SEXUAL CHARACTERISTICS OF THELIA 605 
parasitized Thelia all explanations based upon the experimental 
evidence gained from operations on vertebrates. As was pre- 
viously stated, vertebrates and especially mammals castrated as 
immature individuals often retain juvenal characteristics and 
continue to grow beyond the normal size, and males may store 
up a large amount of adipose tissue. Throughout the descrip- 
tion of the changes incurred by adult parasitized Thelia, the 
normal nymphal characteristics have been presented for com- 
parison, and it may safely be stated that in no case were the 
alterations of males referable to the retention of juvenal charac- 
teristics. Only in one case was arrest of development encoun- 
tered, and that in the genitalia of the fifth instars which contained 
large Aphelopus larvae. These individuals had genitalia resem- 
bling those of the fourth instar. It is unfortunate that such 
individuals are always killed by the emergence of the eruciform 
larvae, for it would be extremely intersting to know, could they 
become adults, whether they would possess genitalia similar to 
those of the fifth instar instead of reduced adult genitalia, such 
as are met with in all infected adults. 
It was seen-that parasitized male Thelia grew to a greater size 
than normal males, but this can hardly be ascribed to continued 
growth similar to that occurring in castrated mammals. Such 
an explanation would not account for the fact that parasitized 
female Thelia are not larger than normal females and parasitized 
females lack gonads even more frequently than do infected 
males. Likewise, no increase in size of experimentally castrated 
insects has ever been reported, and it is quite safe to say that 
parasitized male Thelia are larger than normal males, not be- 
cause the testes may be degenerate, but because the parasites 
exert some positive effect upon the soma which in part develops 
certain female extragenital secondary sexual characteristics of 
which greater size is one. This increase in size of parasitized 
males was noted in the pronotum, wings, head, acrotergites, 
hind legs, and abdomen. Not merely, therefore, is the region 
containing the parasites, the abdomen, enlarged, but the most 
remote portions of the body respond. As far as could be ascer- 
tained, this is the first case of this sort found in parasitized 
