Growing up in Poland, a country with relatively mild but still not particularly pleasant winters, I was always on the lookout for the first signs of the return of insect life, and one of the most welcome sights in the spring was a beautiful, metallically violet oil beetle (Meloe proscarbeus). I was so used to associating these insects with early spring that when I first saw an oil beetle in the fall in New England I was positively shocked. But unlike their European counterparts, North American oil beetles appear as adults in the August-October, and thus signal not the beginning, but rather the end, of the insect season. Yesterday, while enjoying a walk with our furry children in Estabrook Woods near the town of Concord, MA, my wife and I found several pairs of Meloe impressus, a local oil beetle species.
A couple of interesting facts make oil beetles stand out among their inordinately species-rich relatives. Species of the genus Meloe, along with many related members of the beetle family Meloidae, exhibit a particularly complex life cycle. It starts, innocently enough, with a female digging a hole in the ground and laying her eggs. Lots of them – a single female of certain species may lay up to 10,000 eggs in multiple clutches. Why so many? This is where things start to get interesting.
The eggs take a few weeks to develop, after which time tiny larvae known as triungulins emerge from the sand. They behave like shrews on amphetamines, and run up and down plants, seemingly without any purpose. But after a while the triungulins begin to aggregate and eventually form a dense cluster on a stem or a blade of grass, and the number of these larvae on a single plant can be quite startling. (Some years ago I woke up to see that my bedroom window was suddenly half black: turned out that the glass was absolutely covered with triungulin larvae, which emerged from a forgotten container where I had kept a female oil beetle a few weeks earlier.)
Once clustered on the plant the larvae begin producing waves of pheromones. These have not evolved to attract beetles, but rather male solitary bees, who mistake the clump of triungulins for a female bee. While trying to mate with it they get covered with the larvae and later, once they finally chance upon a real bee female, they pass the larvae onto her like some six-legged venereal disease. She then carries the triungulins to her underground nest, where they first feed on her supplies of pollen, stocked up for her own children, and once the pollen is gone, the oil beetle larvae start chomping on the developing bees. While this is happening, their morphology changes dramatically. No longer are they agile triungulins, but after the first molt they transform into more sedentary, fat grubs. Eventually, having consumed enough pollen and bee larvae, they turn into pupae, and after a while emerge as adults. This life cycle, with an extra larval stage, is known as hypermetamorphosis, and is typical of parasitic insects. It can be found, among others, in eucharitid wasps and strepsipterans, animals whose chances of finding the right host are extremely low, and having thousands of highly mobile larvae increases the odds.
Another interesting thing about oil beetles is that they are deadly toxic. Like all species in their family they are capable of synthesizing cantharidin, one of the most poisonous compounds known to man. In minute doses this substance apparently acts as an aphrodisiac, and one of the meloid beetles, Lytta vesicatoria, is well-known as the Spanish fly – probably the earliest attempt at creating Viagra. But even a slight overdose can kill you, or any other vertebrate, and for this reason oil beetles have few natural enemies. Cantharidin is present in the blood of adult beetles, and they defend themselves through reflexive bleeding from their leg joints. The blood is bright yellow and, if applied to human skin, causes painful blisters (hence the name “blister beetles” often used for this group of insects.) Interestingly, adult females of oil beetles cannot synthesize cantharidin, and must rely on the residual supplies created when they were still in their larval stage and, consequently, their defenses get weaker as they get older. But a single romp can restore their defensive powers – male oil beetles are very good at synthesizing the compound and transfer it to females through a spermatophore during copulation.
Although it may seem that there is nothing particularly good about oil beetles, a recent study (Verma and Prasad, 2012, Cell Biol. Toxicol. 28: 133-147) revealed that cantharidin is an incredibly powerful anticancer agent, and laboratory experiments have shown an 82% increase in lifespan of mice suffering from carcinomas after a treatment with an extract obtained from meloid beetles. As far as I know tests on human cancer have not yet been done, but I will not be surprised if these beetles eventually become powerful allies in our battle against this dreadful disease. Another great argument for saving, and fully appreciating, all members of the natural world.
(You can learn more about oil beetles and see a video of their larval development on Adrian Thysse’s blog, Splendour Awaits.)