281] NOCTUID LARVAE— RIPLEY 39 



to 1.61. The increase from first to second stadia is usually greater, from 

 penultimate to last more often less than for other molts, this latter condi- 

 tion being explainable by the fact that the more rapid development of 

 adult structures in the later instars leaves proportionately less energy 

 available for growth. The other noticeable differences in indices of growth 

 within a species present no uniformity. In one species, for instance, the 

 index from second to third stadia is greater than from third to fourth, in 

 another the reverse may be true, or in a third species these indices may be 

 equal. The question must arise, then, as to whether the relation between 

 these different indices within a species be definite to any extent. 



An examination of the successive exuviae of isolated individuals, shows 

 us that the variations in the index of growth for any species are of no uni- 

 formity, with the exception of the tendency toward largeness of the first 

 and smallness of the last. All other variations are to be accounted for, 

 then, by environment, all indices but the first and last probably tending to 

 be equal under uniform external conditions. Any influences affecting the 

 rate of activity of the moulting mechanism differently from that of the 

 general metabolism must necessarily either increase or decrease the index 

 of growth. Thus, if growth be impeded without interfering proportionately 

 with hypodermal activity, or at least with the molting mechanism, a small 

 index will result. Wodsedalek, by starving larvae of Trogoderma (Der- 

 mestidae), obtained many exuviae, some of the last of w^hich were actually 

 smaller than the earlier ones. In like manner factors favoring growth more 

 than molting necessitate a large index. 



The effect of external factors on the index is characteristic. Starvation 

 and parasitism, of course, greatly reduce growth, but do not retard the 

 molting processes proportionately, since the number of stadia is not affected, 

 larvae continuing to molt when very little growth is undergone. Favorable 

 climatic factors, on the other hand, increase the index. The unusually 

 large increase from second to third stadia in Agrotis ypsilon is to be ex- 

 plained by the fact that the individuals upon which the given figures are 

 based were reared simultaneously under like natural conditions, which were 

 evidently optimum for growth, or nearly so, while these individuals were 

 passing the second stage. 



We have demonstrated the fact that some species increase in size from 

 first to last stages considerably more than others, the total amount of 

 growth being characteristic for the species, although variable according to 

 external factors. It may vary widely in closely related species as in 

 Lycopholia margaritosa and infecta. Although molting has been generally- 

 considered to be primarily a phenomenon necessitated by growth, somel 

 entomologists have been inclined to question this point, tending ratherl 

 to emphasize its excretory significance. It is to be noted, however, that 

 the seven-staged species grow more than the six-staged. This obvious 



