42 ILLINOIS BIOLOGICAL MONOGRAPHS [212 



In order to get some light on this problem, a study was made of 

 the labium, particularly the submentum and mentum, of a number of 

 generalized insects of the more common orders. The literature of this 

 subject was examined, but no satisfactory results were obtained from 

 this source. After a careful study of a number of labia, the following 

 general characteristics which bear upon the labium of Diptera, were 

 noted. The submentum is the large proximal segment, while the mentum 

 is usually small and in some cases very thinly chitinized and almost 

 obsolete. The sutures separating the mentum from the submentum 

 and the ligula are only represented by small remnants in Melanoplus. 

 The ligula, so far as observed, comprises the movable parts of the labium, 

 while the mentum and submentum are more or less firmly united with 

 the head-capsule. The proximal part of the ligula is usually well de- 

 veloped and gives rise to the palpigers, palpi, paraglossae, and glossae. 

 The glossae are located between the paraglossae, and in a number of 

 forms a distinct depression or thickening extends proximad between the 

 glossae and the proximal margin of the ligula. 



With these observations as a basis for comparison, the labium of 

 such generalized Diptera as Chironomus (Fig. 371), Simulium (Fig. 

 366), Trichocera (Fig. 365), Dixa (Fig. 375), and others may be inter- 

 preted as follows. The mesal membranous area of the caudal aspect 

 of the head, which is bounded by the postgenae (po), the occipital 

 foramen (o. f), and the proximal chitinized piece of the labium (the), 

 is made up of the submentum, mentum (su. me), and the cardines (ca) 

 and stipites (st) of the maxillae (mx). Since this area is largely mem- 

 branous, it is impossible to determine the boundaries of these sclerites. 

 The areas laterad of the cardines and the stipites apparently belong to 

 the maxillae, while the area mesad of these parts is made up of the 

 submentum and mentum (su. me). The important feature concerning 

 this mesal membranous area is the fact that the maxillae and the 

 labium both play a part in its formation. This undoubtedly indicates 

 that the submentum and mentum, of a more or less fixed nature in 

 generalized insects, have been more extensively fixed in the Diptera, 

 and that the submentum and mentum are included in the membrane 

 developed from the stipites and cardines. Such an interpretation is 

 altogether possible, since the proximal portions of the maxillae are adja- 

 cent to the submentum and mentum in generalized insects. 



The ligula (lg) of the generalized Diptera agrees with the ligula 

 of generalized insects in that it is the movable part of the labium. 

 Structurally it is composed of a well-developed proximal area which 

 gives rise to two large bulb-like paraglossae (pgl) and to two small 



