Our Insect of the Week is Lydomorphus sp. of the beetle family Meloidae. Meloidae is one of the most interesting Coleopteran families. The great majority of meloid species are poisonous and advertise this to potential predators, warning them away with a display of aposematic colouration, mostly on their abdomens which are often decorated with spots or larger circles, or with lengthwise or transverse stripes of various shades of yellow, orange or red. In contrast, the orange or red colouration of Lydomorphus species is limited to the head and pronotum, the abdomen being black. Lydomorphus species are sometimes referred to as redneck blister beetles.
Meloid species differ from nearly all other beetles in having soft and pliable elytra (the modified forewings of beetles). Other beetles usually have a strongly reinforced exoskeleton meant to protect them from attacks by predators. Meloidae, though, have evolved a completely different strategy in matters of individual security. Male meloids, but not females, synthesize the noxious toxin Cantharidin. Females acquire Cantharidin from the males during copulation. Experiments have demonstrated that predators such as reptiles and large insects are repelled by the toxin. Presumably, in Meloidae the metabolic investment required to produce hard elytra is made unnecessary, and molecular resources can be directed elsewhere during the beetle’s life cycle. Cantharidin is present in the meloid’s hemolymph. When threatened, it is secreted from the base of the legs in a process called reflex bleeding.
Although not a serious health threat to humans, cantharidin in contact with skin acts as a vesicant producing small to large blisters (hence the common name) at the point of contact with the beetle, as happens with Nairobi fly (Paederus sp. [Insect of the week 66]). Crushing a meloid against the skin magnifies the effect. Blisters develop within a day or so, leaving no long-term disfigurement (but potentially ruining a prom date).
Cantharidin has been of cultural value among diverse peoples. It has many pharmaceutical applications, the most notorious being its use as an aphrodisiac, known worldwide as Spanish Fly. This reputation is based on the occurrence in men of priapism (predating the blue pill) following Cantharidin ingestion (not recommended). There are reports of horses dying after ingesting meloids while grazing in grassy or herbaceous areas with dense meloid populations.
Léon Marc Herminie Fairmaire (1820–1906) described the genus Lydomorphus Fairmaire, in 1882. Fairmaire was a prolific French entomologist, and an expert in the Coleopteran fauna. Over his working life, Fairmaire published about 450 scientific papers. Perhaps the most important insect described by Fairmaire is the emerald ash borer, Agrilus planipennis, a jewel beetle of the family Buprestidae, indigenous to Asia but invasive in the United States and elsewhere, wreaking havoc among ash trees (Fraxinus spp.). Without the natural enemies that keep it under control in Asia, the borer quickly reaches epidemic levels elsewhere. Much of the ash forest in the United States has been destroyed by this insect.
Note: Of all the genera of animals in the world, Agrilus contains the most species (ca. 3000)!