There are several types of coleopteran larvae. Carabid larvae have a tapering, flattened, smooth body, as do those of staphylinids (rove beetles) and silphids (carrion beetles); larvae of the Dytiscidae (diving beetles), although somewhat similar to those of carabids, have a lobed air float at the end. Larvae of click beetles (Elateridae) are cylindrical or flat and slender and have a hard surface. Some click beetle larvae, called wireworms, feed on newly planted seeds and roots of plant crops (e.g., maize, cotton, potatoes); others feed in deadwood or on wood-boring beetle larvae (Cerambycidae). Larvae of Buprestidae (metallic wood borers), which are soft-bodied and slender, bore under the bark of trees or burrow beneath the surface of leaves.

Dermestid larvae, somewhat tapering and cylindrical, have whorls of short bristles and some longer ones and resemble hedgehogs or porcupines. Coccinellid larvae—flattened, broad in the middle, and tapering at the back—sometimes have a few low projections (tubercles) bearing short hairs and are often strikingly coloured with red or yellow and black. Larvae of the plant-feeding epilachnines often are yellow with black bristles. Scarabaeid larvae are soft-bodied, thick, strongly C-shaped, and somewhat flattened beneath and round above. Cetonine larvae, similar to those of geotrupids and lucanids, are often short, less C-shaped than most scarabaeids, hairy, active, and capable of locomotion on their backs through movement of body segments. Passalid larvae, white and slender, are found with their parents. Chrysomelid larvae are short, are flattish or fat, and sometimes have lobes at the sides or appendages at the hind end. Cerambycid larvae are long and slender, with swellings at the sides of the segments. They are pale, almost hairless, and fairly soft; they have either minute legs or none at all; and the eyes are poorly developed. Weevil larvae, usually white and soft, are fatter in the abdominal region than at the head end. The head capsule may be hardened, be brown in colour, and have strong mandibles.

Some beetles undergo hypermetamorphosis, in which they have different larval types in different instars (the stages between molts). The early larval stages usually are active, and the later stages are parasitic on other organisms. The active young larvae of most Meloidae (blister beetles), called triungulins, for example, hatch from eggs laid on flowers, become attached to bees visiting the flowers, and thus are carried to a bee nest, where they become parasitic on bee larvae.

Pupae

Pupae of beetles usually have a form similar to that of the adult except that the elytra are represented by pads on the exterior of the body; the colour, generally white, is sometimes pale brown or patterned. As the time for emergence of the adult approaches, the pupa may darken, especially the mandibles and eyes. After emerging from the pupal skin, the adult rapidly assumes its final adult form and coloration, although metallic colours may take some days to develop their final appearance.

Larvae that bore in wood, cones, or seeds and those that live in the ground or in excrement chew or dig a cavity, or pupal cell. In some cases the pupa lies on a cushion of frass (chewed or torn wood fibres) or other material; in others it is enclosed in a cocoon of frass or other material (e.g., a smooth, white, hard covering similar to the shell of a bird’s egg). Sometimes material is used only to seal off the open end of the tube, gallery, or cell for protection from ants and other predators. After the body of the adult has hardened, the adult breaks or dissolves the barrier and emerges. Many wood borer adults must chew through solid wood to emerge, although the larva usually chews close to the surface of the tree, under the bark, before pupating.

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Typical life cycle

The adult Aspidomorpha furcata, a tortoise beetle of South China, feeds on the leaves of the host plant Ipomoea (sweet potato), where the entire life cycle takes place. Eggs, laid in small groups, are cemented together in a thin paperlike egg capsule whose thin brown layers separate and camouflage them. The larva, which hatches in four to six days, burrows directly through the egg capsule and feeds on the leaf epidermis for about three days before it molts. Later it eats through the entire leaf. When the fifth instar larva is fully grown, it goes through a resting prepupal period before pupation occurs. During each molt the old larval skin is pushed back and attached to processes at the hind end. The dried and shrunken skins plus extruded feces combine to camouflage the larva. When pupation takes place, the combination of exuviae (molted skins) and feces becomes attached to a paired process at the hind end of the pupa, thus camouflaging and shielding it. The last larval skin is used to attach the end of the abdomen of the pupa to the leaf surface. The pupa usually rests with its camouflage flat over its back, although it may erect the camouflage and turn it back to discourage an enemy.

The pupal stage lasts four days or longer. The life cycle from egg to adult requires 21 to 27 days in mild weather and longer in winter; adults may live more than 230 days. Females lay from 63 to 228 egg cases, with an average of about 3 eggs per case. There may be several generations per year.

In cooler temperate areas, life cycles may occupy much longer periods, even up to four years or more. In general, wood-boring beetles and root feeders have the longest life cycles, while leaf-feeding species have shorter ones. Several generations per year are possible with subtropical and tropical species.

Beetles as prey

In general, beetles are very well-armoured insects and thus are reasonably protected against enemies; most, however, have parasites. Some tachinid flies, for example, lay their eggs on adult beetles, and the larvae feed inside their bodies. Beetle larvae also often have hymenopterous parasites—e.g., wasps. Long-horned beetle larvae (Cerambycidae) are parasitized by wasps that lay their eggs directly on or in beetle larvae. Beetles are probably attacked by fewer predators than many other insects; birds that often feed on various kinds of insects may not eat some kinds of beetles. Swifts and other birds, such insectivorous mammals as bats, reptiles, frogs, and other insects may act as beetle predators. Some beetle predators feed particularly on beetle larvae, although many beetle larvae that feed on plants and in the ground probably are distasteful to birds and other predators.