321] NOCTUID LARYAE—RIPLEY 79 



During the growth of noctuid larvae the number of crochets increases 

 markedly on all of the larvapods. This process in Agrotis ypsilon may 

 serve as a typical example. The formula for the first instar is -, 3, 5, 7, 8, 

 the first pair of larvapods being absent, for the second it is -, 5, 7, 7, 8, for 

 the third 3, 7, 9, 10, 14, for the fourth 9, 13, 13, 13, 16, for the fifth 12, 13, 



15, 16, 19, for the sixth 16, 18, 19, 19, 20, and for the fully grown larva it is 



16, 20, 20, 21, 25. Like the ocellarae and the sensoria the crochets are 

 relatively much larger in the earlier than in the later stadia. Consequently 

 there is sufficient space on the larvapods of the first instar to accommodate 

 but relatively few crochets. A survey of the number found in the larvae 

 of the more generalized famihes in the order, as well as in caterpillars gener- 

 ally, offers absolutely no evidence favoring the view that this change is 

 recapitulative. The increase in the number of crochets is apparently to be 

 accounted for by the mechanics of growth. 



Generally throughout the order the larvapods whose distal ends have 

 the greater diameter bear the larger number of crochets. This relation is 

 also clearly revealed by the condition found in an individual larva whose 

 different pairs of larvapods differ in size. Since, in such an individual, the 

 crochets are of approximately the same size on both large and reduced 

 larvapods, there is naturally a larger number of them on the former. Hence 

 with the increase in the relative size of the first two pairs, previously 

 discussed, the number of crochets increases proportionally. But this 

 change in the relative size of the larvapods is due to unequal function, as 

 already determined. Therefore, the increase in the number of the crochets 

 on the first two pairs is unquestionably the expression of two factors, the 

 mechanics of growth and unequal function. This change is, therefore, to 

 be classified as mechanical-adaptive. 



The analogous process on the other pairs is, on the other hand, not 

 influenced by the adaptive factor, since there is no appreciable change in 

 the relative size of these larvapods during larval growth. Hence the 

 mechanics of growth alone is responsible for this change. The fact that 

 the increase in the number of the crochets of the first two pairs is con- 

 siderably greater than that of the others is consistent with the compound 

 nature of the former postembryonic change as opposed to the simple one 

 of the latter. 



