SEXUAL CHARACTERISTICS OF THELIA 569 
increase in width approaching that characteristic for the female. 
This same relation of size increase is seen to exist when we com- 
pare lengths of labia of normal males, normal females, and para- 
sitized males (fig. 23). As was the case in the pronota and 
wings, parasitized females show no marked changes in either 
size or pigmentation of the head. 
C. Extragenital abdominal characteristics 
The effects of the parasites are also very marked upon the 
abdomens of Thelia. The changes occurring in the extragenital 
secondary sexual characteristics or tertiary sexual characteristics 
will be described first. 
The abdomens of the two sexes present very different appear- 
ances and, directly or indirectly, many of these differences are 
associated with reproduction. The female abdomen must be 
capable of holding, on the average, thirty-five ova, 2.4 mm. in 
length—a bulk much greater than that formed in the male 
abdomen by the testes, seminal vesicles, and accessory glands. 
In the male the reproductive apparatus is mature when the 
nymph molts to an adult, and is contained in the abdomen 
without distention; but in the female the ova are very minute 
at the beginning of imaginal existence and continue to grow, 
filing the abdomen and extending the chitinized abdominal 
sclerites to such an extent that the intersegmental membranes 
show as lighter bands between these pigmented plates. Thus 
the female abdomen is much larger than that of the male, 
and its cuticula is far more pliable than that of the male. This 
pliability, a necessity in accommodating the growing ova, is 
accompanied by a pigmentation of the abdominal sclerites, 
lighter than that of the male abdomen (figs. 24and 28). Strength, 
firmness, and rigidity of chitinous parts in insects is always 
accompanied by a heavy melanic pigmentation, as in mandibles, 
ovipositing apparatus, and muscle attachments; whereas the 
absence of melanin leaves the chitin much weaker and more 
pliable. 
