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Rh years; (2) certain stages of the life that are naturally “resting stages” may be in exceptional cases prolonged, and that to a very great extent; in this case no food is taken, and the activity of the individual is almost nil; (3) the life of certain insects in the adult state may be much prolonged if celibacy be maintained; a female of Cybister roeselii (a large water-beetle) has lived five and a half years in the adult state in captivity. In addition to these abnormal cases, the life of certain insects is naturally more prolonged than usual. The females of some social insects have been known to live for many years. In Tibicen septemdecim the life of the larva extends over from thirteen to seventeen years. The eggs of locusts may remain for years in the ground before hatching; and there may thus arise the peculiar phenomenon of some species of insect appearing in vast numbers in a locality where it has not been seen for several years.

Number of Species.—It is now considered that 2,000,000 is a moderate estimate of the species of insects actually existing. Some authorities consider this total to be too small, and extend the number to 10,000,000. Upwards of 300,000 species have been collected and described, and at present the number of named forms increases at the rate of about 8000 species per annum. The greater part by far of the insects existing in the world is still quite unknown to science. Many of the species are in process of extinction, owing to the extensive changes that are taking place in the natural conditions of the world by the extension of human population and of cultivation, and by the destruction of forests; hence it is probable that a considerable proportion of the species at present existing will disappear from the face of the earth before we have discovered or preserved any specimens of them. Nevertheless, the constant increase of our knowledge of insect forms renders classification increasingly difficult, for gaps in the series become filled, and while the number of genera and families increases, the distinctions between these groups become dependent on characters that must seem trivial to the naturalist who is not a specialist.

Orders of Hexapoda.—In the present article it is only possible to treat of the division of the Hexapoda into orders and sub-orders and of the relations of these orders to each other. For further classificatory details, reference must be made to the special articles on the various orders. As regards the vast majority of insects, the orders proposed by Linnaeus are acknowledged by modern zoologists. His classification was founded mainly on the nature of the wings, and five of his orders—the Hymenoptera (bees, ants, wasps, &c.), Coleoptera (beetles), Diptera (two-winged flies), Lepidoptera (moths and butterflies), and Hemiptera (bugs, cicads, &c.)—are recognized to-day with nearly the same limits as he laid down. His order of wingless insects (Aptera) included Crustacea, spiders, centipedes and other creatures that now form classes of the Arthropoda distinct from the Hexapoda; it also included Hexapoda of parasitic and evidently degraded structure, that are now regarded as allied more or less closely to various winged insects. Consequently the modern order Aptera comprises only a very small proportion of Linnaeus’s “Aptera”—the spring-tails and bristle-tails, wingless Hexapoda that stand evidently at a lower grade of development than the bulk of the class. The earwigs, cockroaches and locusts, which Linnaeus included among the Coleoptera, were early grouped into a distinct order, the Orthoptera. The great advance in modern zoology as regards the classification of the Hexapoda lies in the treatment of a heterogeneous assembly which formed Linnaeus’s order Neuroptera. The characters of the wings are doubtless important as indications of relationship, but the nature of the jaws and the course of the life-history must be considered of greater value. Linnaeus’s Neuroptera exhibit great diversity in these respects, and the insects included in it are now therefore distributed into a number of distinct orders. The many different arrangements that have been proposed can hardly be referred to in this article. Of special importance in the history of systematic entomology was the scheme of F. Brauer (1885), who separated the spring tails and bristle-tails as a sub-class Apterygogenea from all the other Hexapoda, these forming the sub-class Pterygogenea distributed into sixteen orders. Brauer in his arrangement of these orders laid special stress on the nature of the metamorphosis, and was the first to draw attention to the number of Malpighian tubes as of importance in classification. Subsequent writers have, for the most part, increased the number of recognized orders; and during the last few years several schemes of classification have been published, in the most revolutionary of which—that of A. Handlirsch (1903–1904)—the Hexapoda are divided into four classes and thirty-four orders! Such excessive multiplication of the larger taxonomic divisions shows an imperfect sense of proportion, for if the term “class” be allowed its usual zoological value, no student can fail to recognize that the Hexapoda form a single well-defined class, from which few entomologists would wish to exclude even the Apterygogenea. In several recent attempts to group the orders into sub-classes, stress has been laid upon a few characters in the imago. C. Börner (1904), for example, considers the presence or absence of cerci of great importance, while F. Klapalek (1904) lays stress on a supposed distinction between appendicular and non-appendicular genital processes. A natural system must take into account the nature of the larva and of the metamorphosis in conjunction with the general characters of the imago. Hence the grouping of the orders of winged Hexapoda into the divisions Exopterygota and Endopterygota, as suggested by D. Sharp, is unlikely to be superseded by the result of any researches into minute imaginal structure. Sharp’s proposed association of the parasitic wingless insects in a group Anapterygota cannot, however, be defended as natural; and recent researches into the structure of these forms enables us to associate them confidently with related winged orders. The classification here adopted is based on Sharp’s scheme, with the addition of suggestions from some of the most recent authors—especially Börner and Enderlein.

Class: HEXAPODA.

Sub-class:

Primitively (?) wingless Hexapods with cumacean mandibles, distinct maxillulae, and locomotor abdominal appendages. Without ectodermal genital ducts. Young closely resemble adults.

The sub-class contains a single

Order: Aptera,

which is divided into two sub-orders:

1. Thysanura (Bristle-tails): with ten abdominal segments; number of abdominal appendages variable. Cerci prominent. Developed tracheal system.

2. Collembola (Spring-tails): with six abdominal segments; appendages of the first forming an adherent ventral tube, those of the third a minute “catch,” those of the fourth (fused basally) a “spring.” Tracheal system reduced or absent.

Sub-class:

Hexapoda mostly with wings, the wingless forms clearly degraded. Maxillulae rarely distinct. No locomotor abdominal appendages. The wing-rudiments develop visibly outside the cuticle. Young like or unlike parents.

Order: Dermaptera.

Biting mandibles; minute but distinct-maxillulae; second maxillae incompletely fused. When wings are present, the fore-wings are small firm elytra, beneath which the delicate hind-wings are complexly folded. Many forms wingless. Genital ducts entirely mesodermal. Cerci always present; usually modified into unjointed forceps. Numerous (30 or more) Malpighian tubes. Young resembling parents.

Includes two families—the Forficulidae or (q.v.) and the Hemimeridae.

Order: Orthoptera.

Biting mandibles; vestigial maxillulae; second maxillae incompletely fused. Wings usually well developed, net-veined; the fore-wings of firmer texture than the hind-wings, whose anal area folds fanwise beneath them. Jointed cerci always present; ovipositor well developed. Malpighian tubes numerous (100-150). Young resemble parents.

Includes stick and leaf insects, cockroaches, mantids, grasshoppers, locusts and crickets (see ).

Order: Plecoptera.

Biting mandibles; second maxillae incompletely fused. Fore-wings similar in texture to hind-wings, whose anal area folds fanwise. Jointed, often elongate, cerci. Numerous (50-60) Malpighian tubes.