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The number of identified hexapod species surpassed one million by 2016. But most specialists admit that only a fraction of those in their areas of expertise have been positively identified. This is especially true of the parasitic Hymenoptera whose roles are critical in the population regulation of other insect species. Among the majority of insect species the actual numbers of individuals is enormous and can rarely be determined experimentally. There are many more species of insects than in all other animal groups combined, and they are regarded as the most successful group of terrestrial animal life. Nevertheless, ignorance of insect numbers and life styles remains widespread. The result is that such species as mosquitoes continue to breed in close proximity with humans that are unaware of breeding sites that could easily be reduced.
Insects possess morphological and behavioral characteristics that readily assist in the study of evolution. Therefore, it is possible to separate primitive from advanced forms where increased complexity is generally found as evolution advances. Occasionally characteristics that are regarded as primitive do reappear in more advanced forms, however.
Most insects are small with a body that consists of transverse segments grouped into three distinct areas: (1) head, (2) thorax, and (3) abdomen (See Insect Morphology). The segments are more obvious in the abdomen and thorax, while the head segments are strongly fused. The usual number of segments remaining from the primitive arthropod is six in the head, three in the thorax and eleven in the abdomen. Although primitively each segment bore appendages, in evolution some have been modified for antennae, eyes, genital and mouthparts, while others have been lost. Wings are usually present on the thorax in two pairs, although a few groups like the Diptera retain only one pair, with the second having been greatly reduced for other functions. The insect surface or body wall is known as the integument. There is a specialized tracheal system for breathing that reaches all over the insect body. Air enters via spiracles located on the body sides.
The integument consists of a single layer of cells and a basement membrane that covers the entire body extending partially into the mouth and intestines (See Integument). The integument cells produce and are covered by a cuticula, which is divided into three layers: epi- and exocuticula consisting of chitin and a epicuticula, each with varying thickness. Projections emanating from the integument are various scales, spines, hairs and setae that are formed from the cuticula itself. Stouter spines and hairs originate from beneath the epidermis.
The insect head is made up of about six fused segments (= sclerites), which are grouped into a hard case or capsule (See Grasshopper Head for example). Although much modified from the primitive ancestor, the six segments gave rise to the compound eyes, antennae, mandibles, maxillae, labium and maxillae. In the grasshopper the mouthparts are a simple "Mandibulate Type" adapted for chewing, but in more highly evolved species such as the mosquito they serve to pierce and suck fluids. Other modifications adapted to serve different functions are discussed and diagrammed in detail by Matheson (1950) and Legner (See Insect Morphology).
Among insects there is an array of different kinds of antennae, which are evolved from appendages of the 2nd head segment (See "Insect Antennae" for common examples). Special names depicting the various types include filiform, capitate setaceous, clavate. The number of segments in antennae varies greatly. The first segment is the scape, the second the pedicel and the rest make up the flagellum. Insect antennae are believed to serve as sense organs for smell, touch and taste.
The insect thorax is joined to the head by a neck, which is not strongly sclerotized (= "thickened") (See Grasshopper Thorax & Mosquito Thorax for examples). There are three thoracic segments, which bear wings and legs. These segments are the prothorax, mesothorax and metathorax. Paired legs are borne on each segment of the thorax, and are composed of five parts: coxa, trochanter, femur, tibia, tarsus, the latter consisting of usually 5 segments (See "Insect Legs" for two examples).
Insect wings are appendages formed from the meso- and metathorax. These are easily observed in the more primitive species, as their development is external. In more highly evolved species development is internal in the larvae, becoming visible during the pupal stage. Most insects hve two pairs of wings, although the flies or Diptera have only one pair for flight while the other pair, called "halteres" are modified for balance. Some insects such as fleas and lice, do not possess wings, even though they are believed to have had winged ancestors.
Comstock and Needham of Cornell University presented what is believed to be the hypothetical type of venation where diagnostically useful wing venation is assigned names (See "Hypothetical Wing"). Later modifications of this system were made for use in identification of certain orders (See "Wing Venation Modified).
Insect abdomens are made up of segments, which are believed to have kept most of the primitive ancestors. On each segment there is a large tergum and a highly developed sternum. The lateral regions are typically membranous, but some species have developed specialized sclerites. There are eleven segments, with the final 3-4 having been modified with special appendages used for clasping or oviposition. All eleven segments are not always distinguishable, however (See "Grasshopper Abdomen").
Matheson, R. 1950. Medical Entomology. Comstock Publ. Co, Inc. 610 p.
Service, M. 2008. Medical Entomology For Students. Cambridge Univ. Press. 289 p
Legner, E. F. 1995. Biological control of Diptera of medical and veterinary importance. J. Vector Ecology 20(1): 59-120.
Legner, E. F.. 2000. Biological control of aquatic Diptera. p. 847-870. Contributions to a Manual of Palaearctic Diptera,
Vol. 1, Science Herald, Budapest. 978 p.