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Tuesday, 19 February 2013

Beetles & Bugs 1: Purple Jewel Beetle

S.africana oertzeni
According to legend, when the great biologist JBS Haldane was once asked what his studies had taught him about God, he replied that he had an inordinate fondness for beetles. There are actually several variants of the quote, largely because JBS repeated the story on numerous occasions (it was too good to throw away). Personally, I take issue with the inordinate part, as beetles represent the largest of all insect groups and have diversified immensely in size, habitat, and lifestyle, with in all probability well over 400,000 different species. Just because people mostly ignore them (unless they eat our crops) is no reason God has to share our prejudice.


Their diversity is a feature of their age. The oldest known beetles date back to before the Permian, well over 300 million years ago, and have become ever more diverse since then. One especial boost to their diversity was the evolution of flowering plants, and many species feed on them either as larvae or as adults.

Beetles and bugs are often confused, but they are not at all closely related. Bugs belong to the superorder Exopterygota (“external wings”) and their eggs hatch into nymphs, tiny versions of the adult, which develop wings progressively as they grow until the adult form is reached. Beetles belong to the Endopterygota, and have a life cycle the same as true flies, butterflies, caddisflies and others, with a life cycle of egg, larva, pupa, and finally adult.

At various times we have had a variety of beetles on show at Bristol, but currently the first ones you will meet as you enter Bug World are a case displaying Purple Jewel Beetles, Smaragdesthes africana. These originate from tropical Africa, and occur in numerous colour varieties over their vast range. Related species range into South Africa, and north into sub-Saharan Africa where suitable habitat can be found.

S.africana africana
The eggs are laid in the soil or leaf litter, and hatch into white grubs which feed on rotting wood, other plant material, and anything else nutritious in their environment, eventually forming a cocoon out of silk which incorporates dead wood. How fast they develop depends on temperature, but around 5 months is probably typical. Adults will live around 3 months. With that development rate, in the wild they probably have one or two generations a year. Whether they go into diapause at any point in their life cycle does not seem to be recorded – in captivity at least it seems they do not.

When they emerge, adults seek out ripe fruit to feed on, as do many of their relatives. I have not found a lot of observations on their behaviour, despite them being widely kept, but males probably fight over access to females. Any contests seem to be fairly minor however, as unlike their larger relatives the Rhinoceros and Goliath Beetles they do not have extravagant weaponry to contest for mates. It is likely that males simply find females when feeding, but it is possible that pheromones may also be involved.

One of the reasons we have Smaragdesthes on show is that these beetles are diurnal and active, so they make a good display. They are quite widely available in the pet trade in the UK at least as well, and are fairly easy to care for. A simple plastic tank, half-filled with a damp mixture of dead leaves and rotting wood, with some additional dry food with more protein (dog biscuits are often used) are all that is needed to raise the larvae. Adults need sunlight or a lamp to show off their iridescent colours, but be warned – they fly well so their tank needs to be covered.

(images from wikipedia)

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