176 THE ENTOMOLOGIST. 



ON THE VARIATION OF INSECTS. 



By T. D. a. Cockerell. 



(Continued from p. 149.) 



Class III. — Varieties of Size. 



CoUas edusa major, expanse 57 mill. ; C. edusa minor, exp. 

 32 mill. (Entom. xi. 51). Samia cecropia, wing-expanse varies 

 from 4^ to 7j inches (Worthington, Can. Entom. 1876, 160). 

 Lyccena corydon minor, confined to one spot near Lewes 

 (A.. W. Gush, Entom. xx. 265). Lyccena icarus minor, exp. less 

 than 20 mill. (Sabine, Entom. xx. 287). Spilochalcis maria, 

 Riley, female, varies from 4 to 8 mill, in length, and the male 

 from Sg- to 6 mill., while Chalcis ovata, Sa)^ varies from 6 to 

 3 mill, long (L. 0. Howard, Bull 5, U. S. Dept. of Agr. Bureau 

 of Entom. 1885). Euchloe cardamines minor, " about half normal 

 size in both sexes" (Ent. Mo. Mag. 1888, 81).— We find that 

 when insects vary in size it is more commonly the case that tliey 

 become smaller than larger, — that is to say, the influences which 

 act upon the size of an insect are more powerful or more frequent 

 in the direction of dwarfing than in that of adding to the normal 

 growth. The reason of this can be understood if we consider 

 what are the factors in this matter ; they are probably three, 

 nutrition (quality or amount of food), length of the period of 

 development, and poiver of assimilation. 



Guyot Daubes, in an admirable paper on " Variations in 

 Human Stature " (translated in ' Popular Science Monthly,' July, 

 1887), has laid down a general law, which seems also largely 

 applicable to insects. He argues that nutrition is the main thing 

 affecting human stature. In high latitudes the power of assimi- 

 lation is great, but the food- supply scanty, and this gives rise to 

 a dwarfed race. On the other hand, in hot climates the food- 

 supply may be abundant, but the power of assimilation small, 

 and this again dwarfs stature. But in temperate climates there 

 is a good supply of food, and the conditions are favourable for 

 assimilation ; and here we get the maximum of stature. It is to 

 be remembered, though, that recent investigations have shown 

 . that tall and dwarf races dwell side by side in equatorial Africa, 

 and I do not know whether the reason of this has been satisfac- 

 torily explained. 



Now, generally speaking, the power of assimilation, at least 

 of herbivorous insects, is taxed to the utmost: they have all they 

 can eat, and they eat all they can digest ; but on the other hand, 

 if the food-supply falls short or dries up, they possess great 

 powers of vitality, and manage to reach the imago stage in the 

 form of dwarfs. This, I think, will explain why we see more 

 dwarfs than giants among Lepidoptera. Il will sometimes happen 



