There's Something on Your Back (Taxon of the Week: Prioninae)

The Palo Verde borer, Derobrachus hovorei, a member of the Prioninae from North America. Photo by Alex Wild.

A brief respite from Amoebozoa to present the new Taxon of the Week, the beetle subfamily Prioninae.

The Prioninae are a subgroup of the Cerambycidae, the longicorn beetles. The common name refers to the elongate, back-swept antennae that are a feature of most members of this family. Cerambycidae are a simply huge family - over 20,000 species have been described, of which about 700 belong to the Prioninae (Bílý & Mehl, 1989). The most familiar longicorns are large beetles, but longicorns come in all sizes from the very large to the very small. Larvae of most longicorns live in and feed on dead wood (females may oviposit in live wood; the process of oviposition kills off the oviposition site and rot spreads through the tree allowing the larva to feed, making some of these species serious horticultural pests), and can take as long as two years to develop. Adults, in contrast, are relatively short-lived.

The most obvious feature distinguishing Prioninae from other longicorns is the presence of a sharp lateral keel on either side of the pronotum (the anterior shield of the thorax). Prioninae are large longicorns, dark in colour and crepuscular or nocturnal in habits. Adult Prioninae probably don't feed (Willemstein, 1987), but they nevertheless possess impressive jaws with which the males engage in vicious battles to win females. The larvae are generally polyphagous (that is, they're not particularly picky over exactly what type of wood they're eating), though some exceptions occur, and oviposition by the females is usually little more complicated than pushing the eggs into already rotting wood (Bílý & Mehl, 1989).


Witchety grubs, dressed for the table. Photo by David Hancock.

Among the better-known members of the Prioninae are the witchety grubs of the genus Cnemoplites which were eaten by Australian Aborigines (Lawrence & Britton, 1991) - still are, in fact, by those who have the nous to know where to find them (I've tried them on one occasion, cooked in wood ash. To be honest, I thought they tasted a bit like snot, but the woman who had brought them assured us that they were wonderful when spread across bread in lieu of butter). The most notorious of all Prioninae, however, is probably the rather self-explanatorily named Titanus giganteus, the Titan beetle of northern Amazonia. At twenty centimetres in length, Titanus is one of the largest of all insects - technically, the Goliath beetles of the scarabaeid genus Goliathus are larger, but for some reason - probably their more compact build and less prominent mandibles - Goliath beetles don't seem quite so bowel-openingly intimidating as Titanus. Titan beetles are a rare sight even within their native range (outside Amazonia, they have only ever been recorded as specialised parasites on time-travelling comediennes) which may just be all for the best; I suspect that even an avid entomophile like myself would be hard-pressed not to go into a blind panic and start screaming like a little girl if one of those suckers started crawling up my arm.


Titanus giganteus, the largest of the Prioninae. Photo by Bruno Ramos.

David Attenborough, who is seemingly a braver man than I, did handle a specimen of Titanus giganteus on an episode of Life in the Undergrowth. In that episode, Attenborough commented on the point that the larval stage of Titanus has not yet been conclusively identified. It might seem unusual that something as doubtlessly impressive as a Titanus grub would be should go unnoticed, but when you consider the concealed habitat of the larvae, the short lives of the adults, and the extreme difficulty of identifying a holometabolous larva with its corresponding adult, this situation becomes much less surprising. The words of Francis Pascoe, commenting in 1866 on a collection of longicorns from Penang in Malaysia, are just as appropriate today as they were 140 years ago:

If we consider that the Longicorns in their perfect [i.e. adult] state are generally short-lived, and that a great majority of the species frequent particular plants or families of plants, so that only where these plants occur can we expect to find the insects, it will be readily understood how this limited range and brief existence make it almost impossible for any collector to obtain more than a portion of those that inhabit even a moderately extensive district. And thus it is that sometimes perhaps half the species of a large collection are represented each by one or two individuals only. The number of species, therefore, and the many superb novelties which Mr. Lamb has had the good fortune to capture, whilst it excites our imagination, shows us how much more might be expected if all those rich tropical lands were as thoroughly worked by entomologists as Europe has been.

REFERENCES

Bílý, S., & O. Mehl. 1989. Longhorn beetles (Coleoptera, Cerambycidae) of Fennoscandia and Denmark. Fauna Entomologica Scandinavica 22. E. J. Brill.

Lawrence, J. F., & E. B. Britton. 1991. Coleoptera. In The Insects of Australia vol. II (CSIRO, ed.) pp. 543-683. Melbourne University Press.

Pascoe, F. P. 1866. Catalogue of longicorn Coleoptera collected in the island of Penang by James Lamb, Esq. Part I. Proceedings of the Zoological Society of London 1866: 222-267.

Willemstein, S. C. 1987. An Ecological Basis for Pollination Ecology. E. J. Brill.