Nature Notebook
A skimmer dragonfly laying eggs in a small pool in Mikocheni, Dar es Salaam - Photo by Anne Outwater
Dragonflies Migrating to Tanzania have arrived.
Dragonflies (kereng’ende in Swahili) are strong fliers. In Tanzania, when looking out the windows of even the tallest buildings, it is common to see dragonflies cruising outside. In fact Pantala flavescens has been found flying at a height of 6300 meters!
Once on a boat far out in the Indian Ocean, a dragonfly landed and rode with us for a while; at the time I assumed it had been accidentally blown by the wind and if we hadn’t been there, it would have become exhausted and drowned in the sea. But recently it has been discovered that dragonflies migrate long distances! Of the 5200 species of dragonflies world wide, at least 25-50 of them are migratory.
Pantala flavescens, often called Globe Skimmers, are probably the most widespread dragonfly. They live along the equator, occurring between the 40th parallels of latitude worldwide. Several populations of this species are known to migrate, and there are recorded migrations of Globe Skimmers from South America to Easter Island in the middle of the Pacific Ocean (about 3,600 kilometers). Annually they appear on the Galapagos Islands, about 1000 kilometers from the South American coast. They have been seen recorded flying over the Chinese Yellow Sea at night and over the Hindu Kush mountains (at altitudes of 6,500 meters) between Pakistan and Afghanistan.
Recently it has been hypothesized that one population migrates across the Indian Ocean between East Africa (Tanzania, Mozambique, and Uganda) and India via the Maldives and the Seychelles every year. This was figured out by a biologist named Charles Anderson who lives and works in the Maldives, small islands about 400 kilometers southwest of India.
Each year, millions of Globe Skimmers arrive on the Maldive Islands in October, an event which is well known to people living there. Maldivians consider that the dragonflies herald the coming of the north-east monsoon. Their arrival coincides with the high-altitude winds of the Inter-Tropical Convergence Zone (ITCZ) weather system – winds which blow steadily between India and East Africa.
Ahead of the ITCZ the wind blows towards India, but above and behind it winds blow from India. So it seems that the dragonflies are able to reach the Maldives by flying on these winds at altitudes above 1000 meters.
Anderson was at first puzzled by their appearance. Dragonflies require fresh water in which to lay their eggs and their larval stage takes place in fresh water. On the Maldives there is a lack of surface fresh water on the islands, so they would be unable to survive for long.
He noted that they leave about a month later. And then they appear on Aldabra Island in the Seychelles. By January they usually reach Tanzania and Mozambique. In April they re-appear in southern India. Because the species is known to breed in temporary pools of water, Anderson believes the dragonflies are following the rains from India to Africa and back.
This dragonfly has a larval stage lasting only five weeks, so they can reproduce in temporary pools and ponds. The dragonflies are “following the rains, taking sequential advantage of the monsoon rains of India, the short rains of East Africa, the summer rains of southern Africa, the long rains of East Africa, and then back to India for the next monsoon," says Anderson. Taking advantage of the moving weather systems created by the ICTZ and the Somali Jet, they complete an annual immigration loop of about 18,000 kilometers. This feat is probably accomplished in successive generations, perhaps four.
I cannot know for sure from where the dragonflies came, but many appeared in my small garden nature reserve in April this year. I looked on the website www.dragonflies.org to discover what species they were. I learned that they were probably of the family of Libellulidae, known as Skimmers.
I noted them dipping their abdomens into the water as they were flying – a sign that they were laying eggs. Not long after, I noticed the larvae. They are aggressive and voracious. I have watched them eat tadpoles, young frogs, and the small fish I had put in to eat mosquito larvae! Several dragonfly larvae would latch onto one corpse until it was finished. They have grown large very quickly.
Anderson believes that the dragonflies survive the ocean flights by gliding on the winds, feeding on other small insects. He also notes that the dragonfly migration follows the same migratory paths as many bird species, including cuckoos, nightjars, Amur Falcons and bee-eaters.
"These birds fly at the same time and altitudes as the dragonflies. And what has not been realized before is that all are medium-sized birds that eat insects, insects the size of dragonflies," he says.