Fire-colored Beetle Larvae
Peek under the loose bark of rotting logs, both deciduous and coniferous, and you’re bound to find the larva of a beetle known as a Fire-colored Beetle (family Pyrochroidae). Its common name is derived from the Greek word pyros (“fire”) + chroma (“’color”), a reference to the bright color, often red, of the adults of some species. From one to several years are spent in the larval stage. Adults of the Pyrochroinae emerge from late spring to midsummer and are primarily nocturnal.
Many adult Fire-colored Beetles are attracted to cantharidin, a toxic compound produced by Blister Beetles. Males locate a Blister Beetle, climb onto it and lick off the cantharidin that Blister Beetles exude and use the blistering agent to impress a female of their own species. When mating takes place, most of the cantharidin is transferred to the female in the form of a sperm packet. The eggs the female subsequently lays are coated with cantharidin to protect them from being eaten before they hatch. (Bugguide.net)
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Blister Beetles Mating
Blister beetles derive their name from the fact that they secrete a yellow blood-like substance called hemolymph which contains the blistering compound cantharidin. Severe burns and even poisoning can occur if the quantity encountered is large enough. (Baled hay containing the carcasses of blister beetles can be lethal to livestock that eat it.)
Defense isn’t cantharidin’s only function, however. Cantharidin is secreted by the male blister beetle and given to the female as a copulatory gift during mating. Afterwards, the female beetle covers her eggs with it as a defense against predators.
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