Coleoptera

(invertebrate zoology) The beetles, holometabolous insects making up the largest order of the animal kingdom; general features of the Insecta are found in this group.

An order of Insecta (generally known as beetles) with a complete metamorphosis (with larva and pupa stages), the forewings forming hard protective elytra under which the hindwings are normally folded in repose, and the indirect flight muscles of the mesothorax having been lost. The head capsule is characterized by a firm ventral (gular) closure, the antennae are basically 11-segmented, and mouthparts are of the biting type with 4-segmented maxillary palpi. The prothorax is large and free. The abdomen has sternite I normally absent, sternite II usually membranous and hidden, and segment X vestigial, with cerci absent. The female has one pair of gonapophyses, on segment IX. There are four or six malpighian tubules, which are often cryptonephric. True labial glands are nearly always absent.

General biological features

The order has well over 250,000 described species, more than any comparable group, showing very great diversity in size, form (see illustration), color, habits, and physiology. Beetles have been found almost everywhere on Earth (except as yet for the Antarctic mainland) where any insects are known, and species of the order exploit almost every habitat and type of food which is used by insects. Many species are economically important.

Coleoptera. (After T. I. Storer et al., General Zoology, 6th ed., McGraw-Hill, 1979)

A common feature is for the exoskeleton to form an unusually hard and close-fitting suit of armor, correlated with a tendency for the adults to be less mobile but longer-lived than those of other orders of higher insects. It seems to be common in a number of groups for adults to survive and breed in 2 or more successive years, a rare occurrence in other Endopterygota. Adult beetles tend to be better runners than most other Endopterygota, and some (such as Cicindelidae) are among the fastest of running insects. The flight of most beetles is not very ready or frequent, often requiring prior climbing up onto some eminence, and in many beetles occurs only once or twice in an average life.

Apart from a tendency for the sclerotized layers to be thicker, the cuticle of beetles differs little from that of other insects in constitution or ultrastructure. The black coloring which is prevalent in many groups of beetles is produced by deposition of melanin in the process of cuticle hardening. In many groups with adults active by day, the black is masked by a metallic structural color produced by interference in the surface cuticle layers. Pigmentary colors, mainly ommochromes, giving reddish to yellowish colors, occur in the cuticle of many species.

Special defenses against predation are common in species with long-lived adults, particularly the ground-living species of Carabidae, Staphylinidae, and Tenebrionidae. These groups commonly possess defensive glands, with reservoirs opening on or near the tip of the abdomen, secreting quinones, unsaturated acids, and similar toxic substances. In some cases, the secretion merely oozes onto the body surface, but in others may be expelled in a jet, as in the bombardier beetles (Brachinini). Other common defensive adaptations include cryptic and mimetic appearances. The most frequent behavioral adaptations are the drop-off reflex and appendage retraction, in which adults will react to visual or tactile stimuli by dropping off the plant foliage on which they occur, falling to the ground with retracted appendages, and lying there for some time before resuming activity. In death feigning, the appendages are tightly retracted, often into grooves of the cuticle, and held in this position for some time. In such conditions, beetles often resemble seeds and are difficult for predators to perforate or grasp.

Sound production (stridulation) is widespread in beetles and may be produced by friction between almost any counterposed movable parts of the cuticle. Stridulatory organs may show sex dimorphism, and in some species may play an important role in the interrelations of the sexes.

Anatomy

The head of beetles rarely has a marked posterior neck, and the antennal insertions are usually lateral, rather than dorsal as in most other Endopterygota, a feature possibly related to an original habit of creeping under bark. Antennal forms are very various, with the number of segments often reduced, but rarely increased, from the basic 11. Mandible forms are very diverse and related to types of food.

The prothorax commonly has lateral edges which separate the dorsal part from the ventral part of the tergum. The mesothorax is the smallest thoracic segment. Dorsally, its tergum is usually exposed as a small triangular sclerite between the elytral bases, and ventrally, its sternite may have a median pit receiving the tip of the prosternum. The form of the metathorax is affected by far-forward insertions of the hind-wings and far-posterior ones of the hindlegs, so that the pleura are extremely oblique and the sternum usually long.

