40 ILLINOIS BIOLOGICAL MONOGRAPUS [282 



correlation between the total growth index, obtained by dividing the width 

 of the head of the last instar by that of the first, and the number of molts 

 lends weight to the former more general view. In Caenurgia, moreover, 

 where there is a sexual difference in the number of stadia the females, 

 ■which often molt once more than the males, average larger in size. The 

 number of individuals of this species used in Table II is too small to justify 

 our drawing conclusions from the fact that the growth index of the males 

 exceeds that of the females. The fact that Cirphis unipuncta, which pre- 

 sents the greatest total growth index of the six-staged species, has been 

 known to pass seven stadia in one instance, is of especial interest in this 

 connection. 



It follows mechanically that species with a large total increase, in other 

 •words, those with a first instar whose head is proportionately small for that 

 of the last instar, produce eggs relatively small for their adults, the smaU 

 first instar being correlated with a small egg, and the large last instar pro- 

 ducing naturally a large moth. The egg measurements in Table I have 

 been made from alcoholic specimens and are, therefore, somewhat larger 

 than certain corresponding published measurements based on fresh eggs. 

 The figures given represent the diameter of the largest circumference, the 

 periphery of the typical noctuid egg being circular. In those species whose 

 eggs have one diameter slightly greater than the one at right angles to it, 

 an average has been given. Altho the correlation between egg-diameter 

 and the width of the head of the first instar is clearly demonstrated in 

 Table I, the ratio between these two measurements varies considerably 

 according to the species. Ceramica pida presents an extreme condition 

 where the egg is small relative to the larval head, the ratio being 1.65: In 

 the catocalas, on the other hand, we find the width of the egg proportionate- 

 ly large for that of the larval head, the ratio reaching 2.62 in C. illia. This 

 condition may possibly bear a direct relation to the habit of hibernation in 

 the egg, which is general in this genus. 



The shape, as well as the general internal structure of the abdomen of 

 all noctuid moths is very nearly uniform, approximately the same propor- 

 tional amount of space being used for egg-carrying in all species. It follows, 

 therefore, that a moth producing eggs proportionately small for its size 

 must bear a larger number than one whose eggs are large relative to the 

 size of the adult. We should remember when considering this point that 

 the moths of this family have but a short period for oviposition, usually 

 laying all their eggs in a few successive nights, which permits of no egg- 

 development during the life of the adult, such as occurs in the queen bee. 



The data for the fecundity is based upon the number of fully developed 

 eggs in the abdomens of reared moths and represents potential fecundity. 

 Since the number of eggs actually laid in breeding cages is determined by 

 external stimuli, all eggs in the abdomen being oviposited only under opti- 



