SEXUAL CHARACTERISTICS OF THELIA 613 
ovaries had been implanted would as adults possess abdomens 
capable of accommodating the mature ovary. In sacculinized 
male crabs and in male Thelia parasitized by Aphelopus the 
abdomen is enlarged, but there is little evidence to support the 
idea that the enlargement is called forth by an influence similar 
to that supposedly exerted by an ovary. The physiological 
activity of the somatic cells, although correlated with gamete 
production, does not, in insects, seem to be governed by the 
gonads, but is probably to be referred to the genetic constitution 
of the cells upon which the gametes themselves may be partially 
dependent for their normal development. ‘This is illustrated by 
the anomalous Thelia nymph described in part 7. This individ- 
ual, a nymph of fourth instar, had perfect female soma and con- 
tained two testes which were undersized for this stage and con- 
tained many degenerating cysts. If we compare the normal 
development of the gonads in the two sexes, we see that the testes 
develop rapidly and are full sized at the final molt, but the ova- 
ries progress very slowly and are smaller than the testes during 
the entire nymphal life. In the adult female they grow to rela- 
tively enormous size, but in the fourth instar they are consid- 
erably smaller than the testes. Thus, in the anomalous nymph, 
I believe, we must ascribe the smallness of the testes to the 
fact that they were provided with only as much nutriment as 
a female soma normally provides to its contained ovaries. The 
degenerating cysts indicate that cell division proceeded faster 
than materials for growth were supplied. Here surely the testes 
failed to influence the soma to change its normal metabolism 
or to produce any male characteristics. If, then, the develop- 
ing soma of arthropods is entirely independent of any influ- 
ence emanating from the gonads, we cannot explain the modi- 
fications produced by parasites on the assumption that the 
parasites act as do the gonads of one sex or the other. 
The sexual characteristics of insects must, I believe, depend 
entirely upon the chromatic makeup of the cells composing the 
individual. Of all the insects, no groups show a more uni- 
versal visible chromatic difference between the cells of the two 
sexes than do the Hemiptera and Homoptera. It is not my 
