BEETLES (Part 1 of 2)   Leave a comment

 

Giraffe beetle. Also Giraffe weevil.

DEFINITION:1 any of a large order (Coleoptera) of insects, including weevils, with biting mouth parts and hard front wings (elytra ) that cover the membranous hind wings when the hind wings are folded 2 any insect resembling a beetle.

There are more species of beetles than of any other kind of insect. They constitute the largest order of insects Coleoptera which includes almost one third of a million recognized species. About 20 percent of all known species of animals in the world are beetles.

Beetles are found throughout all continents except Antarctica. Although most species are terrestrial, many such as the whirligig, water scavenger, and true water beetles have become adapted to aquatic environments. Some beetles are only about 0.01 inch (0.025 centimetre) long, whereas tropical rhinoceros beetles and Goliath beetles may reach lengths of 4 to 6 inches (10 to 15 centimetres).

Beetles display a remarkable array of colours, forms, and habits. Some are plain black or have brownish patterns that help to camouflage the insects against certain types of wood or soil. Some beetles are brilliant orange, red, or yellow; others are iridescent green or blue or have a metallic sheen. The antennae of some beetles are large and ornate. Some stag beetles have enlarged, hooked mandibles, or lower jaws, that are almost as long as the beetle itself. Male rhinoceros beetles have huge horns projecting over their heads. The shapes of beetles’ bodies vary from round to elongate. Some are flattened; others are domed or cylindrical.

Some beetles are of great significance to humans. Members of the family of beetles known as weevils, or snout beetles, are notorious agricultural pests. They have specialized, elongated heads and down-curved snouts with mouth parts at the end. Some beetles feed on plant materials such as wood, paper, and fabrics. The larvae of some dermestid beetles are destructive pests of clothing and carpets and even of plant and animal specimens in museums.

Many beetles are valuable because they prey on destructive insect pests. Ladybugs, for example, destroy untold numbers of aphids each year and so protect a wide variety of flowers and vegetables. Many other beetles play more subtle but equally important roles in various ecosystems. Dung beetles, or tumble bugs, eat vast quantities of dung in livestock areas. Carrion beetles are scavengers whose larvae feed on dead animals. Many beetles pollinate flowers.

Physical Characteristics

Mandible, from Latin mandere, to chew; term applied to: (1) chewing jaws of insects and other arthropods; (2) the lower jawbone of mammals; (3) the upper or lower part of a bird’s beak.

Like other insects, beetles have three major body segments: the head, with a single pair of antennae and a pair of compound eyes; the thorax, which bears two pairs of wings and three pairs of legs; and the abdomen, where the reproductive organs are housed. Beetles have chewing jaws called mandibles and paired structures known as maxillary and labial palpi (singular, palpus) that are used for feeding or handling food. The bodies of beetles and other insects are covered by a usually hard layer known as the cuticle that supports the internal organs and protects the body. The cuticle is hard because it contains a substance called chitin. Each defined, plate like area of the cuticle is called a sclerite.

A distinctive feature of beetles is their front pair of wings, which are thick, hard, and opaque, without the veins characteristic of most other insect wings. These fore wings, called elytra (singular, elytron), serve as protective wing covers for a second pair of functional wings underneath. The hind wings are membranous and translucent. These are ordinarily used for flying, while the heavy elytra are held out of the way. When the beetle is at rest, the elytra fold over the back and form a straight line down the centre where they meet. Some beetles have shortened wings, and a few species are entirely wingless.

Life Cycle and Behaviour

Like other insects, beetles reproduce sexually by means of internal fertilization. The ovaries of the female and testes of the male are enclosed within the abdomen. In some species, such as the stag beetles, males engage in combat with one another for the right to mate with the females. After mating, the females lay the fertilized eggs in a location suitable for development of the larvae.

Beetles undergo a complete metamorphosis: they develop from egg into active larva into inactive pupa and finally into an adult. The larva, or grub, does not resemble the adult in structure. The pupal stage though soft, pale, and immobile does have the body form of an adult. The life spans of beetles range from a few months in some species to more than four years in others.

Feeding habits. Most beetles feed on living or dead plant materials, but some are scavengers of dead animal matter and some prey on other insects. A few are parasites.

The adults and larvae of a number of beetle species feed on various plant roots, stems, fruits, seeds, and foliage. Some beetles feed only on certain plant species and plant parts, whereas others are less particular in their choice of foods. The adults and larvae of many beetles feed on decaying wood and help break down dead trees and other vegetation in forest habitats.

