Around the doorstep of a spider’s lair – a doily of silken trip wires emanating out from a dark recess in the craggy bark of a veteran oak tree – are the husks of unfortunate dinner guests, a wrecking yard of bug bits. But one spider’s trash is another beetle’s treasure: none is more daring than the tiny and rare Ctesias serra, better known as the cobweb beetle, if it’s known at all.


What is the cobweb beetle?

The cobweb beetle is a member of the Dermestidae, a family of successful beetles that are part of nature's clean-up crew.

Collectively they’re called skin, hide, larder or carpet beetles, their names giving away their predilection for eating dried natural materials. Some are used by museums to clean specimens.

Others eat our soft furnishings and even violins (bow beetles). In cases of infestations in our homes, the hastisetae are blamed for allergic reactions and skin irritation.

What does the cobweb beetle larvae look like?

Cobweb beetle larvae are only a few millimetres long and although they are very much a typical beetle larva in body design – elongated and segmented – they are also somewhat broad and flattened in form, which no doubt helps them slip into hiding. They are also very hirsute: their bodies are banded with belts of blond hairs or setae. These are of varying length, but the feature that makes them most distinctive is on the end of their abdomens.

Towards the rear of a larva’s body, the hairs are longer and fan out, but the most obvious are clusters of slightly shorter ones. These gingery hairs are arranged in four dense tufts. They are held in clumped formation most of the time, but when a larva is disturbed, it turns more porcupine than insect, erecting the hair tufts into expanded rosettes.

The cobweb beetle larvae secret weapon

This hair-raising act hides an even more elaborate detail, which can only be revealed by scrutiny under a microscope. The hairs are not just setae, these are known as hastisetae, a particular kind of hair used in defence.

Not only does each hair divide toward its tip, but rather than being a smooth shaft, like the quill of a porcupine, it is made up of a series of spiny, barbed rosettes all the way up its length, terminating with a sharpened cone-like ‘anchor’ at its tip. Towards the base of each hair is a weak pedicle, which easily snaps, breaking the entire hair off from the beetle’s body. The result is a medieval-looking bit of micro-engineering designed to lodge and entangle the limbs, palps and mouthparts of any invertebrate attacker.

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How it dares to invade spiders web

Loaded with a weaponised derrière, this baby beetle dares to go where few insects would: right into a spider’s web. It is a specialist thief. If the resident spider detects the imposter either scavenging for scraps or even stealing something fresher, it is met with a surprise.

To confuse and jam the spider’s detection system, the fuzzy beetle rapidly vibrates its abdomen, sending a counter message along the mesh of silk threads, which helps conceal its location.

If this first defensive move is thwarted, the beetle can swing its abdomen forward over its head – the hairs, easily dislodged, are thought to be irritating to the spider and so the beetle gets away with its raid. It doesn’t end here, either. When the time comes for the larva to pupate, it does so without fully leaving its last larval skin, which continues to protect it: the pupa sits cosied up inside its lethal furry jacket until the adult gets to emerge.


How rare are cobweb beetles?

To find a cobweb beetle for yourself will require a lot of luck. In the UK, it is listed as nationally scarce. While the beetle is quite widespread, it is limited to sites with ancient trees and is found in many English counties, but with fewer records from the far south west, Wales and Scotland. However, nighttime inspections of the craggy bark of an old oak or beech tree may well turn up this surprising little insect.


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Nick BakerTV presenter and naturalist