20 ILLINOIS BIOLOGICAL MONOGRAPHS [190 



Ptilinum. — A deep, inverted U-shaped groove is present in the 

 heads of all the Calyptratae and the Acalyptratae dorsad of the anten- 

 nae. This groove is called the frontal suture (fr. s) and marks the line 

 of invagination of the large membranous pouch, the ptilinum (pt). In 

 Sphyracephala (Fig. 52) the frontal suture is V-shaped, owing to the 

 peculiar development of the head. The extent of the invagination of 

 the ptilinum (pt) is indicated by a dot-and-dash line in the drawings 

 of the cephalic and lateral views of the head-capsule. 



The origin of the ptilinum has been a mystery to morphologists. 

 After a careful examination of the heads of the Braehycera and the 

 Cyclorrhapha, no definite data were found which would throw any light 

 on its origin. A few forms, however, suggested a possible way in which 

 it might have been developed. The frontal suture and the ptilinum 

 are comparatively small in Tetanocera (Fig. 55), Sapromyza (Fig. 60), 

 Conops (Fig. 67), Ochthera (Fig. 56), and Chloropisca (Fig. 51). 

 These genera gave no clue to the early stages of its development unless 

 the thinly chitinized condition of the fronto-clypeus of Chloropisca has 

 some significance. It seems evident that the frontal suture was once a 

 membranous area which became invaginated to form a membranous 

 pouch or ptilinum. If this is the case, the mesal membranous area of 

 the fronto-clypeus of Sepsis (Fig. 46), Oecothea (Fig. 48), Calobata 

 (Fig. 44), and Desmometopa (Fig. 47) would be very significant. The 

 ptilinum might possibly have originated from some form similar to 

 Scenopinus (Fig. 41), in which the ventral margin of the chitinized 

 vertex is located dorsad and laterad of the antennae. It seems quite 

 possible that the membrane along this margin became invaginated in 

 the early stages of the development of the ptilinum. The above con- 

 jectures may or may not be correct. A real solution of the problem will 

 undoubtedly require a careful study of the pupal development. 



Ldbrum. — The labrum (1) of a hypothetical dipterous head (Fig. 

 1, 140, and 493) is a distinct, chitinized, tongue-like structure connected 

 with the ventral margin of the clypeus. The shape and size of the 

 labrum are identical with the shape and size of the epipharynx, which 

 is located on its caudal aspect. The labrum (1) and epipharynx (ep) 

 are joined together by a membrane along their lateral margins. These 

 two structures thus aet as one organ and they have rightly been called 

 the labrum-epipharynx (1. ep). The above relation of the labrum to 

 the epipharynx and the fronto-clypeus resembles that in the Orthoptera. 



In a general way the labrum of all the genera studied resembles the 

 hypothetical type described above. It varies, however, in shape and 

 in degree of chitinization. In Promachus (Fig. 22), in Psorophora (Fig. 

 10 and 26), and in the female of Tabanus (Fig. 20) it is completely 



