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The antennæ are movable, jointed filaments, attached usually close to the eyes, and varying much in shape in different Insects. They doubtless discharge the functions of tactile organs, but are probably the organ of other more recondite senses in addition.

The sexes in Insects are in different individuals, and most are oviparous. The ovum undergoes partial segmentation; and the embryo has its future ventral surface turned outwards, and its dorsal surface inwards. Generally speaking, the young insect is very different in external characters from the adult, and it requires to pass through a series of changes, which constitute the "metamorphosis," before attaining maturity. In some Insects, however, there appears to be no metamorphosis, and in some the changes which take place are not so striking or so complete as in others. By the absence of metamorphosis, or by the degree of its completeness when present, Insects are divided into sections, called respectively Ametabola, Hemimetabola, and Holometabola, which, though not, perhaps, of a very high scientific value, are nevertheless very convenient in practice.

Section 1. Ametabolic Insects.-These pass through no metamorphosis, and also, in the mature condition, are destitute of wings. The young of these insects (Aptera) on escaping from the ovum resemble their parents in all respects except in size; and though they may change their skins frequently, they undergo no alteration before reaching the perfect condition, except that they grow larger.

Section 2. Hemimetabolic Insects.-In the insects belonging to this section there is a metamorphosis consisting of three stages. The young on escaping from the ovum is termed the "larva;" when it reaches its second stage it is called the "pupa," or "nymph;" and in its third stage, as a perfect insect, it is called the "imago." In the Hemimetabola, the "larva," though of course much smaller than the adult, or "imago," differs from it in little else except in the absence of wings. It is active and locomotive, and is generally very like the adult in external appearance. The "pupa," again, is a little larger than the larva, but really differs from it in nothing else than in the fact that the rudiments of wings have now appeared, in the form of lobes enclosed in cases. The "pupa is still active and locomotive, and the term "nymph" is usually applied to it. The pupa is converted into the perfect insect, or "imago," by the liberation of the wings, no other change being requisite for this purpose. From the comparatively small amount of difference between these three stages, and from the

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active condition of the pupa, this kind of metamorphosis is said to be "incomplete."

In some members of this section, however-such as the Dragon-flies the larva and pupa are aquatic, whereas the imago leads an aerial life. In these cases (fig. 158) there is necessarily a considerable difference between the larva and the adult; but the larva and pupa are closely alike, and the latter is active.

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Section 3. Holometabolic Insects. These - comprising the Butterflies, Moths, Beetles, &c.-pass through three stages which differ greatly from one another in appearance, the metamorphosis, therefore, being said to be "complete." In these insects (fig. 162) the "larva" is vermiform, segmented, and usually provided with locomo

tive feet, which do not correspond with those of the adult, though these latter are usually present as well (fig. 162). In some cases the larva is destitute of legs, or is "apodal." The larva is also provided with masticatory organs, and usually eats voraciously. In this stage of the metamorphosis the larvæ constitute what are usually called "caterpillars" and "grubs." Having remained in this condition for a longer or shorter length of time, and having undergone repeated changes of skin, or "moults," necessitated by its rapid growth, the larva passes into the second stage, and becomes a 'pu- Fig. 162.-Metamorphosis of the Magpiepa." The insect is now permoth (Phalana grossulariata.) fectly quiescent, unless touched or otherwise irritated; is incapable of changing its place; and is often attached to some foreign. object. This constitutes what-in the case of the Lepidopterais generally known as the "chrysalis," or "aurelia" (fig. 162). The body of the pupa is usually covered by a chitinous pellicle, which closely invests the animal. In some cases (e.g., in many Dipterous insects) no traces of the future insect can be detected in the pupa by external inspection; but in the Lepidoptera the thorax and abdomen are distinctly recognisable in the pupæ;

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whilst in others (e.g., Hymenoptera) the parts of the pupa are merely covered by a membrane, and are quite distinct. In some cases the pupa is further protected within the dried skin of the larva; and in other cases the larva-immediately before entering upon the pupa-stage-spins, by means of special organs for the purpose, a protective case, which surrounds the chrysalis, and is termed the "cocoon.”

Having remained for a variable time in the quiescent pupastage, and having undergone the necessary development, the insect now frees itself from the envelope which obscured it, and appears as the perfect adult, or "imago," characterised by the possession of wings.

