The air miles of a dragonfly

IN HER book Wild Dublin, Éanna Ní Lamhna mentions an unusual visitor to one of the city’s parks.

On October 12 1913, a man named Douglas saw an odd-looking insect on the grass in Herbert Park. He promptly took a swipe at it and sent the specimen to the Natural History Museum in Kildare Street, where it remains to this day. The insect turned out to be a dragonfly but no ordinary one. The Irish dragonfly and damselfly list runs to more than 30 species but this one was not among them. It was identified as a ‘vagrant emperor’, a native of India and sub-Saharan Africa.

Dragonflies, particularly the hawkers, are powerful fliers, able to stay in the air for long periods, but flights of thousands of miles must surely be impossible for them. So how did this little creature manage to get here? Recent research by a naturalist living in the Maldives, may provide part of the answer.

The Maldives are a group of coral islands around 900km south-west of India. There are more than 1,200 of them. Every year, thousands of dragonflies suddenly appear on the Maldives, only to vanish soon afterwards. British biologist Charles Anderson, who lives there, became intrigued by the insects and, in 1996, began a detailed study of them.

He records the first dragonflies each year between October 4 and 23. Several species are involved. Most are globe skimmers but vagrant emperors, the Herbert Park species, are also present. Numbers peak in November and December. Then the insects vanish. Are they born on the islands, having surviving for most of the year as eggs or larvae? Or are they migrants? If so, where do they come from?

Dragonflies need fresh water in which to lay their eggs. They spend most of their lives as nymphs at the bottoms of ponds and lakes. The flying-insect form only develops at the end of their lives when they take wing-seeking partners with whom to mate. There is, however, almost no surface freshwater on the Maldives and nowhere for dragonflies to breed.

Any dragonfly on the islands must be a visitor. The nearest landmass is India, so Anderson turned his attention there. Southern India, it turned out, also experiences a dragonfly invasion and this occurs at about the same time, or a little before, the one on the Maldives. Clearly, the visitors are of Indian origin. They seem to come in waves, each group remaining for a few days.

The next question to be addressed was even more difficult to answer; where do the insects go when they leave the Maldives? It would make no ecological sense for them to return immediately to India.

The only possible destination is the Seychelles, a group of 92 islands, 1,300km to the south-west. The idea of their crossing such a huge expanse of open ocean seemed ridiculous, but when Anderson examined the matter further, he found that hordes of dragonflies arrive on the Seychelles in November. The Seychelles, however, was not their final destination; the insects continue to fly for another 1,000km to mainland Africa.

The extraordinary migration to southern Africa and back again to India, a round trip of about 14,000km, would seem to require not just stamina but extraordinary navigational skills. The first leg of the journey, from India to the Maldives, would be equivalent to travelling from Ireland to Iceland without a compass or a map. Even a slight course error would cause the migrants to miss their target and perish in the sea. How could insects manage such a feat?

The Earth is encircled by a belt of moisture-laden air inside which winds, high in the atmosphere, blow consistently from India towards Africa each Autumn. It’s known as the Inter-tropical Convergence Zone Weather System. According to Anderson, the dragonflies hitch rides on this.

The cloud-filled wet winds carry the insects to the Maldives, then on to the Seychelles and finally to Africa, no navigational skills being required for the ride. Of course, this hardly explains how a vagrant emperor dragonfly ended up in Herbert Park but, somewhere along its way, the insect must have got on the wrong bus to be carried on another airflow in our direction.