588 SIDNEY I. KORNHAUSER 
instar the parasites exert an influence upon the host sufficient to 
change the appendages at the succeeding molt, an attempt was 
made to see if this could be carried back to still earlier stages. 
Accordingly, a number of male and female nymphs of fourth 
instar each containing parasites of maximum size for that stage 
(0.75 mm.) were prepared and mounted similar to the normal 
individuals shown in figures 41 and 45. Careful camera-lucida 
drawings were then made of twenty individuals, ten parasitized 
and ten normal. The width and length of the nymphal append- 
ages were measured on the drawings, and in neither sex could 
any alteration from the normal in the size or form of the append- 
ages or in the pigmentation of the integument be found. From 
this we may infer that in the third instar the parasites are still 
too minute to exert any influence upon the cells producing the 
integument and appendages of the fourth instar. Since the 
genital appendages are laid down even before the third instar, 
we can see that the parasites could in no way interfere with the 
formation of the anlage of the gonapophyses. Only during the 
fourth and fifth instars do the parasites affect the genitalia, and 
this effect appears in the retardation of development and reduc- 
tion in size found in the fifth nymphal instar and the great 
reduction in size in the adult. Even minute parasites, still 
spherical embryos just becoming separate individuals, cause the 
adult genitalia to be reduced. ‘This is probably due to the fact 
that the greatest step in the formation of the gonapophyses 
comes in the development of the adult from the fifth nymphal 
instar. That the genitalia are reduced in size but retain their 
general form in parasitized individuals is probably to be ascribed 
to their history. 
It is generally conceded that organs formed early in ontogeny 
are phylogenetically older than those appearing late in develop- 
ment. The extragenital secondary sexual characteristics which 
arise during the fifth instar certainly belong to the species. The 
genital appendages are relatively older and appear early in 
ontogeny. In the later stages of development various specific 
modifications may slightly alter the gonapophyses. Systema- 
tists have shown that the Membracidae possess a relatively 
