990 



THE NATURE BOOK 



A REMARKABLE INSTANCE OF AGGRESSIVE MIMICRY (ALLURING COLOURS), 



AFFORDED BY A SPIDER OF THE GENUS THOMISUS. 



This spider mimics both in colour and form the unopened bud of the bramble. 



the precise benefit which any particular 

 colour-briUiancy confers on any particular 

 insect. A sombre habit, which harmonises 

 so closely with the insect's normal sur- 

 roundings as to make it practically 

 invisible at rest, may be explained as a 

 result of natural selection. Insects which 

 are so protectively coloured as ordinarily 

 to escape the notice of their enemies, 

 must, we imagine, have a better chance of 

 survival than insects which are markedly 

 conspicuous. If we are right in assuming 

 that dull-coloured insects have been 

 benefited in the struggle for existence by 

 their invisibility, we must also be right 

 in assuming that brilliantly coloured 

 insects have been prejudiced by their 

 visibility, and we must, in their case, look 

 for some additional means of defence 



which has enabled them to survive in 

 spite of a severe handicap. The theory 

 of " warning colours " though it cannot 

 be said to resolve the difficulty com- 

 pletely — the final word must come from 

 observers in the field rather than from 

 experimenters in the vivarium — has at 

 least furnished us with a useful working 

 hypothesis. According to this theory a 

 brilliant colour scheme (the black and 

 yellow banding of the Wasp for exam])le) 

 is associated with some noxious qualit}' 

 (the Wasp's sting in this instance), and 

 serves io advertise Ihe existence of tJuit 

 quality to all whom it may concern. 



At first sight it would appear that 

 " warning " colours can only benefit the 

 prospective enemy, and that they cannot 

 therefore in any sense be the result ol 



