SEXUAL CHARACTERISTICS OF THELIA 541 
brought about by Stylops. The head showed a reduction in 
size, the abdomen became more globose with the puncturing less 
strongly marked, and the villosity increased. The scopa, or 
pollen-carrying apparatus, of the hind tibia of the female was 
reduced in parasitized females. These individuals also lost the 
pollen-gathering instinct. In the males, on the contrary, the 
narrow hind tibia was increased by the presence of Stylops. 
The clypeus of the female lost its black color and gained the 
yellow color of the male, whereas the clypeus of parasitized 
males showed merely a reduction in the extent of the yellow 
pigment. The sting of the female was reduced greatly in size. 
This was also the case with the external genitalia of the male. 
Perez contends, therefore, that not only is there a loss of sec- 
ondary sexual characteristics due to the parasites, but also in 
certain cases there is the assumption of characteristics of the 
opposite sex. A study of the gonads showed that in the male 
one testis might continue functional, whereas in the female only 
a minute rudimentary ovary remained. 
Wheeler (10) studied the effect of Xenos on Polistes. He 
also gives a most excellent review of work done on the castra- 
tion of insects. The parasitized Polistes failed to give the inter- 
esting series of changes we might expect. They merely assumed 
a reddish tinge to the abdomen and face. Wheeler’s work does 
not, of course, invalidate that of Perez; it merely fails to extend 
the known changes on bees to the wasps. Smith (14) studied 
three species of Andraena infested with Stylops. A large part of 
his study is devoted to the development and habits of the para- 
site and the remainder to the internal and external changes 
wrought in the host. He reviews in detail the work of Perez and 
reproduces several of his figures. Of the parasites themselves, 
two facts are of especial interest. Stylops carries on its respira- 
tion with the external world through two tubercles on the head, 
which extend between two abdominal segments of the host, and 
therefore does not use the haemolymph of its host for respiratory 
changes. The sex of the parasite is not a factor in considering 
the changes brought about in Andraena. Smith does not de- 
scribe modifications as extensive as those given by Perez, but in 
