Animals/Insects

Insects: Variation and Adaptability

No other terrestrial group of animals has as many members as the class Insecta. More than 900,000 species of insects have been described - about 500,000 of these alone are beetles (order Coleoptera), and there are many thousands more yet to be identified and named. The success of this group is partly due to its tremendous adaptability and huge variation in lifestyles. Insects live in almost every habitat.

Golden Tortoise Beetle

General Features

The exoskeleton which covers the entire insect body is composed of chitin and hardened by proteins. It is made up of several parts: the tergum, covering the back; the sternum, on the underside; and 2 pleura, which link the tergum to the sternum. The pleura are considerably thinner than the rest of the cuticle and generally contain spiracles - the openings to air tubes (tracheae).

The body is clearly divided into a head, thorax, and abdomen. The head is derived from 6 segments, but they are fused together and are not obvious in adults. Typically, the insect head bears 2 pairs of sensory antennae, 1 pair of compound eyes which may be color sensitive, and 1 more ocelli (clusters of light-sensitive photoreceptors).

Pallid-winged Grasshopper

The thorax is composed of 3 segments, each bearing a pair of walking legs; this characteristic is unique to insects. The legs are modified in different species for grasping, swimming, jumping or digging. The winged insects (subclass Pterygota) also bear a pair of wings on the dorsal surface of each of the second and third thoracic segments, whereas those of the subclass Apterygota are primitively wingless. The abdomen is made up of 11 segments, but the 11th is often reduced.

All insects have a heart which lies dorsally within the thorax and, in most species, in the first 9 abdominal segments. The blood circulates in a blood space (called the hemocoele) and bathes all the tissues.

Respiration in most insects takes place by means of a system of internal tubes called tracheae, which open to the exterior via paired spiracles. The insect nervous system is much like that of other arthropods, with a brain and a system of linked and individual and fused ganglia connected to a ventral nerve cord. Apart from the eyes, sense organs such as chemoreceptors and tactile hairs occur all over the body, but are most concentrated on the appendages.

Puss Moth Caterpillar

Insect Flight

One of the most successful adaptive features of insects is flight. Most pterygotes have wings, although secondarily wingless insects do occur in this group. Various species, such as ants (order Hymenoptera) and termites (order Isoptera) have wings during certain stages of their lifecycle. Many insects have 2 pairs of wings. These may move independently, as in damselflies or they can be hooked together so that they move as a single structure, as in many bees and wasps. Beetles have undergone a further change, the first pair f wings has been hardened to form the wing cases (elytra) that protect the membranous hind wings, which are used for flight.

The number of beats per second varies greatly from between 4 and 20 for butterflies (order Lepidoptera) to 190 beats per second in bees, and up to 1,000 per second in a small fly. The mode of flying also varies in different groups. Butterflies tend to have a slow, fluttering style, whereas bees and flies can hover and dart. Flight is controlled by a complex interaction of feedback from sensory hairs on the head, stretch receptors at the base of the wing, visual cues and the wing muscles themselves. There is no nervous center for the control of flight. Flight speed appear to be controlled by the flow of air against the antennae or by sight.

White Peacock

Life Cycle and Development

Most insects lay eggs. Once hatched the primitive wingless insects do not metamorphose, they gradually mature through a series of molts. The winged insects, however, do metamorphose. They are divided into 2 groups: the Endopterygota, such as flies and butterflies, whose larvae do not resemble the adult and whose wing development is internal; and Exopterygota, such as cockroaches, grasshoppers and some bugs, which hatch as miniature versions of the adults, having external wing buds but without being sexually mature and gradually develop by incomplete metamorphosis.

Many insects have complicated lifecycle, especially parasitic insects. Parasitic wasps, for example, have a form of reproduction called polyembryony, in which the embryonic cells give rise to more than one embryo. This means that from 1 egg deposited in the body of a host many larvae can be formed, and the resulting embryos can use the host's body as both a refuge and a food source.

