THE ACTION OF LIGHT, ETC. 



375 



idoptera and Coleoptera, while yellows and browns 

 of different shades are so exceedingly common, 

 and relatively so permanent. We find also that 

 certain caterpillars, green in the earlier part of 

 their life generally, though not invariably, take a 

 brown color as they approach the time of their 

 assuming the pupa state. 



But even supposing that chlorophyll is de- 

 monstrably assimilated or deposited in the tis- 

 sues of certain insects, the hypothesis we have 

 been suggesting takes us but a very little way. 

 We have still to ask why the green color in cer- 

 tain species remains undecomposed to the mature 

 condition, while in others it disappears in the 

 pupal or even in the larval state, and how, after 

 disappearance or absence in the larva, as in 

 Choerocampa Elpenor, it appears in the perfect 

 insect ? We have to inquire why certain diurnal 

 caterpillars, consuming as much chlorophyll as 

 do any others — e. g., Vanessa lo, V. xanthomelas, 

 V. urticce, etc. — are free from a green colora- 

 tion? At the same time we must admit that in 

 caterpillars of this class a yellow pattern is very 

 rarely absent, as if the xanthophyll had already 

 been separated from the cyanophyll. We have 

 to explain the pigment-blues, of which there seem 

 to be two, if not three, the identity of which with 

 cyanophyll must not be too rashly assumed, 

 though in many cases we see both blue and yel- 

 low spots appearing in a butterfly, as if the two 

 colors, which in its earlier state had been blended 

 together, were now separated, as in Papilio Ma- 

 chaon. We have, further, to throw a light upon 

 the origin of the pigment-reds, to two of which 

 Mr. Wallace refers as being different in their 

 chemical constitution and behavior. 



But chlorophyll is not the only substance 

 which has been called into requisition in order 

 to explain the mysteries of animal coloration. It 

 has not escaped the attention of biologists that 

 all those creatures which develop, more or less 

 frequently, beautiful hues, are precisely the same 

 in which uric acid is abundantly secreted — i. e., 

 birds, reptiles, insects — while in the mammalia, 

 in which the secretion of uric acid is trifling, the 

 prevailing colors are dull. It was asserted that 

 while uric acid is abundantly found in the excre- 

 tions of parrots, humming-birds, etc., at other 

 times of the year, during and immediately before 

 the moulting season it was absent. Hence the 

 inference that this compound might play a part 

 in the elaboration of the new plumage was not 

 unwarrantable. In addition came the fact that a 

 beautiful violet color, known as murexide, and 

 capable of producing a variety of shades, was ar- 



tificially obtained from uric acid. 1 Unfortunately, 

 when these investigations were carried on, the 

 distinction between interference-colors and ab- 

 sorption-colors had not been fully apprehended, 

 and the iridescent hues of humming-birds, tro- 

 gons, Belionotw, were supposed to be due to some 

 peculiar pigments of unknown composition. .Nor 

 has it, as far as we are aware, ever been shown 

 that the excreta of splendidly-colored birds are 

 richer in uric acid than those of sea-fowl. For 

 the present, therefore, the uric-acid theory must 

 be considered as useless. 



A consideration of the food of different spe- 

 cies might at first sight appear likely to throw 

 some light upon the nature of their coloration. 

 But we find intense splendor and varied tints 

 alike among carnivorous species (Cicindelidae and 

 certain Carabidae), wood-eaters (Buprestidse), and 

 leaf-eaters (Chrysomelidse). We find dull and 

 sordid colors among many carrion and dung- 

 feeders (Silphidce, Aphodiidae, Staphylinidee), 

 while others addicted to a similar diet — such as 

 most species of the genus Phanceiis — display the 

 most splendid hues. Nor is an examination of 

 the diet of birds more satisfactory. 2 



It may perhaps be thought that in an inquiry 

 into the influence of light upon the coloration of 

 animals a consideration of their diet, or of their 

 peculiar secretions and excretions, is out of place. 

 But whether solar radiations, or local atmospheric 

 influences, or the need of protection take a great- 

 er or smaller share in the development of color, 

 there must be essential differences in the material 

 upon which these causes act. Mammals are ex- 

 posed to the same climatic influences as birds 

 and insects, and are likewise exposed to dangers 

 which they might escape by a coloration favor- 

 able to concealment. Their hair is, chemically 

 considered, a material no less suitable for the 

 display of gay and brilliant colors than are the 

 feathers of birds, the scales of serpents, or the 

 chitinous coating of insects ; yet neither in lus- 

 tre, in varying play of color, nor in delicacy and 



1 Murexide, known in the commercial world as 

 "Roman purple" and " Tyrian purple," was some 

 time ago prepared from guano — i. e., the excreta of 

 sea-fowl — and was in considerable demand among dy- 

 ers and calico-printers. Being costlier than the coal- 

 tar colors, it is now superseded. 



2 In addition to the case of chlorophyll above men- 

 tioned there seem to be individual instances where 

 the coloring-matter of a plant, if eaten by insects, may 

 be traced in their secretions. We do not know 

 whether the deep reddish violet liquid which exudes 

 from Timarcha laevigata, an insect feeding upon bed- 

 straw, a plant of the madder tribe, has ever been ex- 

 amined for alizarin or purpurin. 



