274 THE CAUSES WHICH ' 



a form of life in the earth^s strata^ among insects^ is ascribable 

 to clivers causes; but these in seeming harmony with the law 

 that a species should vary on the confines of possible existence, 

 and that variation is dependent on change of circumstances. As 

 regards evidence of this, these laws of insect modification, as 

 Mr. Bates observes, are nowhere so legible as in the framework, 

 shape, and colour of insects^ wings, especially in those of the 

 butterflies and moths, where these dermal tracheal extensions are 

 clothed with minute feathery scales, coloured pi-evious to expan- 

 sion in consanguineous patterns that, like those of a kaleidoscope, 

 differentiate as regards forai and staining secretions, with the 

 slightest change in condition to which a species is exposed. 



Insects firstly obtain nourishment and secrete their colours 

 from their food, and change of diet often produces variation. 

 The caterpillar of the Large Tortoiseshell Butterfly, reared on 

 willow instead of elm, excludes a pale dwarf smaller than its 

 nettle-feeding congener — the Small Tortoiseshell. Varieties of 

 the Small White Butterfly, with the wings of a sulphur yellow, 

 have occurred in England, and are frequent in Canada; these 

 generally prove to be males, and are obtained at Montreal by 

 feeding the ordinary cabbage caterpillars on mignonette. The 

 Drinker Moth is likewise said to vary with its food-plant. It 

 has been also stated that generally larvae reared upon succulent 

 and overgrown herbage have imagines of large size and paler 

 colour, while dry and semi-withered food produces dark moths of 

 small size. The common Silkworm Moths, fed on lettuce and 

 mull^erry respectively, are adduced in exemplification. f 



Lei)idoptera exhibit seasonable varieties that from experiments 

 instituted appear to us the result of heat and cold accelerating or 

 retarding the pupal stage; and as we proceed northward or 



* Darwin, " Origin of Sjjecies." 



f E. K. Eobinson, Entomologist, 1877, p, 131. The food-plant in very 

 many cases doubtless is the immediate cause of variation. The Chelonida; pro- 

 duce local vai'iety of a white, yellow, or rosy colour, and in a Common Tiger 

 Moth the latter line may be forced by feeding the caterpillar on lettuce and 

 onions. At the moment of correcting the proof sheets I have received from 

 Bareilly, North India, a specimen of the Buff Ermine Moth, taken during 

 February or March, that differs mainly from our summer examples in this very 

 rosy colour. Dr. Staudinger, in his Catalogue, p. 60, also notices a similar 

 variety of the White Ermine. These might surely be bred by some variety 

 fancier. 



