46 ILLINOIS BIOLOGICAL MONOGRAPHS [288 



searches of the histologist. Hence morphological, biological and histolog- 

 ical investigation may be expected to explain for the most part these num- 

 erous problematical changes, such as those in the shape of the body-setae 

 and of the antennae of caterpillars, in the number of facets in the eye 

 of the nymphs of dragon-flies, in the heads of muscid larvae, in the struc- 

 tures of the caudal end of the body of lipulid larvae, in the number of tarsal 

 segments in the Heteroptera, and many others. Such investigations will 

 probably reveal a number of types not listed in our present classification, 

 which is necessarily very restricted, applying only to noctuid larvae. 



The postembryology of the fixed parts of the noctuid larval head reveals 

 the following changes: 



(1) Appearance of the adfrontal sutures; (2) Change in the relative 

 length of the epicranial stem; (3) Mesal extension of the postgenae; 

 (4) Change in the shape of the labrum; (5) Reduction in the relative size 

 of the ocellariae and sensoria; (6) Change in the position of the setae; 

 (7) Change in the shape of the setae; (8) Change in the coloration. 



ADFRONTAL SUTURES 



Although the presence of the adfrontal sclerites has long been regarded 

 as a condition diagnostic for lepidopterous larvae, the fact that this area 

 appears only in the later stadia, at least in noctuid larvae, has apparently 

 not been discovered. Very frequently the coloration of the early instars 

 gives the appearance of adfrontal sclerites where no structural differentia- 

 tion exists, which has most probably been conducive to the general over- 

 looking of the true situation. In the noctuids these sutures are distinct 

 only in the larvae of the two later stadia, very faint indications being some- 

 times distinguishable in exuviae or treated heads of the third from last 

 stage. The adfrontal sclerites have been regarded as bearing a direct 

 structural relation to the infoldings along the epicranial arms. Fracker 

 speaks of them as the "external expression of the attachment of the 

 anterior arms of the tentorium." An inspection of a section thru this 

 region (Fig. 1) reveals absolutely no connection between the adfrontal 

 suture and the epicranial parademe, to which the pretentorium is attached. 

 This suture appears externally as a narrow light-colored line constant in 

 general position throughout the family but varying much in its irregular 

 curving, even within a species. In sections it is not distinguishable from 

 the general cuticle, except by its lighter pigmentation. Since the older ideas 

 of its significance are obviously incorrect, our present problem is to account 

 for its existence. 



The usual place of splitting in the head-capsule at molting and at pupa- 

 tion is along the epicranial stem and arms in all but the more specialized 

 forms such as dipterous or coccinellid larvae or coccid nymphs. We regard 

 this, therefore, as the generalized condition in insects. As was stated in 



