Some time during the middle of October, you will enter one of the well-lit ground floor garages in Dhaka around dusk to find it full of quite large brown butterflies. I have counted more than sixty in our own building in Gulshan. The butterflies sit all over the walls and the roof - some will be low enough for you have a close look at them. You will see that they have a somewhat irregular wing profile and that the main colour is some shade of grey, but with darker and lighter additional markings. They are beautifully camouflaged as dry leaves and the underside of no two specimens are rigorously the same. What you see is the first and only mass emergence of the dry season form of the Common Evening Brown (Melanitis leda).

Between April and October the Evening Brown is widely distributed in Dhaka though usually not in very large numbers. But this is the wet season form which is so different that you could be forgiven for thinking that they were two distinct species. The wet season form differs from the dry in having a less angular wing shape, and a uniform, striated pattern with large, prominent eye-spots towards the edge of the wings.

So, why these two different seasonal forms? It is a fairly complex mechanism which would not have evolved without strong incentives, strong incentives in Darwin’s terms being increased survival. In 1982, Paul Brakefield and I began to sift the literature and came up with the following hypothesis:

During the wet season the Evening Brown, though not very common, is active throughout the day, especially since the monsoon keeps down the heat and much of the day is dark like the evenings that it likes. This means that the predators which would like to eat it are also active, like birds, squirrels, mice, and snakes. They rush up to where they saw the butterfly settle and then have to make a split second decision – where to strike? Well, the eye is the logical place, but the predator ends up with a mouthful of fluff, and the butterfly can fly away with little or no damage; the real eye and head is about as far away from the marginal eye-spots as can be. One thing is certain: You often find adults that show the effect of a bird attack on the margin of the wing, and you don’t do that in the dry season form which lacks the eye-spots.

During the wet season the day is both bright and cool which does not suit the Evening Brown, so cool that even its caterpillar at most eats so slowly that it grows but little. Especially December and January are very poor for all butterflies and very few are on the wing. So the dry season form spends much of its time hiding in caves, hollow trees, or dense undergrowth. Its predators are a very different lot, being lizards, frogs, mice, and other small animals that browse about looking for things to eat. They would spot marginal eye-spots from a great distance, but they are fooled by camouflage.

Some time in February, the surviving dry season forms will start laying eggs, their caterpillars will grow, and finally some fine morning in April they will hatch in the wet season form which will be modestly numerous till the huge hatching of the dry season from in October.