SEXES OF INSECTS.-The great majority of Insects, as is the case with most of the higher animals, consists of male and female individuals; but there occur some striking exceptions to this rule, as seen in the Social Insects. In those organised communities which are formed by Bees, Ants, and Termites, by far the greater number of the individuals which compose the colony are either undeveloped females, or are of no fully developed sex. This is the case with the workers amongst Bees, and the workers and soldiers amongst Ants and Termites. And these sterile individuals, or "neuters," as they are commonly called, are not necessarily all alike in structure and external appearance. Amongst the Bees, all the neuters resemble one another; but amongst Ants and Termites they are often divided into "castes," which have different functions to perform in the general polity, and differ from one another greatly in their characters.

The organs of the two sexes are in no case united in the same individual, or, in other words, there are no hermaphrodite insects. (In some abnormal cases amongst Bees, Lepidoptera, &c., hermaphrodite individuals have been observed.) has been noticed, however, before, asexual reproduction is by no means unknown amongst the Insecta, and the attendant phenomena are often of extreme interest. (See Introduction.)

The great majority of insects, during their adult condition, are terrestrial or aerial in their habits, but in many cases, even of these, the larvæ are aquatic. Many other insects live habitually during all stages of their existence in fresh water. A few insects inhabit salt water (either the sea itself or inland salt waters) during the whole or a portion of their existence. (This is the case with two or three Beetles of the families Hydrophilidae and Dytiscidæ, some Hemipterous Insects, and the larvæ of various Diptera.) Lastly, many insects live

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parasitically upon the bodies of Birds or Mammals, or upon other Insects.

DISTRIBUTION OF INSECTS IN TIME. - The most ancient remains of Insects at present known to us are from the Devonian Rocks of North America. Here occur several forms apparently belonging to the Neuroptera (or Pseudo-neuroptera). In the Carboniferous Rocks the remains of Insects are more abundant, and we find examples of several orders (such as the Coleoptera, Orthoptera, and Neuroptera). The orders Hymenoptera and Lepidoptera do not occur till the Secondary Period is reached; and in the Tertiary Rocks we find representatives of almost all the existing orders. Amber, which is a fossil resin, has long been known to contain many insects in its interior (in certain specimens); and all of these appear to belong to extinct species, though amber, geologically speaking, is not an ancient product.

CHAPTER XL.

DIVISIONS OF INSECTA.

THE class Insecta includes such an enormous number of species, genera, and families, that it would be impossible to treat of these satisfactorily otherwise than in a treatise especially devoted to entomology. Here it will be sufficient to give simply the differential characters of the different orders, drawing attention occasionally to any of the more important points in connection with any given family.

As already said, the Insecta are divided into three divisions, termed Ametabola, Hemimetabola, and Holometabola, according as they attain the adult condition without passing through a metamorphosis, or have an incomplete or complete metamorphosis. The Insects which come under the first head (viz., Ametabola) are not furnished with wings in the adult condition, and the four orders which compose this section are commonly grouped together under the name Aptera. By some, however, this division is entirely rejected, and the orders in question are placed amongst the Hemimetabola, or even grouped with the Myriapoda. Indeed, it is certain that the orders of the so-called Apterous Insects are not, strictly speaking, scientific divisions. It is, however, a matter of convenience to retain them in a separate form, as it is by no means absolutely

certain how they may most naturally be distributed amongst the higher orders.

SUB-CLASS I. AMETABOLA.-Young not passing through a metamorphosis, and differing from the adult in size only. Imago destitute of wings; eyes simple, sometimes wanting.

ORDER I. ANOPLURA.-Minute Aptera, in which the mouth is formed for suction; and there are two simple eyes.

This order comprises insects which are commonly parasitic upon man and other animals, and are known as Lice (Pediculi). The common Louse (fig. 163, A) is furnished with a

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Fig. 163-Morphology of Aptera. A, Pediculus humanus capitis; B, Docophorus hamatus, one of the Bird-lice; C, Campodea; D, Degeeria, one of the Pedurida: E, Scale of a Podurid, as seen under the microscope; F, Degeeria purpurascens. All the figures are greatly enlarged. (After Packard and Gervais.)

simple eye, or ocellus, on each side of a distinctly differentiated head, the under surface of which bears a suctorial mouth. There is little distinction between the thorax and abdomen, but the segments of the former carry three pairs of legs. The legs are short, with short claws or with two opposing hooks, affording a very firm hold. The body is flattened and nearly transparent, distinctly segmented, and showing the stigmata very plainly. The young pass through

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