20 ILU.\OIS BIOLOGICAL MONOGRAPHS [262 



the spinneret has arisen from the hypopharynx, which, as will be shown 

 later, is otherwise represented in lepidopterous larvae. 



The position of the spinneret is that normally occupied by the glossae 

 and paraglossae. It may represent the fusion of either or both of these 

 lobes, altho its mesal position indicates that it is derived only from the 

 glossae. Dampf's homologies, where he regards the paraglossae as repre- 

 sented by the proximal semicircle and the alaglossa by the spinning organ 

 are, therefore, open to no serious objection by those who regard the 

 spinneret as a primary structure. 



Certain biological considerations lend weight to the view that the 

 spinneret has developed secondarily, being without homologue in the 

 typical insectean labium. Since the spinning habit appears in insects 

 only in the larvae of Entopteraria, except in the Embiidae, where the 

 glands open on the legs, it is evidently a secondary acquisition, which was 

 not present in ancestral insect. The widespread occurrence of silk-spinning, 

 however, in the larvae of Lepidoptera, Trichoptera, Hymenoptera, and 

 Diptera seems to justify the conclusion that the common ancestral larva 

 of these orders spun silk, although this habit has been lost in certain groups 

 of each order, as Wheeler has shown it to be in certain families of ants. 

 This acquisition has apparently developed furthest in the Lepidoptera, 

 although it is possible that it was at one time equally extensive in the 

 other orders mentioned, having been subsequently reduced. So far as 

 known the spinneret is well developed only in caterpillars, the opening of 

 the silkduct in silk-spinning hymenopterous, dipterous, and trichopterous 

 larvae being without any well developed spinning organ and usually 

 represented by a small aperture located near the distal end of the labium 

 and surrounded by a chitinized ring. The glossae or paraglossae are 

 rarely, if ever, well developed in these larvae. The condition of these 

 structures leads us to suppose that they were probably vestigial in the 

 ancestral larva of these orders, from which we may reasonably conclude 

 that they are not well developed in caterpillars. The spinneret, therefore, 

 is apparently a secondary development which evolved in correlation with 

 the extensive spinning of silk. The proximal semicircular sclerite in lepi- 

 dopterous larvae appears to correspond to the chitinized ring around 

 the aperture in other orders and was apparently derived from the vestigial 

 glossae. Although these conclusions are by no means certain, they seem 

 to be the most reasonable on the basis of the evidence available. 



The variations presented by the labium and their taxonomic value in 

 the Noctuidae will now be considered. This appendage offers more exten- 

 sive variation in caterpillars than any other structure, both in the form 

 of its sclerites and of its distal lobes, especially of the spinneret, which 

 exhibits the most diverse conditions. As Forbes has shown, the sclerites 

 of the submentum present considerable differences in the extent of their 



