312 Memoirs of the Indian Museufn. [Voi,. Ill, 



Islands we find the genus Aceraius, of which one species, A. grandis, resembles Epis- 

 phenus comptoni and Episphenus indiens in its gregarious habits and its wide range of 

 variability, resembling or rather exceeding the latter in the prominence of the anterior 

 angles of its head, and resembling the former (if not the latter also) in being the most 

 markedly asymmetrical species found in the region which it inhabits. Finally, in the 

 greater part, if not the whole of this area, we find the genus Opkrygonius, apparently 

 less common, and nearly related to the genus Aceraius. 



It will be noticed that whenever one species is markedly more abundant than 

 any of its allies inhabiting the same region, it is always found to be much more vari- 

 able than them in size, and to be more markedly asymmetrical, in addition to which 

 it may have the anterior angles of the head more prominent. Now asymmetry, 

 and the prominence of the anterior angles of the head, are both of them peculiar 

 characters which probably imply a high degree of specialization in their possessor ; 

 from which it will be seen that the dominant species of the sub-family Aceraiinae, in 

 Ceylon and in the continental area east of the mouths of the Ganges, is in each case 

 apparently the most highly specialized species of the sub-family found there, as well 

 as being the most variable in size, and always gregarious in its habits. And it may 

 be inferred that Episphenus indiens, which differs from E. neelgherriensis in having 

 the anterior angles of the head prominent, in its very variable size, and in its 

 somewhat more markedly gregarious habits, either is, or is likely to become, the 

 dominant species of the subfamily in the Indian Peninsula, although at present it 

 does not appear to be markedly more common than E. neelgherriensis. 



The curious processes of the canthus in Aceraius occnlidens suggest that this 

 species, which is very closely allied to A. grandis, is perhaps even more highly special- 

 ized. If this is the case the former will presumably oust the latter, in due course, 

 from its position as the dominant species of the genus. But it is also possible that 

 the higher specialization indicated by these processes is not of an advantageous char- 

 acter, in which case A . grandis may be expected to hold its own until the appearance 

 of some form better fitted to succeed it. 



To explain the distribution of the Aceraiinae one must suppose that a migration 

 of more and more highly specialized forms has taken place from the east towards 

 the head of the Bay of Bengal and then south-westwards towards Ceylon.^ With the 

 exception of the one symmetrical genus Tiberioides , which has been able to hold its 

 own in the Eastern Himalayas and the far east of Assam , the symmetrical or very 

 slightly asymmetrical forms have been driven into Ceylon. There they have been cut • 

 off from further aggression, and so have been able to perpetuate their race, and even to 

 produce one species, Episphenus comptoni, having all the biological and morphological 

 characteristics of a dominant form except perhaps as regards the anterior angles of 

 the head. Similarly the Indian Peninsula forms of the genus Episphenus , both of 

 them more strongly asymmetrical than either of the Ceylon forms, but with the 



' The possibility of the migration having taken place in a reverse direction, the degree of specializa- 

 tion increasing in response to a fresh environment, is precluded by the occurrence of species allied to 

 Episphenus moorei in Australia (see below, p. 315). 



