With their slow movements, distended abdomens and their propensity for grass, oil beetles will forgive me for thinking that they are the insect equivalent of cows. Indeed for the short period that this specimen was kept in a pill bottle, it managed to eject a green liquid mass of partially digested plant bits (out of which end I did not observe) that would best be described as cud. But there is a more fascinating side to these grass-grazing beetles that belies their placid and lumbering bovinity — they secrete poison from the leg joints, the males have grappling hooks on the antennae, and the children are ruthless killers.
Oil beetles are so named because, when physically disturbed, they exude oily droplets of hemolymph from their joints. This secretion contains cantharidin, a poisonous chemical that can cause a painful blistering (hence sometimes called ‘blister beetles’) on the skin. Certainly a handy defence when you are a fat, flightless beetle.
The lifecycle is an example of hypermetamorphosis, a variety of complete metamorphosis (holometabolism) with several distinct larval stages. After hatching, the larvae, called triungulins, actively climb plants to seek out a flower, where they will transfer to visiting bees. In Meloe franciscanus, the bees will cluster on a stem and secrete pheromones that attract male ground bees. When the male tries to mate with the cluster, the triungulins scramble aboard. The male then moves on to mate with other real female bees, and the triungulins can then transfer to those females. Now fertilized and loaded with larvae, she will carry the triungulins back to the nest, where the little blighters†, going through four instars, will proceed to consume the complete contents of the cell that she constructs and provisions, including the larvae. See the segment from Life in the Undergrowth below for the details of how some blister beetle triungulins do their work. (This is probably Meloe franciscanus: see http://www.pnas.org/content/103/38/14039.full)
And the grappling-hook antennae? Male oil beetles use the hooks on the antennae to latch onto the females antennae during courtship. See an image of the antennae in use at BugGuide.
And don’t you think this would be a good choice for Alberta’s Provincial Beetle? 😉
More oil beetle behaviour can be seen at Alberta Oil (Beetle).
For more on oil beetles see page 19 of: Alkali Bees. Their Biology and Management for Alfalfa Seed Production in the Pacific Northwest.
Thanks to BugGuide for the ID
(Photographed 25 May, 2012. Grasslands National Park, Saskatchewan)
†Not an entomological term, although it should be…