Insecta

(invertebrate zoology) A class of the Arthropoda typically having a segmented body with an external, chitinous covering, a pair of compound eyes, a pair of antennae, three pairs of mouthparts, and two pairs of wings.

A class of the phylum Arthropoda, sometimes called the Hexapoda. In fact, Hexapoda is a superclass consisting of both Insecta and the related class Parainsecta (containing the springtails and proturans). Class Insecta is the most diverse group of organisms, containing about 900,000 described species, but there are possibly as many as 5 million to perhaps 20 million actual species of insects. Like other arthropods, they have an external, chitinous covering. Fossil insects dating as early as the Early Devonian have been found. See also Parainsecta.

Classification

The class Insecta is divided into orders on the basis of the structure of the wings and the mouthparts, on the type of metamorphosis, and on various other characteristics. There are differences of opinion among entomologists as to the limits of some of the orders. The orders of insects (and their relatives the parainsects) are shown below.

Superclass Hexapoda

     Class Parainsecta

                    Order: Protura; proturans

                                   Collembola; springtails

     Class Insecta

          Subclass Monocondylia

                    Order: Diplura; telsontails

                                   Archaeognatha; bristletails

          Subclass Dicondylia

               Infraclass Apterygota

                    Order: Zygentoma; silverfish, firebrats

               Infraclass Pterygota

               Section Palaeoptera

                    Order: Ephemeroptera; mayflies

                                   Odonata; damselflies and dragonflies

               Section Neoptera

               Hemimetabola

                    Order: Plecoptera; stoneflies

                                   Grylloblattodea; rockcrawlers

                                   Orthoptera; grasshoppers, katy-dids, crickets

                                   Phasmatodea; walkingsticks

                                   Mantodea; mantises

                                   Blattodea; cockroaches

                                   Isoptera; termites

                                   Dermaptera; earwigs

                                   Embioptera; webspinners

                                   Zoraptera; zorapterans

                                   Psocoptera; psocids, booklice

                                   Phthiraptera; lice

                                   Thysanoptera; thrips

                                   Hemiptera; cicadas, hoppers, aphids, white                                        flies, scales

               Holometabola

                    Order: Megaloptera; dobsonflies, alderflies

                                   Raphidioidea; snakeflies

                                   Neuroptera; lacewings, antlions

                                   Coleoptera; beetles

                                   Strepsiptera; twisted-wing parasites

                                   Mecoptera; scorpionflies

                                   Siphonaptera; fleas

                                   Diptera; true flies

                                   Trichoptera; caddisflies

                                   Lepidoptera; moths, butterflies

                                   Hymenoptera; sawflies, wasps, ants, bees

Morphology

Insects are usually elongate and cylindrical in form, and are bilaterally symmetrical. The body is segmented, and the ringlike segments are grouped into three distinct regions: the head, thorax, and abdomen. The head bears the eyes, antennae, and mouthparts; the thorax bears the legs and wings, when wings are present; the abdomen usually bears no locomotor appendages but often bears some appendages at its apex. Most of the appendages of an insect are segmented.

The skeleton is primarily on the outside of the body and is called an exoskeleton. However, important endoskeletal structures occur, particularly in the head. The body wall of an insect serves not only as a covering, but also as a supporting structure to which many important muscles are attached. The body wall of an insect is composed of three principal layers: the outer cuticula, which contains, among other chemicals, chitin; a cellular layer, the epidermis, which secretes the chitin; and a thin noncellular layer beneath the epidermis, the basement membrane. The surface of an insect's body consists of a number of hardened plates, or sclerites, separated by sutures or membranous areas, which permit bending or movement. See also Chitin.

A pair of compound eyes usually cover a large part of the head surface. In addition most insects also possess two or three simple eyes, the ocelli, usually located on the upper part of the head between the compound eyes; each of these has a single lens. See also Eye (invertebrate).

Insect mouthparts typically consist of a labrum, or upper lip; a pair each of mandibles and maxillae; a labium, or lower lip; and a tonguelike structure, the hypopharynx. These structures are variously modified in different insect groups and are often used in classification and identification. The type of mouthparts an insect has determines how it feeds and what type of damage it is capable of causing.

Several forms of antennae are recognized, to which various names are applied; they are used extensively in classification. The antennae are usually located between or below the compound eyes and are often reduced to a very small size. They are sensory in function and act as tactile organs, organs of smell, and in some cases organs of hearing.

Insects are the only winged invertebrates, and their dominance as a group is probably due to their wings. Immature insects do not have fully developed wings, except in the mayflies. The wings may be likened to the two sides of a cellophane bag that have been pressed tightly together. The form and rigidity of the wing are due to the stiff chitinous veins which support and strengthen the membranous portion. At the base are small sclerites which serve as muscle attachments and produce consequent wing movement. The wings vary in number, placement, size, shape, texture, and venation, and in the position at which they are held at rest. Adult insects may be wingless or may have one pair of wings on the mesothorax, or, more often two pairs. There is a common basic pattern of wing venation in insects which is variously modified and in general quite specific for different large groups of insects. Much of insect classification depends upon these variations. A knowledge of fossil insects depends largely upon the wings, because they are among the more readily fossilized parts of the insect body.

Internal anatomy

The intake of oxygen, its distribution to the tissues, and the removal of carbon dioxide are accomplished by means of an intricate system of tubes called the tracheal system. The principal tubes of this system, the tracheae, open externally at the spiracles. Internally they branch extensively, extend to all parts of the body, and terminate in simple cells, the tracheoles. Many adaptations for carrying on respiration are known.

