283] NOCTUID LARVAE— RIPLEY 41 



mum conditions, which for many species are difficult to obtain artificially, 

 the published records of the number of eggs laid by various species are un- 

 reliable as indices to the potential fecundity. The undeveloped eggs, which 

 are never laid, occupy a quite uniformly small space in the abdomen. Ac- 

 curate data on fecundity can be obtained only with difficulty, since counts 

 must be made of the eggs contained in the abdomens of moths emerging in 

 captivity only. Moreover, a large number of individuals should be exam- 

 ined, because of the great individual variation m this respect. In spite of 

 the regrettable insufficiency of data, the column headed potential fecundity 

 presents significant indications. 



A consideration of the mechanical relations already discussed enables 

 us to understand the significance of the correlation between high fecundity 

 and large total growth. Although a general relation between these con- 

 ditions is clearly indicated, we note that certain irregularities occur. Feltia 

 subgothica and Ceramic a pi da lay an exceptionally large number of eggs 

 relative to the amount of their larval growth. An examination of the ratios 

 between the diameter of the egg and the width of the head of the first in- 

 star in these two species reveals the fact that both bear eggs proportionate- 

 ly small for the size of their first instars. This condition enables the moths 

 to lay a large number of eggs relative to the amount of larval growth for 

 the species, accounting for the irregularity in the correlation. The relation 

 between fecundity and growth is not direct, since the former increases 

 more rapidly than the latter, as is evident when we read from top to bottom 

 in these columns, a relatively slight increase in growth corresponding to a 

 large increase in fecundity. It is highly probable that the factors determin- 

 ing fecundity are many. Of these factors the amount of growth is an im- 

 portant one in this family. 



It should be noted that Cirphis unipunda, with the largest total growth 

 index of the six-staged species, having seven stadia rarely, attains the 

 highest fecundity of those with six stadia. This destructive species has 

 three broods in Illinois. The larvae developing in June grow markedly larg- 

 er than those of the following brood, which pass the larval period in mid- 

 summer, the individuals of the fall-brood being nearly as large as those of 

 the spring-generation. This relation probably holds only for the latitude 

 and climate of Illinois in a usual season. Since the eggs of all generations 

 are of the same size, the total growth is different for each brood under 

 normal weather conditions. Altho our data as to the fecundity of the 

 moths of different broods is inadequate, it seems quite evident that those 

 developing from the large September larvae must have relatively high 

 fecundity. This is suggested as a factor contributing to the fact that the 

 spring-larvae, offspring of moths from the September larvae, almost al- 

 ways constitute the brood which attains such great numbers in Illinois, 

 accompanied by the well known army-worm devastation. Altho the larvae 



