia (Fig. 368) are reduced and 

 only the caudal pseudotracheae extend over the paraglossae. The para- 

 glossae in these genera are united along the meson and form a single 

 large lobe. The cephalic pseudotracheae are indicated by small rudiments 

 in Mycetophila (Fig. 11). The pseudotracheae in these forms resemble 

 the typical labium in that they are simple, unbranched, chitinized 

 troughs. From the typical labium, or from the p.seudotracheae as they 

 occur in Tipula, it is possible to derive the arrangement and structure 

 of the pseudotracheae as they are found in Tabauus (Fig. 390) and 

 similar forms, where two long pseudotraclieal trunks (m. ps) extend 

 cephalad and caudad from the .glossae (gl) and give rise to many 

 branches on their ventral side. These branches extend ventrad over 

 the entire mesal area of the paraglossa (pgl). The arrangement of the 

 pseudotracheae of most Diptera is readily derived from a form similar 

 to Tabanus. The arrangement in Scenopinus (Pig. 400), Psilocephala 

 (Fig. 403), and many of the Calyptratae resembles that in Tabanus. 

 In such genera as Stratiomyia (Fig. 396), Oecothea (Fig. 453), Coelopa 

 (Fig. 449), and Heteroneura (Pig. 460) no main collecting duets 

 (m. ps) extend beyond the glossae. In many genera, such as Chloro- 

 pisca (Fig. 431) and Chyromya (Fig. 412), no line of demarkation 

 can be drawn between the proximal ends of the pseudotracheae and the 

 glossae. 