Insects possess an alimentary tract consisting of a tube, usually coiled, which extends from the mouth to the anus. It is differentiated into three main regions: the foregut, midgut, and hindgut. Valves between the three main divisions of the alimentary canal regulate the passage of food from one region to another.

The excretory system consists of a group of tubes with closed distal ends, the Malpighian tubules, which arise as evaginations of the anterior end of the hindgut. They vary in number from 1 to over 100, and extend into the body cavity. Various waste products are taken up from the blood by these tubules and passed out via the hindgut and anus.

The circulatory system of an insect is an open one. The only blood vessel is a tube located dorsal to the alimentary tract and extending through the thorax and abdomen. The posterior portion of this tube, the heart, is divided into a series of chambers, each of which has a pair of lateral openings called ostia. The anterior part of the tube is called the dorsal aorta.

The nervous system consists of a brain, often called the supraesophageal ganglion, located in the head above the esophagus; a subesophageal ganglion, connected to the brain by two commissures that extend around each side of the esophagus; and a ventral nerve cord, typically double, extending posteriorly through the thorax and abdomen from the subesophageal ganglion. In the nerve cords there are enlargements, called ganglia. Typically, there is a pair to each body segment. From each ganglion of the chain, nerves extend to each adjacent segment of the body, and also extend from the brain to part of the alimentary canal.

Reproduction in insects is nearly always sexual, and the sexes are separate. Variations from the usual reproductive pattern occur occasionally. In many social insects, such as the ants and bees, certain females, the workers, may be unable to reproduce because their sex organs are undeveloped; in some insects, individuals occasionally occur that have characters of both sexes, called gynandromorphs. Also, parthenogenesis—the process of females giving rise to females—is known in some species.

Metamorphosis

After insects hatch from an egg, they begin to increase in size and will also usually change, to some degree at least, in form and often in appearance. This developmental process is metamorphosis. The growth of an insect is accompanied by a series of molts, or ecdyses, in which the cuticle is shed and renewed.

The molt involves not only the external layers of the body wall, the cuticula, but also the cuticular linings of the tracheae, foregut, and hindgut; the cast skins often retain the shape of the insects from which they were shed. The shedding process begins with a splitting of the old cuticle. This split grows and the insect eventually wriggles out of the old cuticle. The new skin, remains soft and pliable long enough for the body to expand to its fullest capacity before hardening.

Insects differ regarding the number of molts during their growing period. Many have as few as four molts; a few species have 40 or more, and the latter continue to molt throughout life.

Insects have been grouped or classified on the basis of the type of metamorphosis they undergo. Although all entomologists do not agree upon the same classification, the following outline is presented:

1. Ametabolous or primitive: No distinct external changes are evident with an increase in size.

2. Hemimetabolous: Direct metamorphosis that is simple and gradual; immature forms resemble the adults except in size, wings, and genitalia. Immatures are referred to as nymphs or naiads if aquatic.

3. Holometabolous: Complete, or indirect, metamorphosis; stages in this developmental type are: egg→larva→pupa→adult (or imago).

Fossils

Insects and parainsects have a rich fossil record that extends to 415 million years, representing all taxonomic orders and 70% of all families that occur today, Insect deposits are characterized by an abundance of exceptionally well-preserved deposits known as Lagerstätten. Lagerstätten refer not only to the familiar amber deposits that entomb insects in hardened tree resin, but more importantly to a broad variety of typically laminar, sedimentary deposits. These deposits, formed in lake basins, are the most persistent of insect-bearing deposits and document the evolution of insect biotas during the past 300 million years. By contrast, the oldest amber is approximately 120 million years old and extends modern lineages and associated taxa to the Early Cretaceous. Other major types of insect deposits include terrestrial shales and fine-grained sandstones marginal to marine deposits during the Early and Middle Devonian, a proliferation of nodular ironstone-bearing strata of late Carboniferous age from the equatorial lowlands of the paleocontinent Euramerica, and distinctive lithographic limestones worldwide from the Middle Jurassic to Early Cretaceous. More modern deposits are Miocene to Recent sinter deposits created by hydrothermal zones with mineral-rich waters, and similarly aged asphaltum, representing the surface accumulation of tar. Lastly, insects are abundant in many Pleistocene glacial deposits of outwash and stranded lake sediments, formed by the waxing and waning of alpine and continental glaciers.

Various types of fossil documentation are important for understanding insect paleobiology, such as the body-fossil history of mouthparts. A recent study of insect mouthparts reveals a fivefold phase of increasing mouthpart disparity through time. This geochronologic deployment of the 34 basic types of modern insect mouthparts began during the Early Devonian with a few generalized types, was expanded during the late Carboniferous to early Permian to include major modifications of mandibulate and piercing-and-sucking types, and increased significantly again during the Late Triassic to Early Jurassic to include filter-feeding mouthpart types and others involved in the ecologic penetration of aquatic ecosystems, and also intricate interactions with other animal and seed-plant hosts. During the Late Jurassic, there was expansion of mouthpart types involved in fluid-feeding on plant, fungal, and animal tissues and during the Early Cretaceous mouthpart innovation was completed by the addition of a few specialized mouthpart types involved in blood-feeding and other specialized associations. A comparison of taxonomic diversity and mouthpart disparity reveals that the generation of taxa has proceeded overall in a semilogarithmic increase reflected in a concave curve, whereas morphologic innovation, as revealed by mouthpart disparity, is a logistic process evidenced by a convex curve. This suggests that the deployment of basic morphologic types typically precedes taxonomic diversification in insect fossil history.