IN DEFENSIVE COLORATION 323 



in any of the representative combinations in other parts of 

 the tropical New World, while a careful study of the exact 

 changes which they undergo as we pass from one district 

 to another enables us to dismiss any such hypothesis. 



Mr. Thayer's criticism is considered in this place 

 because the preceding five Sections show how subtle is 

 the relationship between two sharply contrasted methods 

 of defence, and how easily and frequently the barrier 

 between them is traversed. The determining factors in 

 each case are to be found in the complex conditions of the 

 struggle for existence. It is these that have decided the 

 degree of conspicuousness attained by Warning Colours, 

 and have kept them far short of an ' intrinsically reveal- 

 ing coloration ' as defined by the artist. 



8. The All-Importance of Instinctive Attitudes and 

 Movements in the Display of Warning Colours. — 

 Warning Colours are, like Cryptic, assisted by special 

 adaptations of the body-form, and especially by move- 

 ments which aid in rendering the appearance as conspicu- 

 ous as possible. On this account forms with Warning 

 Colours generally move or fly slowly, and it is the rule in 

 the wings of butterflies that the warning patterns are 

 similar on both upper and under surfaces, and so far as 

 there is a difference, that the colours of the under surface 

 exposed during prolonged rest should be even more 

 conspicuous than those of the upper surface. The 

 flight of certain American Danaine butterflies appears 

 to be especially adapted to display the conspicuous 

 under surface of the wings. 1 



Other instincts also assist in Aposematic display. 

 Thus the effect of the Warning Colours of caterpillars 

 is often intensified by gregarious habits. Furthermore, 

 many animals (spiders, beetles, caterpillars, &c.) when 

 attacked or disturbed, ' sham death ' (as it is commonly 

 but wrongly described), falling motionless to the ground. 

 Well-concealed animals, when once detected, are thus 

 given a second chance of escape among little fragments 

 of earth, dead leaves, or the roots of grasses. Animals 

 with Warning Colours are, on the other hand, enabled 



1 Ann. jSlag. Nat. Hist., ser. 7, vol. xiii, April, 1904, pp. 358-9. 



Y 2 



