A week or two ago, Dan discovered a large, shiny blue beetle crawling through the grass on our lawn and called me out to see it. Even before investigating I had a fair idea of what it was: a blister beetle, of the genus Meloe. This isn’t the first one I’ve seen, and it isn’t even the first one Dan’s found for me – last fall he brought me a male one. The one above is a female; you can tell the difference by the antennae, since the males have a U-shaped kink in theirs, which they use for grasping the female during mating, and those of the female are straight. Females also have larger abdomens. Blister beetles are so named for the liquid they exude from their joints when startled or threatened, which contains a chemical called cantharidin. There are some third-party beetle species which will collect the cantharidin from blister beetles and use it as a sperm additive (putting it into the sperm packet they transfer to the female); when she lays her eggs, the eggs are coated in this protective chemical. Humans use the chemical too, but for less honest purposes – it’s used in making the aphrodisiac/date-rape drug “Spanish Fly”. I wrote more about blister beetles when I found my first one; you can read a bit more here.
Right. So the purpose of yesterday’s post and all those headstones wasn’t actually to ramble on about life and death, but actually to introduce this guy, who I stumbled across in the grass and pine needles among the stones as I was slowly meandering through the cemetery. Its gleaming blue sheen caught my eye as it crawled through the obstacles. I knew what it was, so I coaxed it onto an oak leaf and gingerly moved it to the top of a stump where I could get better photos.
It’s a blister beetle, in the genus Meloe (oil beetles). There are 22 species in the genus, but they’re difficult to tell apart. All but one occur only in North America. Blister beetles get their name from their characteristic of exuding a skin-blistering chemical when they’re squeezed or unduly disturbed, a behaviour called “reflex bleeding”. The chemical, cantharidin, is contained in their haemolymph, or blood, and is exuded from pores at their joints. Members of the genus are generally quite plump, and flightless, though they have shortened, residual wing-covers.
You’d think that would be interesting enough, but even more so is that individuals go through multiple larval life stages (not just instars, but actual different types of larva), more than the average insect, which is called “hypermetamorphosis”. Eggs are laid on vegetation, and when the larvae, called triungulins at this stage, hatch they crawl up flower stems to wait on flowers for a passing bee. They hitch a ride on a female bee when she comes around, and are taken back to the burrow she’s created for her own eggs and larva(e). There, the triungulin moults into a grub and sits and consumes the pollen and nectar that the female bee brings to provision her own larva, and then eat the bee’s larva itself once it hatches from its egg. Each Meloe species generally targets a single species of bee, and while it will eat the larva, it can, if necessary, survive just on the pollen and nectar and so doesn’t qualify as a true parasitoid.
BugGuide.net also adds, “In at least one Meloe species, the larvae climb to the top of a grass or weed stalk as a group, clump together in the shape of a female solitary ground bee, exude a scent that is the same as, or closely resembles, the pheromones of the female bee, and wait for a male ground bee to come along. When he does, he tries to mate with the clump of larvae, whereupon they individually clamp onto his hairs. He then flies away, finds and mates with one or several real female bees, and the larvae transfer to the female(s).”
I presume this to be a female, both because she’s exceptionally plump, but also because her antennae are straight. Males have little hooks in the middle of their antennae which they use to grasp the female when mating.
Among other uses for the blister beetle – the males of some beetle species “collect” the cantharidin from the blister beetles and use it to impress females, and then includes the chemical in the sperm packet that’s transferred to the female. When she lays her eggs, they’re coated in the chemical, which protects them from predators. The chemical has also been used by humans for centuries in the production of the aphrodisiac “Spanish Fly”, frequently used nowadays as a date rape drug.