|Soldier Beetle – Trypherus frisoni|
Order Coleoptera / Family Cantahridae
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Live adult beetles photographed at Ogle and DuPage Counties, Illinois.
These charming, ethereal beetles are numerous in the forest understory at White Pines State Park near Oregon, Illinois.
Members of the approximately 3,500 species of the widely distributed insect family Cantharidae are commonly known as soldier beetles or leatherwings. It is thought the brightly-colored elytra (hardened outer wings that cover the soft flying wings), often striped or outlined in black, resemble soldiers' uniforms; the leathery appearance accounts for the other less common moniker.
There are 16 genera and 455 species of Cantharidae in North America. Adults are abundant on flowers and foliage where they feed on nectar, pollen, or other insects. Larvae of most species are carnivorous, a few species feed on plants.
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. 
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, MÃ©loÃ© 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 are immunite to the chemical and remain unharmed. 
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.
Order Coleoptera: Beetles are the dominant form of life on earth: one of every five living species is a beetle. Coleoptera is the largest order in the animal kingdom, containing a third of all insect species. There are about 400,000 known species worldwide, ~30,000 of which live in North America. Beetles live in nearly every habitat, and for every kind of food, there's probably a beetle species that eats it.
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