Ravenous rove beetles

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Catching prey is far from easy, that’s why predators have evolved a host of means of catching and subduing their food. Some of the most bizarre of these are in the insect world where evolution has run riot. A bit like chameleons, some insects have dispensed with having to pounce on their prey, instead relying on mouthparts that can be shot out to bring the prey within chomping range of the mandibles.

Stenus rove beetles are just such animals. These tiny insects have a structure called a labium that can shot out by forcing blood into it – much like the unfavourable creatures in the Alien films. In absolute terms, the distance it can shoot this structure out to is nothing to write home about, but for the beetle it’s the difference between securing an appetizing aphid or a succulent springtail and going hungry.

Stenus fossulatus
A Stenus rove beetle (Stenus fossulatus) looking for something to shoot its mouthparts at

The SEM images below show what this structure looks like at increasing magnifications (taken from the paper below). You can see in the last photo the business end of the labium is covered with brush-like setae, which probably ensure the prey is well and truly stuck when the beetle retracts its telescopic face.

Stenus rove beetle
The telescopic labium of Stenus comma (Thomas Bauer and Martin Pfeiffer) prm = praementum, mt = membranous tube
Stenus comma labium tip
The fearsome looking business-end of Stenus comma’s protusible labium (Thomas Bauer and Martin Pfeiffer) pgl = paraglossa, pm = palpus maxillaris
Stenus comma paraglossa
The setae covered paraglossa of Stenus comma (Thomas Bauer and Martin Pfeiffer)  
Stenus comma
The brush-like setae on the paraglossa of Stenus comma’s labium (Thomas Bauer and Martin Pfeiffer)
Here’s a Stenus sp. sitting very still with its labium fully extended. It’s roughly half length of the whole beetle (Ross Piper – specimen from the Picos de Europa, Spain)

Just what they use these elaborate mouthparts to catch is more of a mystery, as it’s not very good for catching springtails – the primitive six-legged animals that share the beetle’s habitat. Springtails are often covered in scales that come off very easily giving predators the slip, even those with abilities worthy of a science fiction film. The beetle’ telescopic mouthparts may come into their own when the insect is crawling around in low growing vegetation – enabling it to grab aphids or other insects on nearby stems and leaves that would otherwise be out of reach. There’s also the possibility the mouthparts are used to probe crevices in to which the beetle can’t quite squeeze.

There’s more about these beetles in the book, Extraordinary Animals

Further reading

Bauer T, Pfeiffer M (1991) Shooting springtails with a sticky rod the flexible hunting behavior of Stenus comma (Coleoptera, Staphylinidae) and the counter-strategies of its prey. Anim. Behav. 41: 819-828

Betz, O. Comparative studies on the predatory behaviour of Stenus spp. (Coleoptera: Staphylinidae): the significance of its specialized labial apparatus. Journal of Zoology 244, (1998) 527-544.

Betz, O. Life forms and hunting behaviour of some Central European Stenus species (Coleoptera, Staphylinidae). Applied Soil Ecology 9, (1998) 69-74.

Betz, O. A behavioural inventory of adult Stenus species (Coleoptera: Staphylinidae). Journal of Natural History 33, (1999) -1712.

Betz, O. and Fuhrmann, S. Life history traits in different life forms of predaceous Stenus beetles (Coleoptera, Staphylinidae), living in waterside environments. Netherlands Journal of Zoology 51, (2001) 371-393

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