626 SIDNEY I. KORNHAUSER 
showed the presence of only small parasites, and it seemed reas- 
onable to believe that the degree of change was largely due to 
the size of the parasitic larvae during the fifth instar. Not by 
their size merely, but by their greater physiological effect, would 
the presence of the large parasites greatly alter the internal 
conditions. The degree of change in the host’s form would be 
directly referable to the degree of internal alteration effected by 
the parasites. It might be expressed as a quantitative reaction 
depending on the concentration of the metabolic products of the 
Aphelopus larvae. The sexual characteristics of Thelia which 
appeared early in ontogeny would be subjected to the products 
of the parasites’ metabolism during several molts, although the 
concentration of these products must be rather less in the earlier 
instars. Yet in spite of this the older characteristics we found 
were not reversed. Spermatogonia were surely subjected to 
conditions where the growth of odcytes would be possible and 
still only spermatocytes developed. The external genitalia 
were greatly reduced in size, but still retained the general form 
found in the Membracidae. Their reduction in size is probably 
due to the fact that the step from the genitalia of the fifth 
nymphal instar to the adult form is a big one, the size difference 
being rather remarkable in the two stages; and, just while this 
growth is taking place in parasitized individuals, the Aphelopus 
larvae must exert a greater influence than they did in the pre- 
vious instar or instars when smaller in size. It may also be true 
that it is difficult to reverse the development of an organ old 
ontogenetically and well established in the group after it has 
once got under way. There also seems to be good reason to 
believe that the more recent the characteristic and the more 
specific it is the more it will be altered. The details of sexual 
differentiation often represent various specializations of phylo- 
genetically older differences. The specific characteristics of the 
gonapophyses come under this category, and thus we find that 
the oedagus and ovipositor of Thelia bimaculata are altered in 
regard to these details when parasites are present. Gliard (’89) 
showed that the elaborate oedagus of Typhlocyba hippocastani 
lost its specific characteristics when parasitized by Aphelopus, 
