24 ILL! KOI S BIOLOGICAL MONOGRAPHS (266 



prairie spun fairly dense cocoons, while a mere network of threads covered 

 the pupae of the individuals collected in the forest. All four pupated on the 

 surface of the ground among grass on the same day under approximately 

 the same external conditions. Since both sexes were represented by those 

 of the prairie, the difference in the amount of silk spun was not a sexual one. 

 A question of considerable interest arises as to whether this striking 

 biological variation is to be explained by the direct effect of different food 

 upon the activity of the silk-glands or upon the basis of physiological 

 adjustment to environment, the pupae of the prairie requiring more protec- 

 tion from the sun than those of the forest. Instances of much less pro- 

 nounced individual variation in this respect have been frequently noted 

 with various species reared. 



Contrary to what would perhaps be the natural supposition, the sub- 

 terranean mode of life frequently exhibited by noctuid larvae, exemplified 

 by the cut-worms, appears to bear no definite relation to the habit of 

 pupation beneath the soil. Larvae which never enter the earth during the 

 feeding period often pupate in earthen cells, while some species showing 

 pronounced subterranean tendencies as larvae always spin slight cocoons 

 among the debris on the surface of the ground. Similarly the Sphingidae, 

 which usually undergo pupation in the earth, are never subterranean as 

 larvae so far as known. The habit of pupation in the soil is a fundamental 

 one which remains constant throughout large groups, whereas the degree 

 of development of the subterranean mode of life in larvae is variable in 

 closely related species. 



Caterpillars which spin much silk are generally provided with a long, 

 slender, tubular, tapering spinneret. The short depressed type is apparent- 

 ly found only in those groups whose larvae spin little or no pupal covering. 

 The Sphingidae and Noctuidae which pupate in earthen cells and certain 

 leaf-miners which undergo this process in their mines without spinning silk 

 present this reduced type of spinneret. It has not been found to occur 

 where the spinning habit is well developed. The general accompaniment 

 in the Noctuidae of the short flat spinning organ by the marked reduction 

 or entire loss of silk-spinning is unquestionable. Some species with the long 

 type of spinneret, however, pupate in the soil, as instanced by Chloridea 

 artnigera. This condition is to be expected in those species whose last 

 instars spin silk during the feeding period. Sidemia devastalrix has been 

 observed by the author to spin a cocoon in which to undergo ecdysis, a 

 peculiar habit, which, so far as known, has not been recorded for any other 

 caterpillar, except for certain leaf-miners studied by Tragardh. 



The peculiar fringe borne on the distal end of the spinneret has been 

 found only in noctuid and sphingid larvae with subterranean pupae. 

 Although the function of this strange modification has not been definitely 

 determined it seems probable that it is used as a brush to distribute a 



