Springtime beetles, legs go spring

Having a garden pond means occasionally finding dead things floating. No, not hedgehogs or other larger fauna - the sides are profiled so that mammals can get out - but sometimes incautious invertebrates drown. Naturally, if they look interesting (and haven't been eaten by pond skaters and other small predators), I fish them out for a closer look. When I saw a flea beetle (in the Chrysomelidae, my specialist group - they are in the tribe Alticini within the subfamily Galerucinae) I identified it easily enough as Longitarsus pellucidus, a common species, though it's always worth checking. It lives on bindweeds (Convolvulaceae) which are growing near the pond so presumably jumped into the water by accident when disturbed by a spider, bird, cat, human or other potential threat. However, it did give me an opportunity to look at why flea beetles are so named.

Longitarsus pellucidus - the swollen himd femora can be seen, as well as the elongate tarsus that gives it its generic name.

The hind femur in flea beetles is swollen to accomodate jumping muscles and the 'metafemoral spring' which is visible through the cuticle as a slightly paler comma-shaped structure taking up much of the inside of the femur.

Hind femur and metafemoral spring of L. pellucidus.

The spring is a long-oval chitinous structure coiled much like a loose fist. Muscles squeeze the spring closed and then a 'catch' is released and the stored energy is released, extending the leg so the beetle can jump.Here you can see a ligament extending from the spring to the tibia so the leg is folded when the spring is compressed. This mechanism differs from that in fleas which have a blob of resilin (a natural rubber) which is compressed instead of a spring. Although fleas may be more familiar for their leaping ability, some alticine beetles are actually able to jump comparatively further, and of course some of these beetles are no bigger than a large flea - smaller beetles jump further relative to their size (Schmitt 2004). Also, the spring may be useful for identification and for grouping species taxonomically by looking at similarities and differences in the form of their springs. However, this is not a mainstream technique, largely because of the difficulty of dissecting out the spring and the lack of readily available published material forming a guide to identification this way, although there is useful information in Furth (1988) along with images and drawings from various species.

References

Furth, D.G. (1988). The jumping apparatus of flea beetles (Alticinae): the metafemoral spring. In: Biology of Chrysomelidae, eds. P. Jolivet, E. Petitpierre & T.H. Hsiao, pp. 285-297. Kluwer, Dordrecht.
Schmitt, M. (2004). Jumping flea beetles: structure and performance (Insecta, Chrysomelidae, Alticinae). In: New developments in the Biology of Chrysomelidae, eds. P. Jolivet, J. Santiago-Blay & M. Schmitt, pp. 161-169. SPB, The Hague.