Hydroglyphus pusillus, the Tiny Tiger

Hydroglyphus pusillus, copyright Udo Schmidt.

Let's take another visit to the world of diving beetles. Above is Hydroglyphus pusillus, one of the few representatives in northern Europe of a genus that otherwise includes close to ninety species spread through the Old World, primarily in the tropics. Hydroglyphus species are tiny diving beetles, only about two or three millimetres in length, with an elongate oval body shape. Characteristic features of the genus include basal striae on the pronotum and elytra, sutural striae on the elytra, and no transverse stria on the top of the head (Watts 1978, as Guignotus, a subsequently synonymised name). Species are often marked with distinctive colour patterns of streaks and blotches.

Hydroglyphus pusillus attacking larva of mosquito Culex pipiens, from Bellini et al. (2000).

Despite their small size, Hydroglyphus species are (like other diving beetles) voracious predators of other aquatic insects. Bellini et al. (2000) investigated the possible role of H. pusillus in controlling mosquito larvae in flooded rice fields in Italy. The larvae of H. pusillus mostly kept to the bottom sediment (so might be expected to be hunting prey other than mosquitoes) but adults were the most abundant diving beetle in the water column at the surveyed locations. One might expect that H. pusillus would not be effective predators of mosquito larvae that greatly outsized them. One would be wrong: not only are they indeed capable of taking down mosquitoes, Bellini et al. went so far as to describe their effects as "a real slaughter". A diving beetle latching onto a mosquito larva would soon find itself joined by others seemingly scenting haemolymph in the water. Between them, this mob of beetles could destroy a larva in a matter of seconds. Tiny, but terrifying.

REFERENCES

Bellini, R., F. Pederzani, R. Pilani, R. Veronesi & S. Maini. 2000. Hydroglyphus pusillus (Fabricius) (Coleoptera Dytiscidae): its role as a mosquito larvae predator in rice fields. Boll. Ist. Ent. "G. Grandi" Univ. Bologna 54: 155–163.

Watts, C. H. S. 1978. A revision of the Australian Dytiscidae. Australian Journal of Zoology, Supplementary Series 57: 1-166.