A Fantastic Dragonfly Migration
Indian Ocean Route Over 2X Longer Than the Monarch Butterfly's Route
Oct 3, 2009
The appearance of migratory birds is often considered to foretell approaching rains. But migratory insects also may arrive in huge numbers, either singly or in swarms staggered over the days preceding the arrival of rains. Arrival of these less conspicuous organisms often goes unnoticed. This oversight may soon be changing with the recognition of a migration of dragonflies across the Indian Ocean that was first announced in early 2009.
Long Migration Described
Until the spring of 2009, the 4300 mile (7000 kilometer) journey of the monarch butterfly (Dania plexippus) was the longest known migration made by insects. But dragonflies have sporadically landed on ships or isolated islands hundreds and sometimes thousands of miles from the mainland. Observers of these incidents have previously assumed the insects had been blown off course, would become exhausted, and drown in the ocean. But Charles Anderson of the Maldives has deduced that dragonflies might make an 11000 mile (18000 kilometer) round trip journey over the Indian Ocean from India to Africa and back, and that the routes are used year after year.
Wandering Glider Dragonflies
The wandering glider (Pantala flavescens), also called the Globe skimmer, is found all over the world and has been known to make extensive journeys over water. It is one of only six dragonflies in the Galapagos, making seasonal visits, and the only one on Easter island, possibly a resident population. This dragonfly has a nymphal stage lasting only five weeks, so can reproduce in short lived pools and ponds where mosquito larvae often teem.
From October through November, waves of wandering gliders arrive in the tropical but arid Maldive Islands in the Indian Ocean. The lack of surface freshwater means the Maldives are unsuitable for dragonfly reproduction, yet millions of them descend on these islands each year – leaving two or three weeks later. Past observations indicate that the dragonflies move down the coast of India, reappearing in the northern Maldives about 300 miles (500 km) across open ocean, then island hopping to the southern Maldives for another 300 miles (500 km) away. The gliders disappear from these islands by the end of December.
Flocks of dragonflies start arriving in the northern Seychelles, some 1800 miles (2700 km) from India, in November, and in Aldabra in the Seychelles, 2200 miles (3800 km) from India, a few weeks later. They start arriving in Tanzania and Mozambique in Africa from late December through January, and twice in Uganda: in March or April and again in September.
Finally, wandering gliders reappear in smaller numbers in the Maldives during May and June and in India during late June or July. In India, their population increases exponentially over the next few months so that millions are available to initiate the migration by October.
Weather Patterns and Dragonfly Migration
During October, the monsoon rains in the Indian Ocean turn southward triggering the Inter-tropical Convergence Zone (ITCZ). The ITCZ moves southerly slowly from India to East Africa, bringing alternating waves of rain across the region and reaching Africa in late December or January. Mass dragonfly migration arrivals typically occur on the heels of cold fronts and storms.
According to Charles Anderson, the dragonflies fly from India with north-easterly tail winds of the ITCZ at altitudes over 3000 ft (1000 m). His deduction is predicated on the concordance of arrival dates of the gliders and the ITCZ at each point along the route.
The alternating waves of monsoon rains moving southward in the fall produce temporary rain pools along the migratory route that make excellent dragonfly nurseries, and three more generations of dragonflies are produced before the final migration leg brings them back to India. In May, the monsoons reverse direction and come from the southwest. Soon tattered dragonflies begin to reappear in the Maldives, and the bedraggled insects reach India in June and July.
Thus, the monsoons appear to serve as the driving force for the migration, providing:
- alternating waves of rain across the Indian Ocean that form temporary pools for dragonfly reproduction
- and appropriate winds for the insects to traverse the 2100 miles (3500 km) of mostly open ocean between India and Africa.
Bird and Dragonfly Migration
Concurrent with the migration of dragonflies is an exodus of birds from India to Africa. Examination of the species of birds that accompany the dragonflies indicates that among them are the Pied Cuckoo (Clamator jacobinus), Lesser Cuckoo (Cuculus poliocephalus), Eurasian Cuckoo (Cuculus canorus) and the Amur Falcon (Falco amurensis). These are all insectivorous birds that feed on dragonflies in both India and Africa and probably only survive the journey by following the migration of and consuming dragonflies along the way.
With a link between dragonfly and bird migratory routes now recognized, it would be advantageous for biologists to examine the paths of other insectivorous bird species to see if insect migrations also provide the fuels for these birds.
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