The tarsi of beetles show all gradations from the primitive five-segmented condition to total disappearance. The number of segments may differ between the legs of an individual or between the sexes of a species.

Most beetles have the entire abdomen covered dorsally by the elytra in repose, and ventrally only five or six sternites (of segments III to VII or VIII) exposed; segment IX is retracted inside VIII in repose. Tergites I-VI are usually soft and flexible, those of VII and VIII more or less sclerotized. A feature developed in several groups of Coleoptera, but particularly in Staphylinidae, is the abbreviation of the elytra to leave part of the abdomen uncovered in repose, but usually still covering the folded wings. One advantage of this may be to give greater overall flexibility to the body. Perhaps surprisingly, many beetles with short elytra fly readily.

Internally, the gut varies greatly in length and detailed structure. The foregut commonly ends in a distended crop whose posterior part usually has some internal setae and may be developed into a complex proventriculus; the midgut may be partly or wholly covered with small papillae (regenerative crypts) and sometimes has anterior ceca. The four or six malpighian tubules open in various ways into the beginning of the hindgut and in many Polyphaga have their apices attached to it (cryptonephric). The central nervous system shows all degrees of concentration of the ventral chain, from having three thoracic and eight abdominal ganglia distinct to having all of these fused into a single mass.

The sense organs of beetles, adult or larval, are generally similar in type and function to those of other insects. For vision, some types are adapted for high visual acuity in bright light, others to high sensitivity in poor light, and many show light-dark adaptations of screening pigment. Color vision seems to be restricted to certain groups. In certain beetles, there is evidence of specialized infrared sensitivity. Antennae are mainly receptors for smell and touch, but detection of aerial vibrations (by Johnston's organ in the pedicel) may be important in some. The olfactory sensilla, which commonly are setae, are often concentrated on expanded apical segments, forming a club. Taste, or contact chemoreception, is usually mediated by sensilla with a single large apical opening. These sensilla are concentrated on the palpi and other mouthparts and are often found on tibiae or tarsi of the front legs.

Taste, or contact chemoreception, is usually mediated by sensilla with a single large apical opening. These sensilla are concentrated on the palpi and other mouthparts and are often found on tibiae or tarsi of the front legs.

Specialized sound receptors, other than Johnston's organ, have not been much studied in Coleoptera, though many species produce sounds and in some cases these play a significant part in courtship. A gravitational sense is evident in many burrowing beetles, which are able to sink accurately vertical burrows despite inclinations of the surface started from. A temperature sense is clearly present in some beetles and larvae, but little is known of the sensilla concerned. What might be called a time sense (otherwise known as a biological clock) clearly operates in many beetles that react specifically to changing daylight lengths marking the seasons in nontropical latitudes.

Food specialization

The biting mouthparts of many adult and larval beetles are adaptable to many types of food, and fairly widely polyphagous habits are not uncommon in the order, though most species can be assigned to one or another of a few main food categories: fungivores, carnivores, herbivores, or detritivores. Included in the last category are species feeding on decaying animal or vegetable matter and on dung. Most parasites could be included with carnivores, but those myrmecophiles and termitophiles which are fed by their hosts, and those Meloidae which develop on the food stores of bees, form special categories.

Water beetles

Beetles are unusual among the higher insects in that aquatic habits, where present, usually affect adults as well as larvae. The elytra may have been preadaptive to the invasion of water. Almost any type of fresh or brackish water body is liable to contain some types of water beetles, though very few species can live permanently in full marine salinities. Almost all aquatic beetles maintain an air reservoir, into which the second thoracic and abdominal spiracles open, under their elytra; and some, mainly small, species may have plastrons or physical gills. Aquatic larvae tend to have a last pair of abdominal spiracles that is large and effectively terminal, or tend to develop tracheal gills. Pupae are terrestrial in the large majority of water beetles, and eggs, when deposited underwater, are commonly laid in contact with airspaces in the stems of water plants, or in an air-filled egg cocoon. Most water beetles are confined to shallow waters, and many of them are ready colonists of temporary pools, through adult flight.

