289) NOCTUID LARVAE— RIPLEY 47 



our consideration of the morphology of the tentorium, the great reduction 

 of this originally supporting structure has been accompanied by the 

 development of a number of deep infoldings, one of which occurs along the 

 epicranial suture. As might be expected, the marked specialization in this 

 region is accompanied by a specialized condition in molting, the entire 

 head-capsule being shed intact. So far as we know the larvae of no other 

 order molt without breaking the exuvia of the head, altho some nymphs do 

 so. It seems probable that the deep infolding along the epicranial suture 

 has rendered the usual splitting impossible. The great change in form 

 undergone at pupation, however, makes a break in the last head-capsule 

 mechanically necessary. This occurs along the epicranial stem and 

 adfrontal sutures. So far as we have been able to determine they have no 

 other function. These structures are to be regarded, then, as a modification 

 for pupation due indirectly to the greatly reduced condition of the ten- 

 torium and to the deep parademe along the epicranial suture, which has 

 taken over the supporting function of the tentorium. 



The well developed condition of these sclerites in the next to the last 

 instar, where they do not function, is paralleled by the general occurrence 

 of adaptive structures in stages earlier than the one in which they are used. 

 Altho the adfrontal sutures appear in larval development as they pre- 

 sumably did in phylogeny, beginning as a very faint line which becomes 

 prominent later, the fact that they function only in the last instar indicates 

 that the factor of unequal function also plays an important part in their 

 development. If recapitulation alone were operating on this postembry- 

 onic change, we should, moreover, expect these sutures to appear in the 

 first instar, as shown by the following facts. Their universal occurrence 

 throughout the order indicates very strongly that they were present in the 

 ancestral lepidopterous larva. Since the first instar of the noctuid larva is 

 typically noctuid, it presumably represents with reference to recapitulation 

 a period in phylogeny later than the one in which the Noctuidae appeared, 

 certainly much later than the period in which the adfrontal sclerites 

 originated. Hence on the basis of recapitulation alone the first instar would 

 exhibit well developed adfrontal sutures. Their failure to appear until late 

 in larval development is evidently due to the fact that they function only 

 in the fully grown larva. This postembryonic change is evidently the 

 expression of the two factors recapitulation and unequal function and is to 

 be classified as recapitulative-adaptive. 



EPICRANIAL STEM 



As has already been shown in our consideration of the morphology of the 

 head, the relative length of the epicranial stem varies widely m the larvae 

 of this family, (Figs. 2, 15, 16, 17) furnishing a character second only to 

 the number of larvapods in conspicuousness. The proportional length of 



