207] HEAD OF DIPTERA — PETERSOX 37 



of the maxillae of generalized Diptera or other insects. Numerous 

 intermediate stages of maxillary development are present in the various 

 species; consequently it is possible, and in fact comparatively easy, to 

 trace thruout the order the main line of specialization and several side 

 lines. 



The hypothetical maxillae of the Diptera (Fig. 257) resemble the 

 maxillae of a generalized insect in their homologous sclerites, their posi- 

 tion between the mandibles and the labium, and their close association 

 with the invaginations of the posterior arms of the tentorium. Struc- 

 turally they are composed of small triangular cardines (ca), long 

 stipites (st), five-segmented palpi (mx.pl), needle-like galeae (g), and 

 short laciniae (la). The cardines and stipites differ from those of gen- 

 eralized insects in that they are embedded in the mesal membranous 

 area ventrad of the occipital foramen. The palpi, galeae, and laciniae 

 are free appendages. The proximal ends of the cardines are adjacent 

 to the invaginations of the posterior arms of the tentorium. The struc- 

 ture and position of the various parts of the hypothetical type have 

 been traced thruout the order. The species in which the ptilinum is 

 wanting are considered first. 



The cardines (ca) are small distinct triangular selerites in Trichoc- 

 era (Fig. 260), Rhyphus (Fig. 261), Dixa (Fig. 262), and the female 

 of Tabanus (Fig. 259). In these genera they are adjacent to the invagi- 

 nations of the posterior arms of the tentorium. The cardines of Simu- 

 lium (Fig. 258), in both males and females, differ from those of the 

 above genera in that they are large and occupy nearly all of the mem- 

 branous area between the postgenae dorsad of the stipites. Their 

 margins are also somewhat indistinct. No other forms figured have 

 distinct sclerites that are homologous with the cardines of the hypo- 

 thetical type. The maxillae of Rhabdophaga (Fig. 268), Bibiocephala 

 (Fig. 269), and Chironomus (Fig. 270) connect with the invaginations 

 of the posterior arms by means of narrow chitinized processes which 

 arise from the stipites proper. Undoubtedly these pieces are reduced 

 cardines which have lost the suture that separates them from the 

 stipites. The presence of this suture is suggested by the suture-like 

 depression in the male of Bibiocephala (Fig. 76). Excepting Promachus 

 (Fig. 276) and the above forms, the cardo is wanting in all the maxillae 

 figured. The maxillae of Psychoda (Fig. 263) and Sciara (Fig. 267) 

 closely resemble some of the above maxillae, but the cardines as chi- 

 tinized pieces are apparently wanting. There is a distinct membranous 

 area between the proximal ends of the stipites and the invaginations 

 of the posterior arms of the tentorium. From forms such as these it is 

 concluded that the cardines have been lost as chitinized areas. No 

 other interpretation seems possible with the evidence at hand. 



