SEXUAL CHARACTERISTICS OF THELIA 571 
Each typical abdominal segment consists ventrad of a sternum 
and two pleura (figs. 25 and 29, s, p) and dorsad and laterad of a 
tergum, bent into an arch which is somewhat more pointed at 
its apex in the female than in the male. The sterna of the 
female are much broader than those of the male. With the 
pleura which bear the spiracles, the sterna form a flat ventral 
surface which, at its union with the terga, forms a sharp ventro- 
lateral angle. Thus the female abdomen is almost triangular in 
cross-section. In the male the sterna are bent slightly dorsad 
and the pleura are also bent upward, making them lateral rather 
than entirely ventral (fig. 25). No sharp angle is formed at the 
union of the male pleura with the terga. The abdomen is rather 
subovoid in cross-section. The sterna and particularly the 
terga of the male abdominal somites are more deeply pigmented 
than those of the female. 
When we examine the abdomens of parasitized males (fig. 27) 
we see that the sterna increase greatly in width and that the 
abdomen in cross-section becomes similar to that of the female. 
The pleura become flattened plates entirely ventral in position 
and form with the terga a sharp ventrolateral angle. All the 
abdominal sclerites show a loss of pigmentation and a corre- 
sponding decrease in strength or rigidity. The terga (fig. 26) 
have even less melanin than those of most normal females, and 
the pigment remaining is restricted chiefly to the regions of 
muscle attachments. 
Two of the changes effected by the parasites on males, namely, 
increase in size of the abdomen and decrease in firmness, strength, 
and pigmentation of the sclerites, are to be found even in indi- 
viduals still showing predominatingly male characteristics in the 
thorax and head (figs. 36 to 39). Broad translucent sterna 
through which greenish fat and some red pigment show are an 
infallible clew to the presence of parasites. These changes are 
requisite to the development of Aphelopus. The larvae must 
have sufficient space in which to develop, and when full grown 
present a bulk quite as great as the ova of a mature female 
Thelia. The narrow abdomen of the male would be insufficient 
for the development of the polyembryonic brood which, before 
