Robber Flies: The assassins of the insect world

Robber Flies eat fellow insects from the larvae to the adult stage in one of Nature’s most carefully crafted mechanisms

Updated - September 26, 2020 11:40 am IST

Published - September 25, 2020 10:13 am IST

The Robber Fly killing its prey by inserting its proboscis

The Robber Fly killing its prey by inserting its proboscis

Throughout my life, and I’m afraid to say, even until sometime after I was introduced to nature and wildlife, I had the misconception that insects were dependent on plants for food. In 2016, I discovered the vicious Robber Fly at the Aravalli Biodiversity Park, Gurugram. With over 7,000 species described, the world of these Robber Flies, is so diverse that an entire family, Asilidae , has been created for them. They’re named for their feeding behaviour: they ‘rob’ other insects of their lives!

They come in various colours but the ones found in Delhi-NCR are mostly brown and black. Although humans may only be able to see two large compound eyes on heads of Robber Flies, they do have three small simple eyes (called ocelli) wedged between those two big eyes. There’s also a bristly moustache — more than a fashion statement, this deters struggling prey from causing damage to the Robber Fly.

Their agile flight, impeccable eyesight, quick reflexes, and strong spiny legs help them catch insects in flight: other flies, beetles, grasshoppers, butterflies, moths, carnivorous dragonflies. They even feast on spiders sometimes. They pierce their strong dagger-like proboscis (tubular straw-like organ to take in food) into their prey, usually between the head and thorax, to subdue them. They inject victims with their saliva containing neurotoxic enzymes to paralyze, and proteolytic enzymes to liquify the innards of their prey. They then fly off with the prey between their legs to an undisturbed branch, where they suck the innards through the proboscis.

They patiently stalk other insects from vantage points and make a swift yet graceful, silent flight on the first opening they see It took me almost two years to actually see this opportunistic assassin hunt another black fly in air. This behaviour has also brought them their other name: Assassin Flies.

Like all insects, they too have a three-segmented body: head, thorax, and abdomen. The females lay eggs using an ovipositor attached to the abdomen, a tubular organ that helps them store and lay eggs. The eggs are laid in plants close to the ground or in nooks and crannies of soil or wooden barks.

When the eggs hatch, the larvae generally lives in soil or in wood and is predatory in nature, feeding on eggs and larvae of other insects. In short, the species in both larvae as well as the adult stage is a bane for other beings. In order to withstand the extreme winter, Robber Flies often overwinter in the soil as larvae or as pupa before emerging when they feel that the climatic conditions are favourable.

I always thought I’d have to be in the wilder areas of the city to see this magnificent creature but I recently saw it in an urban park in front of my house.

These predators maintain a delicate balance in the food chain and also help rid your backyard of pests. Participants on our nature trails have also witnessed Robber Flies being hunted by bigger dragonflies in the vicinity, but these instances are rare.

The writer is the founder of NINOX - Owl About Nature, a nature-awareness initiative. He is the Delhi-NCR reviewer for Ebird, a Cornell University initiative, monitoring rare sightings of birds. He formerly led a programme of WWF India.

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