COLIINM. 
83 
They select the sea-coast, I feel sure, simply to avoid obstacles. The road 
between Trincomalee and Kandy, which runs through dense forest, is also 
largely used by the migrating insects. When travelling south they have the N.E. 
monsoon behind them, but when turning north they meet a stiff wind which really 
seems to drive them to a faster flight. The breadth of the flight is usually not more 
than a quarter of a mile. 
“ The reason for these flights is at present very obscure ; it was probably 
originally a question of food-supply. This instinct might have arisen from the 
necessity for constantly seeking new feeding-grounds for the larvae. As the species 
increased, this tendency to expand would not only preserve the species, but would 
cause in time its very material increase ; the necessity for constantly enlarging the 
feeding-grounds would in time produce an inherited tendency to migrate. But in 
due course, when all available feeding-grounds were occupied, as they soon would 
be, in a small island like Ceylon, some check would be required to keep the 
enormous number of resulting butterflies within due bounds, otherwise the species 
would be in danger of annihilation from their very numbers. This appears to me 
to be effected in the following manner:—The insects of the wet-season migration 
are mostly composed of females, and provided that the males can successfully 
impregnate more than one female, the result would be an enormous number of eggs 
laid, and this I have shown to be the case. The migratory instinct is so strong 
that the females are precluded from taking any precautions for their future 
offspring, as the females of most butterflies do ; and the result is that the struggle 
for existence among the multitude of larvrn subsisting on the food-plant, which is 
quickly diminishing by their voracity, and also slowly by the heat and dry weather, 
is so great that the larvae which would produce female butterflies succumb, and a 
great majority of males are produced which form the dry-weather flights. This 
majority of males would also be another factor in checking the increase of the 
species. During the intervening portion of the year the species would gradually 
increase, until the wet months at the fall of the year favour a luxuriant vegetation, 
and all the female larvae then survive, and possibly being stronger crowd out the 
male larvae. These larvae produce the overwhelming proportion of females in the 
next wet-season flight, with the result shown above. This migratory instinct, 
originally due to a necessity for the increase of the species, is now become a means 
of preventing its undue propagation.” (Trans. Ent. Soc. Bond. 1904, pp. 701-6.) 
Migratory Habits: in Burma.— Col. 0. T. Bingham writes whilst “returning 
O fc3 
down the Salween to Moulmein, on a hot steamy day in October, and when below 
Shwegon, I noticed clouds of butterflies, chiefly Catopsilias, migrating, crossing 
the Salween from East to West in a continuous stream ” (Tr. Ent. Soc. 1902, 
363). 
