(Top photo credit: Andy Ross)
I was told to “Stop, don’t step on the beetle!” by a young man, hand up, traffic cop style, kneeling on the pavement in Explore the Wild. It was a late fall day in December (meteorological winter) with temperatures in the high 40s, though a dry northerly wind made it feel colder. The young naturalist spotted a beetle and he and his family were watching it make its way across the path.
As I got closer, the large swollen abdomen suggested some sort of blister beetle, though I didn’t know which one. An internet search quickly narrowed it down to oil beetle in the genus Meloe. The great size of its abdomen pointed towards female, chock full of eggs. And, as I later found out, males have a crook in their antennae with which they grasp a potential mate’s antennae during courtship.
Why the antennal grasping? I don’t know, but perhaps it interferes with the female’s ability to feel what’s going on, maybe it enhances her sense of the male, who knows? But remember, antennae are sense organs allowing a fine-tuned ability to smell and even detect variations in humidity in some insects. I doubt the male’s antennal crook is merely a convenient way for the male to hang on during mating, they’ve got six legs for that.
These beetles can and do excrete an oily substance from their joints which causes blisters on human skin. I let this beetle walk on me, but it apparently didn’t feel threatened, and I didn’t squeeze the insect. It didn’t excrete the liquid. But, I don’t advise anyone else to handle oil or blister beetles, I hear the blisters can be painful.
Ranger Brooke picked the beetle up (with a leaf) and moved it well off the path so as not to get stepped on. Later the same afternoon as I passed by this location I saw what I assumed was the same beetle only three feet from where we encountered it the first time. It had been stepped on. There was a thick yellow substance on either side of the beetle’s abdomen. Again, I assumed it was the toxic oil squeezed out by the impact of being stepped on. Two assumptions, both wrong.
The first assumption proved wrong when yet another beetle, of the same variety, showed up at the very same location three days later. It too was a female, jam-packed with eggs.
The second assumption, that the yellow ooze was the blister causing oil excreted by the beetles, was actually eggs. A closer look at the photos I took of the crushed beetle revealed tiny, yellow, cylindrical objects in the mass next to the beetle, eggs. These eggs would have been deposited in a shallow hole dug by the beetle had it not been halted in its tracks.
I’m not sure of the species although it may be Meloe impressus.
Oil beetles in general have an interesting life cycle. As mentioned above, the female lays her eggs in a shallow hole dug by herself. The larvae that hatch from the eggs have six legs and are know as triungulins. They actively scamper up and down on grass stems or other plants, eventually forming dense clusters.
The larvae begin to emit pheromones which attract male solitary bees, apparently guided by the pheromones thinking the cluster of larvae a female bee. The larvae attach themselves to the bee as he tries to mate with the cluster of beetle larvae. Once the male locates a “real” female bee and begins to mate, the larvae transfer themselves to the female.
Back at the nest, the oil beetle larvae feed on the stored up pollen provided by the female bee for her larvae. Here, they transform into more sedentary, fat, legless grubs; no further need to move about quickly, everything they require is right there for the feasting. Exhausting the supply of pollen, the larvae begin to eat the bee larvae. The beetle larvae pupate and eventually emerge from the bee’s nest as adults.
These beetles do not fly, they have no hindwings. Their forewings, which typically form the hard back of a beetle (elytra) are reduced, barely covering a fourth of the beetle’s abdomen.