Coleoptera(redirected from coleopterans)
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beetles and weevils, the largest order of in-sects. Coleopterans are characterized by the modification of the first pair of wings into rigid wing casings (elytra) that cover and protect the second, or flying, pair of wings and the soft upper side of the abdomen. The prothorax and the head form the anterior section of the body articulated movably with the posterior section, which is covered by the elytra. The mandibles are suitable for chewing. Coleoptera develop with complete metamorphosis. Larvae have a developed head and mandibles. Almost all Coleoptera have legs. Pupae are usually exarate, soft, and ordinarily white.
Structure. The body dimensions vary from 0.3 mm (family Ptiliidae) to 150 mm (the Hercules beetle of the family Scarabaeidae). The body structure is diverse among adult Coleopterans because they have adapted to various living conditions. The head bears organs of smell and touch (antennae and palps) and of vision—compound eyes (often under-developed in cave species and inhabitants of the forest floor) or, rarely, simple eyes (in Dermestidae, or hide beetles). The antennae vary in shape but principally have 11 segments (the minimum number of segments is two, the maximum 40).
The mandibles are most generally homotypic. Sometimes, however, they are specialized—for example, the upper jaws of the male stag beetles (Lucanidae) serve as weapons during the mating period, and the lower jaws in some oily blister beetles (Meloidae) Jiave become a snout.
The thoracic segments bear well-developed legs; the terminal section of the leg is a tarsus, five-segmented in the basic type. Often the number of segments is reduced to four, sometimes to three or even two. The structure of the legs is extremely diverse; most often they are gressorial or cursorial. Sometimes the front legs are fossorial with strong dentes on the tibia (in the scarab beetle) or prehensile (in the males of certain water beetles). The posterior legs are often natatorial (in true water beetles, Dytiscidae) or saltatorial (in flea beetles).
The metathorax is immovably joined to the mesothorax, and is completely covered on top by the elytra; the mesothorax usually emerges between the bases of the elytra in the form of a scutellum. The elytra in the majority of Coleopterans completely cover the abdomen on top, but in a number of groups are shortened and leave visible two to five segments of the abdomen (rove beetles); rarely, the elytra are completely absent. Only the posterior wings are used in flight. They fold not only longitudinally but also transversely, and they retract under the elytra. The venation of the wings is quite simple, and the type of venation is significant in classifying Coleopterans. The posterior wings are underdeveloped in many beetles and weevils. The abdomen consists of nine segments, of which four to six are visible externally. The anatomy is of the usual type for insects.
Development and way of life. Coleopterans are always dioecious and almost always oviparous; viviparity is found very rarely (in some Chrysomelidae and the Staphylinidae). Parthenogenesis is characteristic of some Tenebrionidae, Chrysomelidae, and especially Curculionidae. The majority of Coleopterans develop in four phases: egg, larva, pupa, and imago. But in some families hypermetamorphosis is known, with six or more phases. The life span of the imago in Coleopterans is most often one to three months, but sometimes is only a few days. If the young beetles hibernate, the life span is six to ten months, rarely more than one year.
The larvae that inhabit the fruits and leaves of plants, dung, and carcasses develop for one to four months, and those that live in soil or wood often develop for one year or more. The pupa phase lasts from several days to one month, rarely longer. In temperate latitudes Coleopterans usually have one generation per year; rarely there are several (in some Chrysomelidae and Coccinellidae). As a rule the principal nutrition of the individual, which ensures its growth and physiological maturity, occurs during the larval phase; however, the imago often needs supplementary nutrition. The larvae are of two principal types. The campodeiform have a well-separated head, a body with compact coverings, and legs of five to six segments, and they are usually free-moving, motile, and often predacious. Eruciform larvae are thick and fleshy, have short legs or no legs, have soft white coverings, are not very motile, usually live in hiding, and are phytophages or saprophages. There are many transitional forms.
There are a number of biological groups among Coleopterans. Three principal groups may be delineated according to the type of nutrition: plant eaters (phytophages); saprophages, which feed on the decomposing tissues or carcasses of plants and animals; and predators.
The phytophages, in their turn, are subdivided into two groups: Coleopterans that feed on the green parts of plants (phyllophages), for example, many Chrysomelidae, some Curculionidae, and the Melolonthinae; and Coleopterans whose larvae inhabit the internal tissues of herbaceous or ligneous plants, for example, the majority of Curculionidae, the Bruchidae, many Scolytidae, the Buprestidae, and the Cerambycidae. A very large number of Coleopteran species develop in dead wood, comprising a transition from phytophages to saprophages (many Cerambycidae, the Buprestidae, and the Scolytidae).
The saprophages form three principal groups: Coleopterans that require plant remains, for example, many Tenebrionidae, the larvae of many Scarabaeidae, and the Elateridae; coprophages, which feed on the dung of vertebrate animals, such as the Coprinae; and necrophages, which eat carcasses or animal remains, for example the Silphidae and Dermestidae.
The predatory Coleopterans—for example, Carabidae, Staphylinidae, Hysteridae, and Cleridae—feed on insects and their larvae, earthworms, and terrestrial mollusks; they catch their prey on the surfaces of soil and plants as well as in the soil or in passages made in wood by wood-eating insects.
