SEXUAL CHARACTERISTICS OF THELIA 587 
inous elevations on the caudal border of the eighth sternum. 
Going back still farther to the first instar, longitudinal sections* 
show thickenings of the hypodermis corresponding to the append- 
ages seen in surface views of later stages, and, since the gonads 
are already differentiated into minute ovaries or testes, one 
knows positively the sex with which he is dealing. 
In considering the effects of the parasites on the gonapophyses, 
it might be inquired whether or not these appendages are modi- 
fied in their nymphal form by the larvae of Aphelopus. As was 
stated on page 552, nymphs stung during their first or second 
instars do not become adults, for the internal larvae reach ma- 
turity in the fifth instar of the host and emerge, thereby killing 
the Thelia. Two such nymphs from which the larvae have 
emerged are shown in figures 43 and 47. In the male (fig. 43) 
there are two changes in the nymphal genitalia. The dorsal 
pockets suffer a great reduction in size and remain in part sim- 
ilar to those of the fourth instar in that the median partition 
still persists. The two pockets are each again subdivided by a 
partition corresponding to those of the normal fifth instar, but 
these are very small and run parallel to the long axis of the 
animal rather than at an angle toward the median plane, as in 
normal nymphs. The condition is, therefore, intermediate 
between the fifth and fourth instars, for the medium partition 
is characteristic of the fourth instar and is not present in the 
fifth instar, whereas the lateral partitions are characteristic of 
the fifth instar and not present in normal fourth instars. The 
persistence of a characteristic of the fourth instar indicates 
retardation of development. In the parasitized female nymph 
(fig. 47) not only are the nymphal genitalia reduced in size, but 
their form corresponds exactly to that of the normal fourth 
instar, each pair of gonapophyses remaining distinct and easily 
differentiated. After having ascertained that during the fourth 
3 At the suggestion of the author, Mr. E. D. Churchill made a histological 
and gross study of the development of both external and internal genitalia of 
the membracid Vanduzea arquata (Say). In this he traced the gonapophyses 
from the first instar in histological sections. His work substantiates exactly 
what has been found true of Thelia. 