Garden Spider

Feeding Adaptations

The mouthparts of insects show great variability. In some, they are adapted for sucking, in other for piercing and biting, and in still others the mouthparts are modified for chewing, crushing and tearing. Chewing mouthparts are found in carnivorous and herbivorous insects. This is regarded as a primitive condition and is characteristic of insects such as dragonflies, grasshoppers and beetles, in both the adult and nymphal stages. Butterflies have chewing mouthparts during the larval stage only.

Social Organization

Communal living is developed to the highest degree in termites, ants, bees and some wasps. The social life of honey bees (Apis mellifera) is controlled by a rigid division of labor and different castes of insects perform different functions within the colony. The nest is made up of a series of vertical wax combs, with hexagonal cells on both sides. Drone cells are larger than those in which workers develop, and the queen cells are more like saclike. Some cells are constructed for soring pollen and honey for use during bad weather or over winter.

A colony consists typically of about 60,000 workers (all sterile females), 200 male drones and a single queen, whose function is to lay eggs. The main purpose of the drones is to mate with the queen. The duties of the worker bees alter as they become older. Newly emerged workers stay in the hive and clean and feed the queen, drones and larvae. At this stage the pharyngeal glands of the workers produce "royal jelly" which is fed to very young larvae (for the first 4 days) and continually to larvae destined to become queens. after about 2 weeks, when the wax gland have developed, the workers' duties change to cell construction and cleaning, and receive stores of pollen and nectar from foraging bees. Three weeks later the worker becomes a field bee with large pollen baskets on the hind legs. Early workers live for about 8 weeks but those emerging in late summer or autumn live over winter. Drones are driven out of the hive by workers in the autumn to conserve food stocks. The queen maintains cohesion in the colony by the production of chemicals called pheromones, which influence the behavior of the bees; these induce the workers to nurse the larvae. When this substance diminishes, as the queen ages, the workers start producing new queens by feeding certain larvae solely on royal jelly. At some stages in the colony's life the old queen leaves with about half the workers. At this time a new queen emerges and the other developing queens are usually killed. The new queen mates during a nuptial flight with the drones and returns to head the colony.

One of the most astounding features of bees is their ability to communicate the position of a rich food source done by a special dance. On returning to the hive, a foraging bee alights on a vertical surface of the comb and begins to dance in relation to the position of the Sun. She moves in a straight line, waggling her body. If she moves upward the food source lies towards the Sun; downward means it lies away from it. Moving at angles to the left or right she indicates that the bees must fly with the Sun on their left or right respectively. The speed of the dance indicates the distance of the food. The other workers follow the progress of the dance by touching her, and then fly off toward the food.

Types of Insects

The following list of types of insects is far from complete and is provided for reference purpose only.

Class Insecta (insects) includes the following groups:

• Subclass Pterygota (winged insects)
  • Order Ephemeroptera (mayflies)
  • Order Anoplura (sucking lice)
  • Order Hemiptera (true bugs)
  • Order Coleoptera (beetles)
  • Order Diptera (true flies)
  • Order Hymenoptera (ants, bees, and wasps)
  • Order Lepidoptera (butterflies, butterflies and moths, and moths)
  • Order Mecoptera (scorpion flies)
  • Order Neuroptera (lacewings and relatives)
  • Order Siphonaptera (fleas)
  • Order Strepsiptera (parasitic insects and twisted-winged parasites)
  • Order Trichoptera (caddisflies)
  • Order Mallophaga (biting lice)
  • Order Orthoptera (grasshoppers, locusts, and relatives)
  • Order Blattaria (cockroaches)
  • Order Dermaptera (earwigs)
  • Order Embioptera (webspinners)
  • Order Isoptera (termites)
  • Order Mantophasmatodea (gladiators)
  • Order Plecoptera (stoneflies)
  • Order Zoraptera (zorapterans)
  • Order Odonata (dragonflies and damselflies)
  • Order Euphasmida (stick and leaf insects)
  • Order Thysanura (bristletails)
  • Order Collembola (springtails)

References:
1. Mike Janson and Joyce Pope (consultant editors). The Animal World
2. Beetles by A. Bochdansky & M. Kriftner