66 ILLINOIS BIOLOGICAL MONOGRAPHS (308 



This postembryological study has provided a source of evidence as to 

 the evolution of habit within this family. The correlation between the 

 reduced condition of the epicranial stem and the subterranean mode of 

 life has already been discussed. We have seen that an anatomical relation 

 exists between the short epicranial stem and the cephalic direction of the 

 mandibles, this latter condition being apparently an adaptation for bur- 

 rowing in the soU. The period in phylogeny in which the shortening of this 

 suture began, as indicated by the curves in Plate I, is to be regarded, then, 

 as the one in which this biological specialization took place. The point of 

 turning upward in one of these curves represents, in other words, the origin 

 of the subterranean habit in the race history of the species in question. It 

 is apparent that this mode of life has originated independently at different 

 times in the phylogeny of different species. Entomology furnishes numer- 

 ous instances of such independent origin of the same biological specializa- 

 tion in various groups of insects. The aquatic and parasitic modes of life, 

 as well as the leaf-mining and wood-boring habits exemplify this situation, 

 the same habit having developed independently at different times in 

 different groups. 



We have demonstrated the accelerative nature of the secondary shorten- 

 ing of the epicranial stem. In the light of the established correlation be- 

 tween this structure and the subterranean habit, it becomes evident that 

 species developing tendencies to enter the soil have gradually become more 

 markedly subterranean at an increasing rate with the passing of time. 

 From this it follows that those forms having developed this habit earliest 

 in race-history must present the most pronounced subterranean mode of 

 life at present. From the data we have collected it is clear that this is pre- 

 cisely the case. The species whose curves turn upward earliest in postem- 

 bryology reveal the greatest degree of "subterraneanness," as evidenced 

 by their resistance to submergence and other biological traits. Those 

 forms which have been subterranean longest, in other words, are the most 

 subterranean now. This point supports further the corollary that cor- 

 responding postembryological stages in different species represent the same 

 phylogenetic period, inasmuch as the conclusions obtained on the basis of 

 this corollary agree with the biological data regarding the relative "sub- 

 terraneanness" of species. 



It has been noted that certain species, typified by Cirphis phragmitidi- 

 cola and Ceramica picta, appear to represent an incipient stage in the 

 development of the subterranean mode of life, entering the soil only under 

 extreme stress during the feedmg period. The curves of such forms turn 

 upward but slightly, in the last unit only. If the development of this habit 

 continues progressively in the future as it evidently has in the past, such 

 slightly subterranean species must eventually become markedly so, like 

 Agrolis ypsilon or Fellia subgothica. This suggests the interesting possibility 