Coleopterans may live in different conditions during various phases of their development—for example, the larvae of Melolonthinae live in soil, while the imago is free-living and feeds on blossoms or leaves. In terms of their relationship to the soil, Coleopterans may be classified as psammophiles (sand-lovers), inhabitants of salt-marshes or of marsh-ridden soils, and so on. Inhabitants of nests and burrows or vertebrate animals and of caves (some Carabidae and others) are classified separately.
Parasitism is quite rare among Coleopterans; the larvae of Meloidae and Rhipiphoridae parasitize clumps of locust eggs, the nests of solitary bees and wasps, and cockroaches; the larvae of some Carabidae parasitize the pupae of other beetles. A few Coleoptera are semiparasites of vertebrates in the imago stage—for example, the beaver flea of the beaver and some South American Staphylinidae of rodents. A considerable number of Coleopterans live as * ‘boarders” in the nests of ants and termites. Aquatic beetles and their larvae are mainly predators (Dytiscidae, Gyrinidae, and the large Hydrophilidae), more rarely saprophages or phytophages (Donaciinae).
The pigmentation of Coleopterans is to a considerable degree connected with their biology. Beetles active at night usually have dark coloring. Cave beetles and many soil beetles are often completely devoid of pigment, having a pale yellow color, which is determined not only by pigments but often also by special structural coverings, which create so-called optical pigmentation of metallic colors. Luminescence is known in fireflies (Lampyridae) and in certain tropical Elateridae (Pyrophorus).
Coleopterans have diverse means of defense against enemies. They include agility (Carabidae), leaping (flea beetles), and rapid takeoff (Cicindelidae, many Buprestidae). Some fall from plants and undergo thanatosis, that is the beetles remain immobile with their extremities tucked away for a long time—they “feign death.” Some have so-called cryptic (camouflage) colors; others spew out caustic or odoriferous liquids, which are sometimes capable (in bombardier beetles) of instantaneously volatilizing with an “explosion.”
Care of the young is often expressed in Coleopterans in the form of preparation of stores for the larvae—dung (dung beetles), leaves wrapped as packages (Attelabidae), and other foods. In the tropical family Passalidae the parents feed the larvae with a gruel of wood pulp, which is first ground and processed with gland secretions. In Passalidae, many dung beetles, and some others, the females and males both provide food for their offspring. (In other insect orders only the females care for the young.)
Distribution. Coleopterans populate all dry land areas except Antarctica, the glacial zone of the arctic, and the highest mountains; they are especially well represented in the tropics. In the USSR the largest number of species live in the broad-leaved forests of the European part, the Caucasus, and the Far East. The specifics of the composition of Coleopteran fauna are significant in determining the character of zoogeographic regions.
Paleontology. The oldest remains of Coleopterans are known from the Lower Permian deposits; many extant families were already represented in the Jurassic. The principal formation of existing groups of Coleoptera apparently occurred in the Lower Cretaceous period, when present-day flora was also formed. Many present-day genera existed in the Paleogene.
Economic significance. There are many pests of fields, forests, and food stores among Coleoptera. These may be divided into the following groups, according to the character of the damage caused by the beetles and their larvae: leaf beetles, which damage the green parts of plants (many Chrysomelidae, the Curculionidae, the June beetles); pests of fruits and seeds (Attelabidae, Bruchidae); trunk pests, which develop in wood and under bark (Buprestidae, Cerambycidae, Scolytidae); soil pests (mainly larvae of Melolonthinae; the Elateridae, or wireworms; and the Tenebrionidae, or false wireworms; and pests of food stores (grain weevil, flour beetles, and Dermestidae, or hide beetles).
At the same time, there are many beneficial species among Coleoptera. The most important are the ladybird beetles (Coccinellidae), which are used successfully in the control of worms, scale insects, and aphids; tiger and ground beetles (Carabidae), which destroy the caterpillars and larvae of other insects, slugs, and other pests; and predacious beetles, which hunt in the passages made by bark beetles, such as ant beetles (Thanasimus), some hister beetles, and many other predators and parasites. Members of Coleoptera are particularly beneficial in soil formation, in processing and humifying plant remains, and in destroying and burying dung and animal carcasses.
Taxonomy. The order Coleoptera is divided into three sub-orders: (1) The Archostemata possess the most primitive structure of abdomen and wings; this suborder includes the present-day family Cupedidae and a number of extinct groups. (2) The carnivorous Adephaga are represented pre-dominantly by predatory beetles, such as the families Carabidae and Dytiscidae. (3) The omnivorous Polyphaga are quite varied biologically; they are divided into a number of families that encompass the majority of beetles, including the following, which have numerous representatives: Staphylinidae, Silphidae, Hydrophilidae, Scarabaeidae, Buprestidae, Elateridae, Coccinellidae, Tenebrionidae, Meloidae, Cerambycidae, Chrysomelidae, Curculionidae, and Ipidae. Twisted-wing parasites (Strepsiptera) are often, included in the beetle order (as the family Stylopidae).
A total of approximately 140 families of Coleoptera are distinguished (in the fauna of the USSR, 108), which embrace approximately 300,000 species; thus, in terms of number of species, Coleoptera is the most numerous animal order. Approximately 25,000 species of Coleoptera are known in the fauna of the USSR.
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O. L. KRYZHANOVSKII