Oil Beetles

Oil Beetles mating.

      Oil Beetles (Meloe spp.) are unusual beetles with a fascinating life cycle. Like their Rove Beetle cousins, the typical hardened wing covers (elytra) in beetles are very short, not covering the abdomen. Their names are derived from their habit of exuding a noxious oily substance from the joints of their legs when disturbed. This substance (including a chemical called canthardin) can cause chemical burns and can protect certain species of beetles such as Blister, Soldier, and of course Oil Beetles. The Meloe males are smaller than the females and some of the 22 or so species in our region have a bend to their antennae. The females attract their mates using pheromones this time of year. Adults feed mostly on plants, but the larvae have a very specialized diet and host.
     After hatching, the new born larvae in some species climb atop foliage and exude a pheromone attractive to certain bee species. Some even aggregate together so they look like a female bee. The male bees then arrive and the Oil Beetle larvae cling to their hair before they fly off. When the male finds a real female bee, the Oil Beetle young transfer over. The female bee then constructs her nest and the larval Oil Beetles eat the eggs and pollen stores. The cycle then starts all over again.
     I recently came across several mating Oil Beetles and noticed some much smaller beetles that seemed to be feeding on their larger cousins. What I had stumbled upon were Fire Colored Beetles from the family Pyrochroidae harvesting that blistering agent called canthardin from the Oil Beetles. The males of certain species of these Fire Colored Beetles detect the canthardin using their combed antennae and find it irresistible.

Fire Colored Beetles (Pyrochroidae family) obtaining canthardin from Oil Beetles.

     Some Pyrochronidae males seek out Blister and Oil Beetles to collect the cathardin they exude. Those who collect the most are also the most attractive to females of their own species. When mating with them, the males transfer cathardin along with the sperm packet to the females who chose them. The females then coat the eggs they lay with the blistering agent, thus hopefully protecting them from many predators.

Three male Fire Colored Beetles harvesting cathardin from the much larger Oil Beetles. 

     These beetles display just some examples of the remarkable use of chemicals and pheromones in the animal world. These are not just used for attracting mates, but in Oil Beetles to attract bees to act as hosts. In Fire Colored Beetles, they steal these chemicals from their larger relatives to be more attractive to their own females and then protect their eggs. There are numerous other examples such as these taking place daily in the world around us.
     Here's a video showing the Fire Colored Beetles attached to an Oil Beetle:

https://www.youtube.com/watch?v=R3U9tFmrVeM&t=8s