152 THIRD ANNUAL REPORT OP 



fly bevies. They certainly do not travel very long distances or we 

 should hear more numerous accounts of them. There are two signifi- 

 cant facts connected with them from which some corollary might be 

 deduced, namely, that only those species which have a very extended 

 range are known to form such flocks, and that they always travel, 

 under these conditions, in a southerly or southwesterly direction. 

 Mr. Bates* gives an interesting account of the uninterrupted pro- 

 cessions of butterflies belonging to the genus Callidryas, which 

 passed from morning to night in a southerly direction across the 

 Amazons ; and as far as he could ascertain these migrating hordes 

 were composed entirely of males. 



If our Archippus flocks should turn out to be all males, this fact 

 may lead to some solution of the cause of their congregating; but I 

 incline to believe the flocks are composed of both sexes. Again, if 

 the swarms occurred during the egg-depositing season, we might even 

 then venture to solve the problem. For it is evident that a species 

 which enjoys such immunity from predaceous animals and which is 

 confined in its diet to a single family of plants, must occasionally 

 multiply in particular districts beyond the capability of the plants to 

 sustain them; and as most female butterflies instinctively refuse to 

 deposit eggs on a plant that has already been abundantly supplied 

 by some other individual, the females of our Archippus would natur- 

 ally roam in vain for fresh plants when once the latter had all been 

 stocked; and would thus congregate together, and, followed by the 

 males, form migrating bevies. Or we might suppose that after the 

 larvae had eaten up all the milk-weeds in a district, the butterflies 

 they produced, finding no plants upon which to lay their eggs, would 

 be forced to migrate in swarms. But neither of these suppositions 

 can have much weight from the fact that the swarms occur either late 

 in the fall or early in spring; and the most plausible solution under 

 the circumstances is that, as these are the seasons when the milk- 

 weeds are either destroyed or have not yet started to grow, the but- 

 terflies, having nothing to confine their attention and keep them iso- 

 lated, naturally congregate together, and that when in motion, the 

 low temperature of the seasons instinctively prompts them to wend 

 their way southwards. The probabilities are that these swarms are 

 eventually destroyed, for no species can multiply beyond a certain 

 limit, and when there is not check to increase in one direction, there 

 will be in another. Of course this is as yet all theory and hypothesis, 

 but hypotheses in such cases are necessary, for they are threads 

 on which to string and combine the known parts of a case so as ulti- 

 mately to arrive at the real truth in the matter. 



* Naturalist on the River Amazons, I, p, 249. 



