46 ILLINOIS BIOLOGICAL MONOGRAPHS [216 



one has iuterpreted these pieces as rudimentary palpigers or palpi. This 

 may or may not be correct. It is possible for palpi to be in such a 

 position ; but since no other genera have similar pieces, and since they 

 are so decidedly dissimilar to the labial palpi and palpigers of general- 

 ized insects, they are here regarded as secondary sclerites. 



The sclerite designated as sigma (si) is present as a chitinized thick- 

 ening at the ventral end of the theca, as in Eristalis (Fig. 443), or as 

 a distinct piece, as in a majority of the Brachycera and the Cyclorrha- 

 jDha. In all genera it is situated between the ventral margin of the 

 theca and the furca. Only a few genera of the Nematocera, such as 

 Tipula (Fig. 388) and Psorophora (Fig. 380), have these sclerites. They 

 undergo some modification in size and structure as can be seen in the 

 following genera: Tabanus (Fig. 391), Mydas (Fig. 397), Conops (Fig. 

 418), Borborus (Fig. 437), Eristalis (Fig. 443), Coelopa (Fig. 448), 

 and Scatophaga (Fig. 470). 



The furca of Bibio (Fig. 315) and that of Tabanus (Fig. 317) 

 closely resemble the typical form. In Bibio, furca-1 (f-1) and furea-2 

 (f-2) are one continuous piece, while furca-3 (f-3) is a distinct arm. 

 In Tabanus, furca-2 and furca-3 are distinctly chitinized areas arising 

 from the distal end of furca-1. Only one chitinized support is present 

 in Sciara (Fig. 314), Ehabdophaga (Fig. 313), Psychoda (Fig. 318), 

 Stratiomyia (Fig. 331), and Trichocera (Fig. 311). In Trichocera this 

 support has a decided dorsal bend near the constriction of the para- 

 glossae. This bend is also present in Psychoda and Stratiomyia, but 

 the constriction is wanting. The distal portion of the furca beyond the 

 bend is homologous with furca-2, and furca-3 is wanting in these forms. 

 Furca-2 is present and furca-3 is wanting in Scenopinus (Fig. 325) ; 

 furca-3, however, is present in more species than furca-2. Such is the 

 case with Borborus (Fig. 342), Chrysomyza (Fig. 341), Coelopa (Fig. 

 337), Tetanocera (Fig. 344), Scatophaga (Fig. 357), Musca (Fig. 351), 

 and Thelaira (Fig. 346). 



Furca-1 (f-1) varies considerably thruout the order. In general- 

 ized forms where the dorso-caudal portions of the paraglossae are not 

 joined together the furcae are always well separated. They are also 

 separated in some forms where the paraglossae are joined, as in Mydas 

 (Fig. 397) and Eristalis (Fig. 443). In Chyromya (Fig. 411), Dro- 

 sophila (Fig. 454), Tetanocera (Fig. 463), and Sepsis (Fig. 439), an 

 intermediate piece joins the mesal ends of furcae-1 while in Sarcophaga 

 (Fig. 477), Musca (Fig. 466), Coelopa (Fig. 448), Sapromyza (Fig. 

 409), Chrysomj^za (Fig. 457), Heteroueura (Fig. 459), and Oecothea 

 (Fig. 452) furcae-1 are united and form one continuous U-shaped piece. 

 This type of furcae is present among the Calyptratae. The furcae of 



