Raindrops on dragonfly wings

Sitting on a balcony in the rain with my camera, I saw a Wandering glider (Pantala flavescens, also Globe skimmer) sheltering under palms. Tiny drops flecked its wings. From the beading of the drops it seems that the wings are water repellent. I’ve seen them before in this season. Their numbers usually peak now, between September and October. They are the commonest of dragonflies in the parts of the world where most human live (which are all the places in the world where the temperature never falls below 20 Celsius). But they are uncommon travellers, as I was told recently (by Kim Smith).

Apparently they migrate between India and eastern Africa. It is believed that around the end of October these 5 cm long dragonflies will begin to journey across the Indian Ocean. On their way to Africa they will mass the Maldives. They are usually visible in the Maldives around November, and their numbers drop off by the end of December. Another peak in their population in the Maldives is in May. These peaks are correlated with the trade winds (monsoon). A clincher to the theory of their cross-oceanic migration would be direct observation, of course. But even if there were peaks in their abundance in Africa in January, or in India in June, it would be corroborating evidence. Since there is no evidence of this kind, this long-distance migration remains a hypothesis, although one with strong support. Perhaps the numbers which make landfall after the migration is small, and are replenished by breeding. There seem to be mysteries behind even the most common observations.