SEXUAL CHARACTERISTICS OF THELIA 629 
complete solution of the problem as to the one important change 
seems rather remote. We would maintain that the changes 
were not effected through the production of some sex hormone 
and that the parasites do not act like an ovary to stimulate the 
development of the female secondary sexual characteristics in 
males. It seems rather that the altered haemolymph failed to 
provide the necessary conditions for the expression of the char- 
acteristics most recent in the evolution of Thelia bimaculata, 
namely, the specific characteristics of the gonapophyses and the 
male extragenital sexual characteristics. These new character- 
istics may have come into the species by the origin of genes 
modifying established characteristics, and the failure of these 
genes to come into activity would produce an organism retain- 
ing ancestral characteristics. The male and femaleindividuals 
would then be more similar in color, size, and in other details of 
their integument. A slight change in the environment would 
permit a partial expression of the newest genes, and thus inter- 
mediates were found in males containing small larvae. 
9. SUMMARY 
1. Thelia bimaculata is parasitized by the dryinid Aphelopus 
theliae. An egg of the parasite may be deposited in an immature 
Thelia of any of the five nymphal instars. The Aphelopus ovum 
undergoes polyembryonic development and gives rise to about 
fifty larvae which reach their maximum development either in 
the fifth nymphal instar of the host or in its adult stage. The 
Aphelopus larvae escaping from their host, thereby killing it, 
drop to the ground, burrow in, pupate, and become mature the 
following summer. 
2. Thelia parasitized by Aphelopus show many alterations. 
Most interesting are the changes wrought in males which reach 
the adult form. These assume either partially or completely 
many sexual characteristics of the female, much as do sacculi- 
nized crabs. The degree of change depends upon the size of the 
parasites during the fifth nymphal instar of the host. If the 
Aphelopus ovum is deposited early in the nymphal life of the 
