297] NOCTUID LARVAE— RIPLEY 55 



phological or physiological, which permit of such striking differences in this 

 respect, is problematical. We find no external structures which throw light 

 on this question. The spiracles offer no variations which seem to bear on 

 this point. Internal structures or histology, a study of which the scope 

 of this work does not permit, may be found to bea^ relation to the develop- 

 ment of the power to withstand submergence. It seems probable that 

 differences in the efficiency of the mechanism for closing the tracheae just 

 entad of the spiracles may be found. 



There are indications that death from drowning in these larvae is 

 caused by two factors, oxygen starvation and mechanical injury due to the 

 filling of the alimentary canal with water. The drowned larvae have 

 always exhibited a black girdle around the body, varying in extent from 

 one segment to five or six, so that in the latter case it extends for half the 

 length of the larva. Those which almost recover from submergence show 

 but a slight ring around the metathorax or first one or two adbominal 

 segments, while individuals which die before their removal from the water 

 often turn black from the head to about the sixth abdominal segment. 

 Larvae killed by pinching have exactly the same appearance. This appears 

 to indicate mechanical injury caused by distending the alimentary canal 

 with water. The expulsion of water from both mouth and anus during 

 recovery has already been mentioned. It is a significant fact that larvae in 

 the prepupal condition and those undergoing ecdysis are much more resis- 

 tant than others. Of these the prepupae swallow water and the moulting 

 larvae do not, owing to the fact that the mouth-parts cannot function 

 during ecdysis; yet the former show at least as great a resistance as the 

 latter. Larvae passing through these two stages are physiologically 

 similar in the following respects: they are quiescent; they are not digesting 

 food, having expelled the contents of the alimentary canal; and they are 

 preparing to shed their cuticle. We have reason to suppose that the oxygen , 

 requirement for both prepupae and molting is relatively low, due to the 

 reduction of motion and to the lack of digestion of food. In the light of this 

 probability the great resistance to submergence of larvae in both of these 

 stages becomes understood. Death by drowning seems to be effected, then, 

 both by lack of oxygen and by mechanical injury due to gorging the digestive 

 tube with water. How subterranean larvae are equipped to withstand 

 either or both of these factors, we do not know. 



The resistance to submergence in different species, as determined 

 experimentally, varies according to the extent to which the larvae are 

 subjected to submergence or to drenching in their natural habitats. This 

 resistance is not only correlated with the proximity of the habitat to the 

 ground during the active life of the larva, but also with the stage in which 

 the hibernation is passed, since larvae passing the winter in the soil must 

 withstand considerable submergence without regard to their habitat while 



