38 ILLINOIS BIOLOGICAL MONOGRAPHS [208 



The stipites (st) are of various shapes and sizes as can be seen in 

 the figures. In Bhabdophaga (Fig. 268), Bibiocephala (Fig. 269), 

 Chironomus (Fig. 270), and possibly Mycetobia (Fig. 90), they have 

 united to form a chitinized strip or plate in the membranous area dorsad 

 of the labium. This piece should not be confused with the submentum 

 of the labium. In all species in which the postgenae have not united 

 ventrad of the occipital foramen, the proximal ends of the stipites are 

 near the invaginations of the posterior arms of the tentorium. In all 

 species where the postgenae form a continuous plate, the stipites are 

 reduced in size and situated at or beyond the ventral margin of the 

 head, as in Mydas (Fig. 319) and Eristalis (Fig. 328). In other words, 

 the usual association between the maxillae and the invaginations of the 

 posterior arms' has been lost. Psilocephala (Fig. 281) and Psorophora 

 (Fig. 96) are exceptions to the last statement. In Psilocephala chi- 

 tinized thickenings (ch. th) are present on the ental surface of the 

 postgenae ventrad of the occipital foramen, and these are undoubtedly 

 rudiments of the stipites. The stipites of Psorophora (Fig. 266 and 

 96) are long, free rod-like structures located entad of the postgenae. 

 They extend between the occipital foramen and the ventral margin of 

 the head. The stipites of Geranomyia (Fig. 382) and Limnobia (Fig. 

 386) are also entad of the postgenae. In these genera their proximal 

 ends are united and they have no connection with the head-capsule. 

 The stipites of Tipula (Fig. 277) resemble those of Geranomyia and 

 Limnobia, but there is greater reduction in size, and they are completely 

 united along their mesal margins, thus forming a single median piece. 



The maxillae of Promachus (Fig. 84) differ from those of all other 

 genera in that the stipites and the cardines are united on the meson 

 and continuous with the postgenae near the occipital foramen. Narrow 

 membranous areas separate the maxillae from the postgenae near the 

 ventral margin of the head. This unique modification of the maxillae 

 agrees with the striking modifications in the other mouth-parts. 



The figures show the variations in other genera belonging to this 

 group. In general it can be said that the stipites have been modified 

 by reduction and by removal to the ventral margin of the head and in 

 some cases are even located on the basiproboscis. 



The maxillary palpi (mx. pi) of the Nematocera figured have from 

 two segments — Geranomyia (Fig. 382) and the female of Psorophora 

 (Fig. 266) — to five segments. The usual number is four or five. In 

 the Brachycera only one articulating segment is present. This segment 

 in Tabanus (Fig. 259) connects with an elongated portion of the stipes 

 which is called the palpifer by some. In this study the palpifer is 

 considered as wanting, since no palpus of the Diptera possesses over 



