40 ILLINOIS BIOLOGICAL MONOGRAPHS [210 



well-developed basiproboscis. This migration of the maxillae in the 

 Cyelorrhapha has not altered their generalized position between the 

 labrum-epipharynx and the labium. 



The stipites of genera having a ptilinum show all stages of in- 

 growth from a turned-in free edge or end (st-e), to forms in which it 

 is entirely entad of the membrane of the basiproboscis, as in Musca. 

 Eristalis (Fig. 286), Eulonchus (Fig. 284a), and Exoprosopa (Fig. 

 285) are the only forms without a ptilinum which show an ental growth 

 of the stipites. These genera make a good starting point for explaining 

 the characteristic development found in the Acalyptratae and the Calyp- 

 tratae. The following scheme of lines and dots has been adopted on 

 the drawings in order to show the degree of ingrowth of the stipes. A 

 continuous solid line on the stipes indicates a definite ectal boundary 

 which connects with the membrane of the basiproboscis. A broken line 

 indicates an ental edge or end which is free of the membrane between 

 it and the observer. The membrane is represented by stippling. For 

 convenience of description and homology the following division of the 

 stipes has been made: st represents the ectal portion of the stipes and 

 st-e the ental portion; and st is further divided into st-1 and st-2 as 

 seen in Coelopa (Fig. 288). 



In Exoprosopa (Fig. 285) and Eulonchus (Fig. 284a) the proximal 

 end of the stipes is free and entad of the membrane, while the cephalic 

 edge and the dorsal end are entad in Eristalis (Fig. 286). From a form 

 similar to Eristalis it is possible to develop a stipes which would resem- 

 ble that of Sepsis (Fig. 287), Coelopa (Fig. 288), and Calobata (Fig. 

 296). In Sepsis the palpus is greatly reduced, but it connects with an 

 ectal portion of the stipes (st) which in turn gives rise to the free ental 

 portion (st-e). The free ental part extends ventrad and is continuous 

 with the galea, which emerges from the membrane near the base of the 

 labrum as a free appendage. The stipes of Coelopa (Fig. 288), Sapro- 

 myza (Fig. 289), and Sphyracephala (Fig. 293) is similar to that of 

 Sepsis, but in these forms the palpus arises from the cephalic margin 

 of the basiproboscis. The palpus is connected with the stipes proper 

 by means of a long chitinized strip (st-1) which is usually covered with 

 setae. This ectal portion of the stipes (st-1) is present in all but a few 

 genera, such as Chloropisca (Fig. 306), Heteroneura (Fig. 298), Chyro- 

 mya (Fig. 299), Loxocera (Pig. 300), and Euaresta (Fig. 292). In 

 a number of forms, particularly in the Calyptratae, a small chitinized 

 area is present ventrad of the palpus. This is regarded as a secondary 

 chitinization. The ectal portion of the stipes (st-2) is present in a 

 majority of the Acalyptratae and in one or two of the Calyptratae. 



The ental portion of the stipes (st-e) is always present in the 

 members of this group. In Desmometopa (Fig. 303), Chloropisca (Fig.. 



