The Cantharidae, commonly called soldier beetles or leatherwings, are soft-bodied, often brightly colored insects known for their aggregating on flowers. They are distributed worldwide, with some 5,000 species in 135 genera. They are slow, lumbering fliers and easily caught; not surprisingly, they have chemical defenses which make up for these apparent evolutionary shortcomings.

Unlike many other beetles, which have a pair of defensive glands at the tip of their abdomen, the soldier beetles have paired glands in the prothorax and on each of their first eight abdominal segments. When molested, the beetle emits droplets of white viscous fluid from pores along their sides. Studies have shown two species of soldiers, Chauliognathus pennsylvanicus and Chauliognathus lecontei, are consistently rejected as prey by birds, mice, other beetles, ants, and jumping spiders. Mantids, assassin bugs, centipedes, and solpugids also avoid them. Chemical analysis has shown the secretion in both beetles to contain (Z)-dihydromatricaria acid, an acetylenic compound. [1]

Blister beetles also produce cantharidin. Stored in the insects' blood, the compound is stable and remains toxic even in carcasses. Animals may be poisoned by ingesting beetles while grazing or eating harvested silage. Cantharidin can also cause severe skin inflammation and blisters.

Cantharidin is absorbed through the intestine and can cause symptoms such as inflammation, colic, straining, elevated temperature, depression, increased heart rate and respiration, dehydration, sweating, and diarrhea. There is frequent urination during the first 24 hours after ingestion, accompanied by inflammation of the urinary tract. This irritation may also result in secondary infection and bleeding.  Taken internally, as little as 10 milligrams can be fatal in humans.

The concentration of cantharidin in adult beetles depends primarily on the sex; males produce the chemical and only pass on small amounts to the females during mating. Cantharidin amounts also depend on species; the striped blister beetle has approximately five times more catharidin than the black variety. In one species, Méloé proscarabaeus, cantharidin makes up fully 1/4 of the insect's blood.

There are other insects, including some flies and bugs that eat live or dead blister beetles to obtain the protective qualities of this chemical defense; these so-called cantharidinophilous insects have acquired immunity from the chemical and remain unharmed. [3]

Male fire-colored beetles in the family Pyrochroidae are known to climb onto blister beetles and ingest the cantharidin exuded by the insect. Completely immune to the effects of the blistering agent, they use the chemical to attract females, who become the recipients of a cantharidin-laden sperm packet with which they coat their eggs.

The process of uncovering and unfurling the flying wings takes time, and this interval sometimes proves fatal for beetles intent on escaping a predator. It is thought the chemical defenses help ameliorate this evolutionary disadvantage.