3011 KOCTVID LARVAE— RIPLEY 59 



dition in these larvae. This suture is fairly long in the root-boring larvae 

 of the three species of Hepialus examined. In the larva of the European 

 cossid, Cossus cossus, we find an unusually short epicranial stem, while 

 Zeuzera pyrina shows the opposite extreme, altho both are borers in live- 

 wood. The bag-worm, Thyridopteryx ephemeraejormis, offers an average 

 condition of the epicranial index. It is probable that none of these species 

 presents a generalized condition with respect to this structure, altho they 

 represent generalized families. Since the turning upward toward the right 

 of the chart, wherever it is found in these curves, evidently expresses a 

 recapitulation of the phylogeny of this structure, it seems reasonable to 

 conclude that the turning downward toward the left in the same curves 

 represents also a recapitulation. The lack of change in epicranial index 

 from first to second instars in the two species of Phytometra examined, as 

 well as the turning upward shown by the curves of various other species, 

 precludes all possibility of explaining this lengthening of the epicranial stem 

 in terms of the mechanics of growth. There is no mechanical force, in 

 other words, producing more rapid growth in the vertex than in the front. 

 Hence, in the absence of knowledge concerning the early phylogeny of this 

 suture, such data as we have indicate that the ancestral noctuid larva 

 possessed a short epicranial stem, altho this condition is found in existing 

 forms only as a secondary development associated with the subterranean 

 mode of life. 



We will consider now the interpretation of the fact that the change in 

 direction in those curves which turn upward takes place in different stadia 

 in different species. This interpretation involves, in the first place, an 

 analysis of the postembryological relation which corresponding stadia in 

 different species bear to one another. Do corresponding instars in species 

 having the same number of stadia necessarily represent identical post- 

 embryological stages? Various mammals at the time of their birth present 

 somewhat different developmental stages. The kangaroo, for instance, 

 brings forth its young in a very immature condition, corresponding to 

 that found in the late embryonic life of the majority of mammals. It seems 

 not unlikely that insects may offer a parallel situation, the early postem- 

 bryonic life of some corresponding, perhaps, to the latter embryonic life of 

 others. Within a group as closely related as the noctuids it seems very 

 improbable that such a condition should exist to any appreciable extent, 

 altho we cannot be sure that all noctuid larvae are equally mature at 

 hatching. However, this may be, it is certain that the passing of cor- 

 responding stadia requires quite different proportional lengths of time in 

 different lepidopterous larvae, even within the same family, suggesting the 

 possibility that the postembryological value of such stadia may differ 

 according to the species. If we find, for example, the first stadium of one 

 species requiring one-third of the total larval life and that of another species 



