279] NOCTUID LARVAE— RIPLEY 37 



POSTEMBRYOLOGY 



Ecdysis is undergone by noctuid larvae four, five, or six times, depend- 

 ing chiefly on the species, but somewhat on other factors. They present, 

 then, five, six, or seven stadia. After each molt the postembryonic changes 

 which have taken place during the previous stadium are suddenly revealed. 

 These changes may be highly conspicuous, but are more often so very slight 

 that careful observation or accurate measurement is necessary to detect 

 them. Before considering the structural changes undergone in larval 

 postembryology, we shall discuss at this point the significance of the 

 number of stadia, and of the amount of increase in size from one stage to 

 the next. 



NUMBER OF MOLTS 



Although not absolutely fixed, the number of molts characteristic for 

 species of lepidopterous larvae is not subject to the considerable variation 

 found in some other orders. Wodsedalek, for example, greatly increased 

 the number of stadia in the larvae of Trogoderma (Dermestidae) by star- 

 vation. It is certain that environmental factors may at times cause one 

 molt more or less in certain lepidopterous larvae. Payne found that those 

 of Ccratnica pida pass but five stadia in both generations in Nova Scotia, 

 whereas those reared in Illinois by the author have uniformly undergone 

 one molt more than these northern individuals. Hibernating butterfly 

 larvae have been known to molt once more than those of the summer broods. 

 This phenomenon has not been found in the noctuid larvae, A gratis c-nigrum 

 and Polia renigera, which have been reared through both winter and sum- 

 mer broods. Weniger reduced the number of stadia in Eacles imperialis 

 and Anlheroca mylitta from the normal six to five, by rearing them at about 

 25 degrees C. coupled with high humidity. By rearing the cutworm, 

 AgroHs ypsilon, at 21 degrees C, 100% humidity, and at 28 degrees C, 

 100% humidity, in ventilated jars, the author has similarly decreased the 

 normal number of molts by one. It is a curious fact that the cutworm, 

 Polia renigera, adds one stadium to its usual number when reared under 

 these same conditions, being affected in an opposite manner by the same 

 stimulus. 



Sexual differences in the number of stadia were first recorded by C. V. 

 Riley in Hemerocampa leucostigma, the males always molting four times, 

 the females either four or five. Payne has recorded the same phenomenon in 