Some beetles, such as tiger beetles, are voracious predators. Adult tiger beetles search for prey that they can subdue with their powerful mandibles. The larvae are sedentary; they live in small tunnels where they wait to capture passing insects. Water scavenger beetles are predators as larvae but are plant eaters as adults. Some species of beetles have highly selective feeding habits: they may eat only mites, ant larvae, aphids, or zoo plankton

Defences Although most beetles are protected by their heavy armour, some species have developed additional methods of defence Blister beetles secrete an oily, blister-causing substance that deters predators. Beetles may also discourage or avoid predators by making a startling noise (see below, “Light and sound production”), secreting or ejecting an obnoxious fluid, biting, hiding (using their natural colouring as camouflage), or simply fleeing on foot or on wing.

Luminescence, emission of light resulting from causes other than high temperature.

Light and sound production. Many beetles are capable of producing light and sound, primarily for the purposes of attracting a mate or for frightening enemies. The familiar fireflies, or lightning bugs, are beetles that have special light organs on the underside of their abdomens. These beetles usually the males flash their lights rhythmically as a signal that they are ready to mate, and the females return the signal. The kind of signal system used by the two fireflies allows males and females of the same species to recognize and locate one another. Some tropical click beetles have large, luminescent eye spots on the back of the thorax that presumably are also used in courtship.

Many species of beetles make sounds by rubbing together hard parts of their bodies a practice called stridulation. The vibration created by the friction of these parts produces a shrill creaking noise. Beetles may stridulate by rubbing the two elytra together, by rubbing a hind leg against an elytron, or by rubbing the head against the front of the thorax. In some species, even the immature grubs can produce sounds. Although stridulation is often used by adult beetles as a mating signal, its purposes in other instances by juveniles, for example are not fully understood.

A wood-boring beetle known as the death-watch beetle strikes its head against the sides of its burrow as a mating signal. The name death-watch is derived from the superstition that the sound was an omen of death. One explanation is that the ticking sound of a death-watch beetle that had made its burrow in an old piece of furniture was most often heard late at night by someone sitting at a sickbed.

When threatened by a predator, bombardier beetles squirt, with a loud popping sound, an unpleasant-smelling liquid from the rear of their abdomens. The noise and the ejection act together to startle and repel the predator and give the beetle time to make its escape. When click beetles fall on their backs, they right themselves by snapping their bodies in such a way that they are tossed into the air with a loud clicking sound that can startle a predator.

Kinds of Beetles

There are many families of beetles about 135, according to some experts. The beetles discussed below represent a sampling of some of the most commonly known as well as some of the most unusual beetle families in the order Coleoptera.

Tiger beetles and ground beetles are the most common beetles in North America. The fierce, long-legged tiger beetles are fast-running, fast-flying, often brightly coloured beetles that capture and eat other insects. Species of tiger beetles occur throughout the world but are especially abundant in the tropics.

Ground beetle, one of a group of the order Coleoptera, family Carabidae; especially the fiery searcher (Calosoma scrutator), one of the largest beetles; if held carelessly will discharge quantities of “fiery” juice.

Ground beetles are also abundant in most parts of the world. Many species are black and shiny; some are iridescent. Like the tiger beetles, ground beetles have long legs. Some have enlarged, pinching mandibles that are used to capture prey. Many are nocturnal.

True water beetles (also known as diving beetles or predaceous diving beetles) are oval-shaped insects that can swim, dive, and fly. They are found in most freshwater habitats worldwide but are most common in northern temperate regions. The hind pair of legs of the true water beetle are long, flattened, and fringed to provide a greater surface area that helps the insect float. The beetle breathes through spiracles openings on the abdomen just under the tips of the elytra. Before diving, the beetle collects an air bubble beneath its elytra and then breathes from the bubble while it is underwater. It is carnivorous, preying on insects and other aquatic organisms, including fish larger than itself. The larvae of the true water beetles are sometimes called water tigers because of their voracious appetites. True water beetles often fly from one aquatic habitat to another and may be seen around outdoor lights at night.

Whirligig beetles, like the true water beetles, are oval-shaped aquatic predators that can swim, dive, and fly. They are known for their gregarious habits they are usually seen in groups, spinning and whirling around on the surfaces of quiet ponds or lakes. They have distinctive, divided eyes a top pair for seeing above the water’s surface and a bottom pair for seeing below.

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Posted 2012/01/25 by Stelios in Education

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