58 ILLINOIS BIOLOGICAL MONOGRAPHS 1300 



undetermined cutworm may inform the field-man whether or not it could 

 consistently attack any well cultivated crop. 



The changes in the epicranial index, length of front divided by length 

 of epicranial stem, undergone in the postembryology of various species are 

 presented in the tables. The percentage of variation has been computed in 

 the same way as in Table I. The measurements were made with an ocular 

 micrometer. It wiU be noted that the greatest individual variation occurs 

 in the last instars of the most subterranean species, which present the most 

 specialized condition of the epicranial stem. The postembryonic develop- 

 ment of the epicranial index has been graphically expressed in Plate I. 

 The horizontal axis has been divided into six equal parts representing 

 stadia, this being the usual number within the family. With those species 

 presenting five or seven stadia, the units on the horizontal axis have been 

 respectively lengthened or shortened so that the total length of this a.xis 

 remains the same for all curves. By this means curves of species having a 

 diflferent number of instars can be more easily compared. The interpreta- 

 tion of this chart presents some very significant points, which we shall con- 

 sider singly. These curves may be conveniently divided into two types. 

 The curve of the first type turns upward toward the right and shows a 

 marked shortening of the epicranial stem in the later stadia, while that of 

 the second type continues downward and reveals a continuous lengthening 

 of this suture. The significance of this turning upward, presented by the 

 first type, will be considered at this point. 



It has been well established in our discussion of the phylogeny of this 

 structure that in this family the short epicranial stem has descended from 

 the longer more primitive one. Since the curves of those species whose last 

 instars present a reduced condition of this suture reveal the presence of a 

 longer one in one or more of the preceding stadia, we must conclude that 

 the postembryology of this structure recapitulates its phylogeny. In our 

 classification of the kinds of postembryonic changes, those involving the 

 relative length of the epicranial stem fall, therefore, under the recapitulative 

 type. 



The curves of all species of Noctuidae examined reveal a lengthening 

 in the stem from earlier to later stadia or to the stage in which the turning 

 upward takes place. In the three species examined representing the 

 families Notodontidae, Liparidae, and Psychidae, we find this same condi- 

 tion, altho not very marked in the first of these, indicating apparently that 

 this suture was short in the ancestral larva of these families, and possibly in 

 all the Lepidoptera. We have, however, no phylogenetic evidence in 

 support of this indication, since the larvae of the most generalized families 

 usually have specialized feeding habits, rendering it unsafe to regard a 

 structure whose condition is correlated with the feeding habit, as we have 

 shown that of the epicranial stem to be, as representing a generalized con- 



