72 ILLINOIS BIOLOGICAL MONOGRAPHS (314 



postcmbryology of Ihe spinneret of Agrotis ypsilon, which typifies this type 

 of development. The proximal sclerile dcscreases in relative width as in 

 the other types. The fold in this species increases as it does in Polia reni- 

 gera but to a much lesser extent. A secondary chitinizalion appears on the 

 fold continuous with the primary scleritc but of a lighter color, again recall- 

 ing the somewhat similar condition in renigera. The fringe develops much 

 as in Lycophotia viargaritosa, its first indication appearing as slight rounded 

 projections on the upper lip of the second instar (Fig. 42). Unlike viar- 

 garitosa, however, the lateral emarginations are well developed in this 

 stadium and the distal projections appear only on the upper lip. In the 

 following instars the lower lip becomes distinctly bilobed and a small 

 fringe, which presents considerable individual variation, develops on the 

 upper one from the projections which appear first in the second instar 

 (Figs. 43-46). 



The essential basis for the recognition of these four types is the differ- 

 ence in the relative length of the spinneret in difierent stadia. The other 

 changes described will be considered later. In Type I the spinneret is 

 longer in the first and last instars than in the others; in II it is long in the 

 first stadium, becoming short in the course of development; the condition in 

 III is exactly the opposite, the first instar havin? a short spinneret which 

 develops into a long one; in IV it is short thruout all stadia. Each of these 

 types of postembryonic development of this structure is correlated with a 

 different distribution of the spinning habit with reference to the instars. 

 The species falling under Type I, long in first and last stadia, spin threads 

 in the first instar and a well developed cocoon in the last. In Type II, 

 long to short, the first instar only spins silk, the cocoon-spinning habit 

 having been entirely lost in correlation with subterranean pupation. Type 

 IV, short thruout, has lost the spinning power in all stadia. 



These changes in the relative length of the spinneret during postem- 

 bryonic development are obviously to be explained by the unequal function 

 of this structure in difierent stadia rather than by recapitulation. Inas- 

 much as the ancestral noctuid larva had a long, slender spinneret, as has 

 been shown on the basis of morphological evidence, the expression of the 

 recapitulative force would result in a relative shortening of this organ from 

 first to last instars in those forms where the spinneret of the last stadium 

 has been reduced. Whereas this condition is found in Type II, where the 

 first instar spins silk and the last one does not, it fails to occur in IV, where 

 the spinneret is short in all stadia, the spinning habit being absent thruout. 

 Similarly species which have preserved the long ancestral spinneret in the 

 last stadium would exhibit this condition in all instars, if recapitulation 

 were the only factor operating, whereas marked inequality in the relative 

 length of this organ in the difierent stadia is found in both Types I and III, 

 where the spinneret is long in the last instar. This situation exemplifies 



