Insectivora

Insectivora, an order of mammals, containing numerous small, generally nocturnal forms, of which the Shrews, Moles, and Hedgehogs offer familial examples. They are very widely distributed, and constitute two sub-orders: (1) Insectivora Vera (True Insectivores), with free limbs, adapted for running, climbing, burrowing, or swimming, and (2) Bermoptera (with claims to ordinal rank) having the limbs united by a membrane. [Flying Lemurs.] The diet of most consists of insects, frogs, lizards, mice, etc., but one form (Potamogale) lives on fish and Galeopithecus on fruit.

Insects are the members of the best known class of all the Arthropoda (q.v.). They belong to the subdivision of this phylum known as the Tracheata, because they breathe by means of a series of internal air passages known as tracheae, which ramify through the tissues. The body is divided into three regions - head, thorax, and abdomen, each of which consists of a series of rings more or less fused together. The thorax bears three pairs of legs and the wings, of which there are usually two pairs. The head carries a pair of antennae or feelers. which are very varied in form. The main function is that of touch, but they may also act as organs of hearing or smell.

Nearly all insects pass through a metamorphosis or series of stages, which are very different in appearance: thus, after being hatched from the' egg, they usually occur as the worm-like caterpillar, then as the fixed, quiescent stage of the chrysalis or pupa, from which emerges the imago or perfect insect. But any of the intermediate stages may be skipped: thus, the egg stage is missed in some beetles, in which the young are born as caterpillars; such are said to be "viviparous." In some flies both the egg and caterpillar stages are wanting, and the }7oung are born as pupae; these are known as "pupiparous." In some plant lice, etc., the young exactly resemble the adults, and there is no metamorphosis. In some few cases, on the other hand, there is an additional stage introduced as in Mantispa: these are said to show hypermetamorphosis.

The metamorphosis is complete when a resting or chrysalis stage intervenes between the caterpillar and adult, as happens in butterflies; these are known as "holometabolous" Ortbemetamorphism may be incomplete when the pupa is active, so that the three stages are not sharply marked off; these are either ametabolous, in which the larvae are like the adults, or hemimetabolous, when it undergoes considerable change.

Parthenogenesis (q.v.) and dimorphism, are both well illustrated and common among insects.

The appendages are the most striking features of the insects.- In most cases the adults have three pairs of legs and two pairs of wings. Both of the latter may be used for flight, or they may be alike as in the Homoptera (q.v.), different as in the Hemiptera, etc.; or only one pair may be used for flying, the anterior pair serving to protect the more delicate posterior pair; or the posterior pair may be rudimentary as in flies. In some insects the wings are absent, and in one group (apterygoaenea) (q.v.) no wings were present even in the ancestors. The legs in the adult are usually six in number, and each consists of five joints, known as the coxae or hips, trochanters, femora (thighs), tibiae (shanks), and tarsi (or feet). The number of legs may be reduced as in the Fritillaries. In the caterpillars there are also a series of claspers or prolegs, which vary in number from 10 to 22. In some cases there are rudiments of true legs on the abdomen, such as the cercopoda (q.v.) of the Orthoptera. The remaining appendages on the abdomen are not true legs; the principal one is the ovipositor, which in many cases also acts as the sting (e.g. in Wasps). Neither are the antenna; true limbs as they are in the crabs. The appendages of the mouth vary considerably in different orders, according to whether the insect feeds by sucking or biting.

The nervous system consists of a series of ganglia connected by a cord along the ventral side of the body; one ganglion (or a fused pair) occurs above the oesophagus, and is united to the main chain by a nerve on each side of the oesophagus. In the imago the nervous system is more concentrated, and the primitive arrangement can be seen best in the caterpillar. A sympathetic system is also present, and controls the opening of the breathing pores or stigmata.

The sense organs are well developed; there is usually a pair of compound eyes and often a few additional simple eyes or ocelli. In some, such as fleas, Collembola (q.v.), etc., only ocelli are present. Many of the parasitic species are blind. Many insects can detect the ultra-violet rays, which are invisible to the human eye. The sense of smell is often very keen, but they have but a dull appreciation of pain.

The circulatory system is fairly simple, and consists typically of a long vessel along the back.

The digestive system is usually complex, but is imperfect in many of those which have a very short life. Thus, in the larvae of Bytiscus and Ephemera there is no mouth, and the animal gains its food by suction through its perforated jaws (or mandibles); in some the stomach ends blindly, as in the larvae of many Hymenoptera; in others there is no digestive system at all, as in the Phylloxera.

The. renal or excretory organs (nephridia) may be few as in butterflies, or numerous as in ants.

This character is of great value in classification. Silk glands, (serictaria) occur in many insects, notably the silkworms.

Respiration is very important, and is effected by tracheae of very various forms. They usually consist of minute tubes ramifying through the body and opening to the exterior by a series of pores, known as spiracles or stigmata; as a rule there are none on the head, but some occur there in a few Collembola (q.v.) and some Lepidoptera larva. In some insects (known as apneartic) the tracheal system is closed, as in the Mayflies and the larvae of Dytiscus, and the respiration is then effected by the walls of the rectum. In other cases there are outgrowths of the tracheae known as "tracheal gills" (q.v.).

The insects are probably always bisexual, the only exceptions being due to malformation.

There are probably about 250,000 described species of insects, and the classification of this great multitude is necessarily complex. The most recent is that of Brauer, based on the embryology and the characters of the nephridia; but the older and simpler classification is here adopted in the main: - Class I. - Apterygogekea : Collembola (springtails). Thysanura.

Class If. - Pterycogenea : 1. Orthoptera (locusts, etc). 2. Neuroptera (dragon flies). 3. Strepsiptera. 4. Rhynchota Hemiptera (plant lice). Heteroptera (bugs). 5. Diptera (flies). 6. Lepidoptera (butterflies and moths) 7. Coleoptera (beetles). 8. Hymenoptera (ants, wasps, etc.).

Insects are not common as fossils, except in amber, resins, and some plant beds. Most of the existing orders begin in the Lias or Trias, but the Apterygogenea are not known before the Ohgocene. In the Palaeozoic (q.v.) there are four extinct orders, the Orthopteroidea including the Silurian Palceoblattina douvillei, Br., the oldest known insect. The Neuropteroidea includes one Devonian species, the rest of it and the whole of the Hemipteroidea being very abundant in the Carboniferous. The richest fossil insect faunas in England occur in the Jurassic limestone.