STUDIES OF VARIATION IN INSECTS 209 



of the head, and the legs and wings, are produced not by a 

 gradual development, growth and transformation of the corre- 

 sponding larval parts, but by a special development in late 

 larval life and during the pupal stage (the beginnings may ap- 

 pear in early larval or even embryonic life) from small groups 

 of previously undifferentiated subembryonic cells, derived in 

 the case of the external parts just named chiefly from invagi- 

 nations of the larval cellular skin-layer. In the larva (maggot) 

 of a house-fly, for example, there are no functional legs or 

 wangs ; there are no external signs (buds, pads) of these organs 

 at any time in the larval stage. In all the larval life there can 

 be no possible moulding influence on these future adult organs 

 of the nature of any direct response or reaction to the immediate 

 environment which we might assume possible if the wings and 

 legs were slowly transforming external structures subject to 

 attempts at or actual functional use in flight or crawling during 

 the larval life. At pupation the wings and legs suddenly ap- 

 pear as external parts, but still equally functionless, and now 

 wholly concealed and protected by the opaque chitinized wall 

 of the puparium. With the final issuance of the adult the wings 

 and legs appear for the first time in functional condition, and 

 with the simple need of unfolding, expanding and drying the 

 outer wall, an operation requiring but few moments, they appear 

 at this time in their definitive fully developed condition. The 

 wings have the arrangement of veins and number of spines and 

 fringing hairs, the legs have the armature of spines and spurs 

 and number of segments which they retain unchanged through 

 the short or longer adult life. The imaginal wings and legs of 

 all insects with complete metamorphosis — and the insects of 

 this category include the beetles (Coleoptera), two-winged flies 

 (Diptera), moths and butterflies (Lepidoptera), ants, bees, wasps, 

 gall flies and ichneumons (Hymenoptera) and some other orders 

 — are exposed during their development to just one set of ex- 

 trinsic influences, namely, those of nutrition, temperature, humid- 

 ity, etc. These influences affect the whole body and metabolism 

 of the developing insect, but have no specific relation to specific 

 parts, as exemplified by the relation of opportunity for use or 

 disuse on the part of locomotory or sensory organs (wings, legs. 



