Click! Goes the Beetle

The click beetle Athous haemorrhoidalis, copyright André Karwath.

There have been occasions when I've found myself complaining of the difficulty of recognising particular beetle families. It almost goes without saying, however, that this difficulty does not apply across the board. Whereas some beetle families may indeed be generically small and brown, there are others that are almost instantly recognisable. One such family, for the most part, is the click beetles of the Elateridae.

Click beetles are a cosmopolitan family with well over 900 known species. The majority of click beetles adhere to a consistent basic form: they have elongate, slender bodies, often with distinct longitudinal grooves running down the elytra. The front part of the body (the prothorax) is relatively large, and more or less acutely pointed at the back corners. Between the prothorax and the next part of the body (the mesothorax), the body is strongly constricted top to bottom so that the beetle can be flexibly bent. This distinguishes the Elateridae from most other beetle families (except for a few close relatives), and this is where the 'click' comes in. If the beetle finds itself lying on its back (or wishes to escape from a threat), it is able to arch itself so that the thoracic junction is lifted upwards. On the rear margin of the underside of the prothorax is a notched peg that sits against the front of the mesothorax, holding the two sections apart like the stick in a cartoon crocodile's jaws. This builds up a lot of potential energy that the beetle is able to hold in place until it suddenly releases the peg, which then slams back into a ventral groove at the front of the mesothorax with an audible 'click'. The effect of this sudden release of energy, followed by an equally sudden stop, is to cause the beetle to rapidly bend forwards, much in the manner of a person folding over as they receive a powerful punch to the gut. This self-inflicted gut punch results in the beetle being flung in the air, somersaulting to a hopeful landing on its feet. Evans (1972) conducted an analysis of the click-jumping of the elaterid Athous haemorrhoidalis, which is about a centimetre-and-a-half in length, and found that it could be lifted over a foot above the ground, tumbling several times over the course of a single jump. He calculated that during the jump it could be subjected to an acceleration of up to 3800 ms^-2, equivalent to a force of 380 G, one of the highest acceleration forces known in the animal kingdom (a human subjected to a similar force would soon end up like a satchel of instant pudding). Ribak & Weihs (2011) subsequently found, however, that the beetle has no actual control over its movements once flung and is just as likely to end up flat on its back again as the right way up, requiring it to attempt a second jump.

Fire beetle Pyrophorus sp., copyright Andreas Kay.

Adult click-beetles feed on the juices from plants but larvae may be more diverse in habits, including phytophagous, saprophagous or predaceous forms. Some burrowing phytophagous larvae, known as wireworms, can be notable pests, feeding on the roots and buried seeds of crop plants. Predatory forms, on the other hand, can be quite beneficial: the eyed elater Alaus oculatus of North America, for instance, has larvae that feed on the larvae of other beetles burrowing in wood. The Elateridae also include the fire beetles of South and southern North America, belonging to the tribe Pyrophorini. These beetles have a pair of large bioluminescent spots on their back at the rear of the prothorax. The larvae and even the eggs of fire beetles are similarly bioluminescent, and my guess is that the bioluminescence provides some sort of defense against predation.

Larva of Drilus on a snail, copyright Cécile Bassaglia.

Not all elaterids match the family's standard morphology, however. An analysis of elaterid phylogeny by Kundrata & Bocak (2011) found that a few groups that had previously been classified as separate families were in fact derived subgroups of the Elateridae. The 'Cebrionidae' (possibly a polyphyletic assemblage in the elaterid subfamily Elaterinae) are softer-bodied than other elaterids and lack the ability to click (presumably because their cuticle does not provide the resistance for a clicking mechanism to work). Female cebrionids may be flightless, with reduced wings and/or elytra. Even more dramatically altered are the females of the tribe Drilini (previously recognised as the Drilidae), the false firely beetles, which are larviform in appearance with only the head metamorphosing to an adult appearance. The larvae of Drilini feed on snails, and have a lifestyle that straddles the boundary between predator and parasite. Rather than killing and eating the prey snail immediately, they crawl into its shell and feed on it slowly; it may take several days for the larva's victim to actually meet its demise.

REFERENCES

Evans, M. E. G. 1972. The jump of the click beetle (Coleoptera, Elateridae)—a preliminary study. Journal of Zoology 167: 319–336.

Kundrata, R., & L. Bocak. 2011. The phylogeny and limits of Elateridae (Insecta, Coleoptera): is there a common tendency of click beetles to soft-bodiedness and neoteny? Zoologica Scripta 40: 364–378.

Ribak, G., & D. Weihs. 2011. Jumping without using legs: the jump of the click-beetles (Elateridae) is morphologically constrained. PLoS ONE 6 (6): e20871. doi:10.1371/journal.pone.0020871.