Masters of the monsoon

Abstract

Locals in the Maldive islands, in the tropical Indian Ocean between southern India and east Africa, are well used to the arrival of millions of dragonflies, most of which are the species known as globe skimmers or wandering gliders (Pantala flavescens), beginning in October. But for biologists their appearance is a puzzle. All 1,200 islands are small coral atolls, with almost no surface freshwater, which dragonflies need to breed. Now a biologist working in the Maldives claims that the arrival of these dragonflies may be part of the longest migration of any insect. If confirmed, this would be the first insect migration across large tracts of open ocean. Charles Anderson, a British-born naturalist, writing in the Journal of Tropical Ecology, noticed the dragonflies when he first arrived in the Maldives in 1983 and started keeping detailed records from 1996. When Anderson compared these observations with those made of dragonflies appearing in southern India, he found a clear progression of arrival dates from north to south, with dragonflies arriving first in southern India, then in the Republic of Maldive's capital, Male, and then on more southern atolls. Dragonfly numbers peak in November and December, before the insects disappear once more. The insects arrive in waves with each staying for no more than a few days. Smaller numbers of dragonflies reappear between April and June. The dragonflies are clearly migrating from India across the open ocean to the Maldives, says Anderson, which is a distance of 600–800 km. But this is puzzling because in October the surface winds in the Maldives are blowing towards India. Anderson believes the answer lies in the Inter-tropical Convergence Zone (ITCZ), a higher altitude weather system that moves southwards across the Maldives in October. Ahead of the ITCZ the wind blows towards India but above and behind it the winds blow from India. So, at altitudes above 1,000 m, the dragonflies could be able to reach the Maldives using these winds. Studies in China have found the dragonflies flying at these heights and studies of air pollutants have shown that they can spread from southern India to the Maldives at speeds of 10 m per second. With such tailwinds, dragonflies might make the crossing from India to Male in 24 hours or less. But Anderson believes the dragonflies' arrival in the Maldives represents a short break in a journey to east Africa and he has gathered considerable circumstantial evidence to back his claim. Large numbers of dragonflies start appearing in the northern Seychelles, 2,700 km from India, in November, and then in Aldabra, 3,800 km from India in December. Dragonflies appear in Tanzania and Mozambique in December and January. These dates strongly suggest that the dragonflies take advantage of the moving weather systems and monsoon rains to complete an epic migration, says Anderson. These dragonflies exploit temporary pools of water where predators are absent but prey, such as mosquito larvae, are present and they can complete their larval life in as little as 40 days, which is remarkably quick compared with most dragonfly species exploiting longer-lasting pools. He believes the dragonflies are then able to exploit the northward movement at the back of the ITCZ in the following spring to migrate back to India, arriving in the Maldives on their way in May and June. This would amount to an epic round-trip migration of 14,000–18,000 km involving four or five generations of dragonfly. He also points out that this proposed migration is similar to that known to be taken by several insect-eating birds, including cuckoos, nightjars, falcons and bee-eaters. The proposed dragonfly migration suggests that the birds might feed on the insects during their migration. The proposed dragonfly migration exceeds even that of the Monarch butterfly, which, over four generations, completes a 7,000 km migration from Mexico to Canada and back. Anderson acknowledges the uncertainties in his hypotheses but the possibility that these insects are making a regular seasonal transoceanic migration “should excite further interest in these extraordinary animals,” he says.