211] HEAD OF DIPTERA— PETERSON 41 



306), Conops (Fig. 305), and the Calyptratae it has no connection with 

 the ectal portion of the stipes (st-2) or the membrane, and by many 

 writers is commonly called the apodeme. The free so-called apodeme 

 is unquestionably derived from the ental ingrowth of the stipes, as 

 illustrated by the modifications found in the following genera : Coelopa 

 (Fig. 288), Sapromyza (Fig. 289), Tetanocera (Fig. 297), Archytas 

 (Fig. 309), Musca (Fig. 304), and others. 



The development of the galea (g) may be traced thruout the Cy- 

 clorrhapha in a manner similar to that of the stipes. In Eristalis 

 (Fig. 286) the galea is a long free appendage arising from the ventral 

 end of the stipes near the proximal end of the labrum-epipharynx. Its 

 length and size are greatly reduced in Sepsis (Fig. 287), but its position 

 is identical with that of Eristalis. Thruout the majority of the Acalyp- 

 tratae the position of the galea resembles that of Sepsis. Its size and 

 form undergo some change, as can be seen in the figures. In the Calyp- 

 tratae and some of the Acalyptratae the galea articulates with the 

 proximal end of the labrum and is more or less firmly connected with 

 the same. The ectal exposure of the galea is very small in these forms. 

 The large galea of the Acalyptratae has been considered as the maxillary 

 palpus by Wesche (1902). This interpretation is highly improbable. 



Labium. — The labium is the most specialized and characteristic 

 appendage of the mouth of Diptera. Its structural modifications are 

 very striking among the specialized genera, such as the Cyelorrhapha. 

 These modifications are largely due to the reduction of the parts and 

 the excessive development of membranous areas, and they agree with 

 similar types of modification in other head- and mouth-parts. 



To explain the unique development of the labium of Diptera, it 

 has been necessary to make a critical study of the generalized condition 

 of this appendage as it occurs in the Nematocera and to compare it 

 carefully with the labia of more generalized insects. As is well known, 

 the labium of a generalized insect is the posterior, independent, flap- 

 like mouth-part, made up of a submentum, mentum, and ligula. The 

 ligula is further divided into palpigers, palpi, paraglossae, and glossae. 

 The labium of a generalized dipteron resembles that of a generalized 

 insect in its caudal position and in its independent condition, but it is 

 very different in structure. It is more or less enlarged and not flat 

 and flap-like, and the palpi and palpigers are always wanting, so far 

 as observed. Since the position of the palpi and the palpigers is very 

 useful in orienting the sclerites of the labium of generalized insects, 

 their absence in Diptera makes it exceedingly difScult to homologize cor- 

 rectly and locate the submentum, mentum, and the parts of the ligula. 

 The membranous condition of the labium also adds to this difficulty. 



