break in the skin or puncture caused by an insect and complicated by introduction into the skin of the insect's saliva, venom, or excretory products. Specific components of these substances are believed to give rise to an allergic reaction, which in turn produces skin lesions that may vary from a small itching wheal, or slightly elevated area of the skin, to large areas of inflamed skin covered by vesicles and crusted lesions. This article encompasses similar wounds inflicted by other small invertebrates, particularly arachnids (spiders, scorpions, ticks, mites, and their allies).

Flying insects, such as flies, gnats, and mosquitoes, attack exposed body parts, each bite resulting in a single itchy wheal that generally subsides within hours. Crawling insects may reach any part of the body, including the covered areas, and are more likely to remain there, generating skin diseases characteristic of each insect. Scabies, or sarcoptic itch, designates the skin inflammation brought about by the itch mite, Sarcoptes scabiei. The female mite burrows beneath the superficial layer of the skin to lay its eggs in a tunnel that can be seen as a dark wavy line. This initial lesion becomes intensely itchy after a few days to about a month, and the scratching leads to secondary skin lesions consisting of papules (solid elevations), pustules, and crusted skin areas. The itchiness is believed to be caused by the accumulation of fecal deposits by the mite in the burrow region. Scabies is most commonly noted on the webs between the fingers, other frequent locations being the natural folds of the skin and pressure areas.

Pediculosis is the skin disorder caused by various species of bloodsucking lice that infect the scalp, groin, and body. The lice live on or close to the skin and attach their eggs to the hair or clothing of the host, on which they periodically feed. Their bite results in a small red spot that is extremely itchy and may become infected after repeated scratching. Chiggers, the larvae of certain mites, also live on humans and feed on blood. Their bite produces a wheal on the skin that is intensely itchy, the itchiness being caused by the digestive juices of the chiggers. Other bloodsucking insects that feed on humans are fleas, bedbugs, and ticks, which do not live on humans as primary hosts but in the ground, bedding, walls, and furniture; the more commonly seen lesions are those of the bedbug, which produces a burning wheal with a central punctured dot, and those of the flea, which produces a cluster of wheals and papules, resulting from the flea's habit of sampling several adjacent spots while feeding on the skin.

Stinging insects produce a painful swelling of the skin, the severity of the lesion varying according to the location of the sting and the identity of the insect. Many species of bees and wasps have two poison glands, one gland secreting a toxin in which formic acid is one recognized constituent, and the other secreting an alkaline neurotoxin; acting independently, each toxin is rather mild, but when they are injected together through the stinger, the combination has strong irritating properties. In a small number of cases the second occasion of a bee or wasp sting causes a severe allergic reaction known as anaphylaxis.

Hornets, some ants, centipedes, scorpions, and spiders also sting. Some insects leave their stinger in the wound. Multiple stings may give rise to severe systemic symptoms and in rare instances may even lead to death; the bites of some spiders are known to be lethal, particularly to young children.