Life History

All beetles exhibit holometabolous development. Eggs are laid singly or in clusters on or in soil, living or dead plant matter, fabrics, water, and carrion and, rarely, on living animals. The larvae of most beetles have a distinct head with simple eyes (ocelli) and chewing, mandibulate mouthparts, and the abdomen has 8 to 10 segments. Beetle larvae exhibit diverse morphological types, from elongate-flattened forms (campodeiform) to cylindrical-flattened forms (elateriform), caterpillar-like forms (erueiform), and somewhat C-shaped soft forms {searabaeiform). The larval body type usually is consistent in a particular family of beetles. In a few families, however, the larval form may vary from instar to instar in a given species, a life history progression called hypermetamorpho-sis. Certain blister beetle larvae, including scavengers in bee nests and ectoparasites or endoparasites of other insects, are hypermetamorphic. They emerge from the eggs as active campodeiform larvae, then molt into erueiform and searabaeiform stages. Most beetle larvae molt at least three times before transforming into pupae.

Although most temperate species undergo only one generation a year, species in warmer climates are often multivoltine. Depending upon the species, any developmental stage may overwinter, but overwintering most often occurs in the pupal or adult stage. Most species exhibit diapause in one or another stage, and those that have developmental cycles exceeding one warm season usually have an obligatory diapause, initiated by changes in photoperiod and temperature. Most adult beetles live for weeks to as long as a year. However, adults of some species may live for years, spending much of their lives in diapause during periods when food is scarce.

Beetles live within all terrestrial and freshwater habitats. The great variation in beetle feeding behavior, whether saprophagous, herbivorous, carnivorous, or omnivorous, reflects the extremely diverse habitats in which these insects live. However, their mouthparts play a minor role in causing discomfort to humans and other animals. Beetle defense mechanisms, which involve the shedding or secretion of physically or chemically irritating materials, and beetle behavior that puts the insects in contact with developmental stages of parasitic helminths and vertebrates can lead to public health and veterinary problems.

Larder or pantry beetles (Dermestidae) are ubiquitous in human and domestic animal environments, where the larval and adult beetles eat stored food, food debris, dead insects, and other organic matter. Setae that cause human skin irritation or act as respiratory allergens are loosely affixed to the larvae of many species. The setae are elaborately barbed so that their firm adherence to many substrates, including human skin, causes them to be dislodged from the crawling, living larvae. In some species, the larvae actively raise the abdomen and make striking movements in response to touch. Other active defensive behaviors are seen in blister beetles, some chrysomelid plant beetles, long-horned beetles, and lady beetles that exude irritant chemicals from the femoro-tibial joints of the legs or from glandular openings around the mouthparts when the beetles are handled or threatened. This reflex bleeding repels predators. One of the most dramatic defensive maneuvers is the explosion of boiling hot, acrid quinones from the anal glands of carabid beetles called bombardier beetles. These forceful expulsions, which are aimed with extreme accuracy at potential predators, cause minimal damage to humans and other large animals but can cause physical and chemical burns in insects and small vertebrates (Evans 1975).

Most of the beetles that serve as intermediate hosts of helminths parasitic in domestic animals and humans are grain or dung feeders. These species ingest helminth eggs present in animal feces or fecal-contaminated food. Because of the proximity of the beetles to feeding animals, whole adult beetles are often incidentally ingested by potential vertebrate hosts.

The tendency of many beetles to fly to artificial lights puts them in contact with human and domestic animal

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