305] XOCTUID LARVAE— RIPLEY 63 



but one or two instances among the noctuid larvae examined has the 

 relative length of this suture remained unchanged after it has ceased to 

 lengthen and never has it grown subsequently longer after once beginning 

 to shorten, but it has continued to become progressively more reduced 

 with the passing of time. The species which began to exhibit this reduction 

 earliest in their race-history generally present the shortest stem in their 

 last instars. This does not necessarily hold true in all cases, however, since 

 some species had a much longer epicranial stem than others at the time 

 when this suture commenced to decrease in length, so that the greatest 

 reduction in the last instar is not always correlated with the earhest appear- 

 ance of this shortening. A comparison of the curves of PoHa renigera and 

 meditala will serve to illustrate this point. Moreover, there is a marked 

 diversity in the angle at which the curves of different species turn upward, 

 so that a form which has been developing in this direction during the last 

 period only may have a shorter stem in its last instar than one in which this 

 suture has been decreasing for a much longer period. To use a convenient 

 analogy, some have run faster than others, some have had farther to go, 

 and some began to run much earlier than others, the latter having won the 

 race in the majority of cases. 



Certain species, such as Cirphis phragmitidicola, apparently represent 

 an incipient stage in this process of reduction, which, if it continues to 

 operate progressively in the future, as it has in the past with other species, 

 must result eventually in reducing the epicranial stem of this species to a 

 fraction of its present length, a condition typified at present by the last 

 instars of A gratis ypsilon and Feltia subgoth'.ca. 



To summarize the conclusion which we have thus far reached regarding 

 the shortening of the epicranial stem in the postembryology of noctuid 

 larvae: This process is a recapitulative one. It represents a secondary 

 development occurring only in certain species with subterranean pro- 

 clivities. It is of independent origin in different species, having begun at 

 widely different times in race history. It is a progressive process, species 

 in which it has begun continually undergoing greater reduction in the 

 length of this suture. The intensity of this process has varied in different 

 species, that is, it has gone on more rapidly in some species than in others. 

 In the following discussion, it will be shown that the rate of reduction 

 in the length of the epicranial stem has been subject to an acceleration. 

 The significance of the slopes and angles of both types of curves will now be 

 considered. Certain of them turn upward at a smaller angle than others in 

 the same period, indicating unequal rates of reduction in the stems of such 

 species, as has been stated previously. A parallel situation may be seen in 

 the left-hand portion of the curves, where some turn downward much more 

 abruptly than others, showing that this primary lengthenmg process has 

 also developed at very different rates accordmg to the species. It should be 



