Nemognatha nigripennis - a beetle with an interesting life history.

Adult blister beetle eats and lays eggs on flowers.  Larvae parasitize ground-nesting solitary bees.

Most of us have no idea about the life cycle of an unknown insect - some are relatively simple, but others are far different than what we could imagine.  Blister beetles are one such insect.

All beetles undergo complete metamorphosis, meaning that they proceed from egg to a larval stage quite different from the adult.  The larva undergoes several moults (each stage is called an instar) to finally pupate, where it casts off its last moult (called an ecdysis) and emerges as an adult.  The adult goes on to mate and lay eggs.  Nothing too different about that, is there?

N. nigripennis, the blister beetle shown in the above photo, eats flowers and pollen as an adult.  It also mates and lays eggs on the flowers themselves.  The larva is very mobile and searches for a ground-nesting solitary bee to visit the flower it is on.  Then, it attaches itself to the bee and travels back to its nest, where it is preparing a burrow for the eggs it will lay.  

Then something strange happens.  The larva, in its first instar, moults into its second instar stage and becomes a legless, grub-like form.  The grub now feeds exclusively on the larder and the eggs prepared by the host bee.  The chamber has been sealed up by this point, with the bee leaving its brood to develop; instead, the blister beetle larva consumes the cell's contents, eggs included.  Several moults later, the adult emerges to start the cycle over again.

Blister beetles produce a chemical called cantharidin.  Most of them deploy this blistering agent by releasing hemolymph from leg joints or sometimes antennae in a process called joint bleeding.  Their blood contains cantharidin, which, on contact with another organism, produces significant blistering.  The same toxin is released if the insect is crushed or eaten.  Consuming cantharidin can be fatal.  Farm animals that eat fodder infected with adult blister beetles are at risk of dying - horses are particularly susceptible.  Dried hay or alfalfa containing blister beetles can kill horses if they eat three to four dozen of the insects.

Cantharidin is harvested for medical use and used in the treatment of warts and benign skin growths.  The old name for the chemical was Spanish fly, its use has been discontinued due to the adverse reactions, especially if taken internally.

Thanks for reading.

Eric Svendsen     www.ericspix.com