14 



the change of season is principally marked by the alternation of 

 rains and drought. 



(5) In many cases, though with our present knowledge not in all, 

 it is possible to discern utility to the life of the species in the 

 seasonal changes undergone by successive generations. As a general 

 rule in tropical butterflies, the dry-season phase tends to be cryptic, 

 or adapted for concealment, and the wet-season phase to be apose- 

 matic, or adapted for the display of warning colours, which may be 

 either true or false. The latter come under the head of Batesian 

 mimicry. The habits of the insect may undergo a change corres- 

 ponding with the difference in aspect. 



(6) The method of concealment is probably on the whole a more 

 efficient mode of defence than the method of display; and accord- 

 ingly we find it more often employed in the dry season, when, from 

 the scarcity of insect food, the need of protection against insect- 

 eating enemies is felt in greatest measure. 



(7) Seasonal Dimorphism is not a case of simple alternation of 

 generations, but it is determined by the influence of some external 

 condition upon an organism so constituted as to react to that 

 influence in a definite direction. 



(8) These external conditions can be imitated by artificial means. 

 In the case of the insects of temperate countries, such as the 

 Araschnias, the needful stimulus appears to be that of temperature; 

 in the case of tropical insects, moisture and dryness appear to be 

 contributing factors. 



(9) The stimulus is not equally operative at all stages of growth. 

 In some instances, as the Teracoli, the larval period is the most sus- 

 ceptible ; in others, as Belenois fiererina, the pupal. 



(10) Though the dependence upon season of these changes in 

 aspect is in manj^ cases well established, it is often the case, 

 especially in dry and arid districts such as the neighbourhood of 

 Aden, that the difterent phases may be found on the wing together. 

 Some of these cases may arise from individual differences of sus- 

 ceptibility between members of the same brood, others from a 

 natural overlapping at the change of seasons, and others again from 

 the effect of a temporary change in meteorological conditions upon 

 insects that happen to be at that particular tmie in the most 

 susceptible stage of their growth. 



This concludes what I have to say on the present occasion about 

 the subject of Seasonal Dimorphism in Butterflies. 



