Dragonflies are bellwethers of earth’s ecosystems

In the imagination of children, flying holds a special place. Folded pieces of paper become airplanes ready to be launched, eager gazes follow soaring eagles, and dragonflies are the helicopters of their fantasies.

Dragonflies (called sitara in Hindi) are indeed wonders of flight. These insects belong to a lineage whose members were among the planet’s first flying organisms. About 300 million years ago, their ancient kin had the same aerodynamic features, but with a wingspan that extended to 72 cm. The spectacle of these giant ancestors of dragonflies would have left any modern observer spellbound.

India is home to around 600 species in the order Odonata, which includes dragonflies and the slender damselflies. The pages of The Hindu sometimes feature colorful pictures of newly discovered Indian species, usually in either the Northeastern States or in the Western Ghats where Karnataka, Kerala, and Tamil Nadu meet. But alas, ecological surveys increasingly report missing dragonfly species and shrinking populations. In a world where public attention is fixated on large mammals, why do these ancient insects merit so much thought?

One notable trait of the Odonata is that they split their life cycle, being aquatic in the early stages and terrestrial and aerial as adults. Nymphs, as their larvae are called, live in water — be it in wetlands, ponds or in flowing streams. The adults hover, accelerate, and swoop on four independently articulated wings. But in all phases, dragonflies are the dominant insect predators. The nymphs feed on the larvae and pupae of mosquitoes and other insects. One growing dragonfly nymph can finish off a hundred mosquito larvae in a day. In flooded paddy fields, nymphs feed on the aquatic stages of crop pests, while dragonflies prey on leafhoppers and other damage-causing insects.

That said, dragonflies are sometimes also the prey.

The Odonata are themselves susceptible to the actions of pesticides, particularly systemic pesticides such as fipronil. These chemical compounds are absorbed by plants and make the whole plant toxic to insects. So over time, excessive pesticide levels lead to a decline in dragonfly numbers. Conversely, the presence of a healthy population of dragonflies and their nymphs in agricultural zones is a sign of well-managed farming without excessive pesticide use.

Away from human-dominated landscapes as well, dragonfly populations are affected by a changing planet. Some species in the Western Ghats have evolved to flourish in forests where water seeps from wet marshy soil into mountain streams. In more recent years, cycles of drought and excessive rainfall have not been kind to several endemic Indian species, such as the blue hawklet (Hylaeothemis apicalis,), a strikingly colored dragonfly with blue-green eyes, but which is very rarely seen these days.

While looking to dragonflies and damselflies as the bellwethers of our ecosystems, we also continue to unearth new facets of what makes them unique. Recent research on the eyes that provide 360-degree vision to dragonflies has shown that there are 16 or more genes that code for visual opsins, the proteins that detect colour and light. We humans make do with just four visual opsins: one each for the red, green, and blue colours in our cone cells and a fourth for low-light vision by the rod cells. The molecular abundance gives dragonflies a very sensitive visual system that even perceives ultraviolet light (PNAS 112 (11) E1247-E1256).

Even as their numbers dwindle, these fascinating creatures continue to provide us with new insights.

The article was coauthored with Sushil Chandani, who works in molecular modelling.

dbala@lvpei.orgsushilchandani@gmail.com