21S] HEAD OF DIPTERA— PETERSON 45 



venience in description and as an aid in tracing the development of the 

 parts of the furca thruout the order, it has been divided into furca-1, 

 which is the stem of the Y, furca-2, which is the dorsal arm of the Y, 

 and furca-3, which is the ventral arm. The furca articulates with a 

 small sclerite which is located between the proximal end of furca-1 and 

 the distal end of the chitinized furrow on the meson of the theca. This 

 piece has been called the sigma (si). Another smaU, independent 

 sclerite is located in the membrane just laterad of the sigma and this 

 may be known as kappa (k). Each paraglossa has on its mesal aspect 

 two trachea-like structures which arise from the proximal portion of 

 the glossa. These structures are commonly called pseudotracheae (ps). 



A general survey of the characteristics of the paraglossae of the 

 various labia shows that they are usually bulb-like, membranous, and 

 somewhat flexible. In these respects they differ decidedly from the 

 firmly chitinized, flap-like labia of many generalized insects. Their size 

 and shape vary greatly, as can be seen in Bibio (Fig. 364), Leia (Fig. 

 368), Promaehus (Fig. 376), Geranomyia (Fig. 382), Tipula (Fig. 384), 

 Tabanus (Fig. 390), Conops (Fig. 417), Empis (Fig. 421), Siphona 

 (Pig. 458), Musca (Fig. 467), Stomoxys (Fig. 479), and Olfersia (Fig. 

 488). The use to which the labia are put seems to have some influence 

 on their form. The main line of development thruout the genera figured 

 is toward the type found among the Calyptratae, in which the labia 

 are usually large, decidedly membranous, and joined together on the 

 dorso-caudal areas, as in Hydrotaea (Pig. 475), Sarcophaga (Pig. 477), 

 Sepsis (Fig. 439), Loxocera (Fig. 461), Tetanocera (Fig. 463), and 

 many other genera. 



The membranous development of the paraglossae is not always a 

 good indication of the main line of specialization. In a number of 

 scattered genera, Chironomus, Rhyphus, Aphiochaeta, Chloropisca, 

 Platypeza, Leptis, Psiloeephala, and Lonchoptera, it is next to impossi- 

 ble to make out the chitinized pieces, such as kappa, sigma, and furca, 

 because of the membranous condition of the entire labium. Outside of 

 the above-named forms, the chitinized pieces of the paraglossae are 

 usually distinct when present. These supports may be secondary in 

 origin or they may be remnants of former chitinized parts of the para- 

 glossae. It is possible to show how the various chitinized pieces of the 

 majority of the labia may have been developed from the typical form. 



The sclerite designated as kappa (k) on the typical labium is only 

 present in Tabanias (Fig. 390 and 391), Tipula (Fig. 388), and Bitta- 

 comorpha (Fig. 85). No other dipteron gives any evidence whatever 

 of such a sclerite. In the above-mentioned genera the pieces are em- 

 bedded in the membrane laterad of the ventral ends of the theca. Some 



