320 TiiK placp: of mimicry 



larvae may remain inconspicuous while undisturbed, but 

 nevertheless the approach ot an enem\ determines 

 united movements in the colon)- which render the whole 

 strikingl)- conspicuous, and . . . may be attended later by 

 the emission of an ollensive smell from the numerous 

 ventral glands of all the individuals simultaneously (e. g. 

 Croesus scptcntrioiialis)! 



5. Scdsotia! Transition from Cryptic to Aposeniatic 

 Defence. — It has been pointed out on p. 317 that in 

 latitudes like our own the insects with Aposematic 

 colouring are compelled to hide during the periods when 

 the food of insect-eating animals is scarce. An ana- 

 logous alternation ma)- be produced by the wet and dry 

 season broods of certain species. In some African 

 butterflies of the Xymphaline Genus Precis, the wet 

 season broods are distinguished by the more or less 

 conspicuous under sides of the wings, while those of the 

 dry season are highly Cr)ptic. The South and East 

 African species Precis arcJusia was the one which most 

 impressed the present writer, and led him to seek for an 

 interpretation in the conditions of life peculiar to the two 

 seasons. 



It appeared, on comparing a series of specimens,^ 

 that, as described on p. 208, the feature which chiefly 

 renders the wet season form conspicuous is derived from 

 the very marking which is the principal element in the 

 concealment of the other ! This led directl)' to the 

 conclusion that there must be some i/reat difference in 

 the conditions of the two seasons which made a certain 

 measure of conspicuousness an advantage in the wet, and 

 concealment a necessity in the dry. Some account of 

 these differing conditions and the manner in which such 

 an astonishing alternation in this and many other species 

 may have arisen is given on pp. 206-1 1. 



6. Geographical Transition fro)}i Aposematic to 

 Cryptic Jkfence. — The much -mimicked, conspicuous 

 Limnas clirysippns — perhaps the commonest butterfly 

 in the world — tends to be replaced on desert areas, and 



' For >uch a iransilional series see Trans. Ent. Soc, Lond.. 1902, 

 plate xiii, figs. 5-8. Consult also pp. 428-9 of the same memoir. 



