2851 KOCTUID LARVAE— RIPLEY 43 



seven stadia as a specialized condition, merely on account of its exceptional 

 occurrence. 



Since the two species of Phytomelra, brassicae and biloba, which have 

 been reared through all larval stadia by the author, present but five 

 stadia and contexta, according to Thaxter, passes six, it would seem that 

 this biological character is not a fundamental one. The persistence 

 of the generalized condition of molting but four times in this structurally 

 specialized group is parallelled by the situation found in Hepialus, whose 

 moth is very generalized structurally, but whose larva has developed the 

 specialized habit of root-boring. 



In Caennrgia erechtea the number of stadia presents an interesting 

 secondary sexual character, the larva undergoing ecdysis but four times in 

 the male and four or five in the female. The males of this species offer the 

 only instance known to us outside of the Phytometrinae where a noctuid 

 larva molts but four times. Parallel instances have been found by C. V. 

 Riley in Hemcrocampa leucosligma and by Payne in other liparid larvae, 

 in which the male passes five stadia and the female either five or six. This 

 phenomenon is most probably to be explained by the fact that the female 

 larvae generally attain a larger size than the male. The fact that the 

 female varies in the number of molts indicates further that the larger 

 number of stadia represents the more specialized condition. 



The species passing seven stadia, Agrotis ypsilon, Lycopholia margaritosa 

 and Nephelodes emmedonia, do not constitute a phylogenetic unit, but have 

 developed an extra molt independently, since each is more closely related 

 to different six-staged groups than they are to each other. Lycopholia 

 infecta undergoes ecdj'sis but five times. Specific differences in this respect 

 in the genus Phytometra have already been mentioned. 



POSTEIIBRYONIC CHANGES 



A study of the postembrj-ology of noctuid larvae, as well as a consider- 

 ation of the ontogeny of animals in general, convinces us that the structural 

 changes exhibited in ontogeny are not all an expression of the same biolog- 

 ical factor, but are of a number of distinctly different kinds. The structural 

 changes appearing in the postenbryonic development of caterpillars may 

 be conveniently classified as follows:- (1) Recapitulative; (2) Non-recapit- 

 ulative; (a) Adaptive to unequal function; (b) Necessitated by the me- 

 chanics of growth; (3) Compound; (a) Recapitulative-adaptive; (b) Recapit- 

 ulative-mechanical; (c) Adaptive-mechanical. 



The Law of Recapitulation is of quite general but by no means of uni- 

 versal application, ontogenetic sequences which do not conform to the law 

 being many and well known. The fact that a mammal at birth has a head 

 large relative to the size of its body does not lead us to regard the ancestor 