Special habitats and adaptations

More or less unusual adaptations and modes of life are manifest in dung and carrion beetles, ambrosia beetles, cave and subterranean beetles, desert beetles, and luminous beetles. Dung and carrion beetles exploit sporadic and very temporary food resources in which competition (particularly with Diptera larvae) tends to be fierce. Many dung beetles avoid the difficulties by burying dung stores in underground cells where their larvae can develop safely, and some beetles (Necrophorinae) adopt a similar strategy for carrion. They may also develop phoretic and symbiotic relations with specific mites preying on fly eggs.

In ambrosia beetles, the adults usually excavate burrows in wood, carrying with them spores of special fungi, which proceed to develop along the walls of the burrows and are fed on by larvae developing from eggs laid there. This type of relation between beetles, fungi, and trees exists in various forms in a number of families and may be of ecological importance if the fungi concerned are liable to kill trees.

More than one family of beetles contains highly adapted types living exclusively in deeper cave systems (cavernicolous) or the deeper layers of the soil (hypogeous). Such species are usually flightless and eyeless, poorly pigmented, and slow-moving, and of very restricted geographical distribution. Another marginal habitat in which beetles are the principal insect group represented is the desert. Here, too, wings are usually lost, but the cuticle tends to be unusually thick, firm, and black. Also beetles may be found in hot springs. A few water beetles are adapted to underground (phreatic) waters, showing parallel features with the cavernicolous and hypogeous terrestrial ones.

More or less parasitic relations to animals of other groups have developed in a number of lines of Coleoptera. The closest parallels to Hymenoptera-Parasitica are to be seen in the families Stylopidae (Strepsiptera of many authors) and Rhipiphoridae. Stylopid larvae are exclusively endoparasites of other insects, while the adult females are apterous, often legless, and remain in the host's body, while males have large fanlike hind-wings and the elytra reduced to halter-like structures. A considerable variety of Coleoptera develop normally in the nests of termites and social or solitary Hymenoptera-Aculeata, some being essentially detritivores or scavengers, but most feed either on the young or the food stores of their hosts. Ectoparasitism on birds or mammals is a rare development in beetles.

The beetles are notable in that some of them manifest the highest developments of bioluminescence known in nonmarine animals, the main groups concerned being Phengodidae, Lampyridae, and Elateridae-Pyrophorini, the glowworms and fireflies. In luminescent beetles, the phenomenon always seems to be manifest in the larvae and often in the pupae, but not always in the adults. In luminous adult beetles, there is often marked sex dimorphism, and a major function of the lights seems to be the mutual recognition of the sexes of a species.

Another possible function of adult luminosity, and the only seriously suggested for that of larvae, is as an aposematic signal. There is definite evidence that some adult fireflies are distasteful to some predators, and the luminous larvae of Phengodidae have dorsal glandular openings on the trunk segments that probably have a defensive function. See also Bioluminescence.

Phylogenetic history

Coleoptera are older than the other major endopterygote orders. The earliest fossils showing distinctively beetle features were found rather before the middle of the Permian Period. By the later Permian, fossils indicate that beetles had become numerous and diverse, and during the Mesozoic Era they appear as a dominant group among insect fossils. Fossils in Triassic deposits have shown features indicative of all four modem suborders, and the Jurassic probably saw the establishment of all modern superfamilies. By early Cretaceous times, it is likely that all “good” modern families had been established as separate lines. In the Baltic Amber fauna, of later Paleogene age (about 40,000,000 years ago), about half the fossil beetle genera appear to be extinct, and the other half have still-living representatives (often in remote parts of the world). Beetle fossils in Quaternary (Pleistocene) deposits are very largely of still-living species.

Geographical distribution

Almost every type of continuous or discontinuous distribution pattern which is known in any animal group could be matched in some taxon of Coleoptera, and every significant zoogeographical region or area could be characterized by endemic taxa of beetles. In flightless taxa, distributional areas are generally more limited than those of comparable winged taxa. Distinct distributional categories can be seen in those small, readily flying groups (in Staphylinidae and Nitidulidae, for example) which are liable to form part of “aerial plankton,” and in those wood borers which may survive for extended periods in sea-drifted logs, both of which are liable to occur in oceanic islands beyond the ranges of most other beetle taxa. Climatic factors often seem to impose limits on the spread of beetle species (and sometimes of genera or families), and beetle remains in peats have been found to be sensitive indicators of climatic changes in glacial and postglacial times. See also Insecta.