LECTURE XVI.

INSECTA.

ALTHOUGH Spontaneous locomotion is the peculiar attribute of the Animal Kingdom, we have seen that the lowest members, the Zoophytes as they were termed, are, for the most part, fixed and motionless, like plants we have seen that the first manifestations of locomotion were of the feeblest and simplest character, a rowing of the body through an element of equal density with itself, or a trailing of the body along the ground, which supported it at every point. As we advanced to the survey of the Articulate series of animals, we saw the integument progressively hardened, divided into segments which were united by flexible joints; at length supported upon moveable jointed limbs, consisting of hollow columns of integument hardened into a dense exterior crust, capable of serving the office of levers and fulcra, whereby the animal could raise its belly from the dust, and swiftly traverse the surface of the ground.

We now come to a class of Articulata, in which the highest problem of animal mechanics is solved, and the entire body and its appendages can be lifted from the ground and be propelled through the air. The species which enjoy this swiftest mode of traversing space, breathe the air directly: but their organs of respiration are peculiarly modified in relation to their powers of locomotion; they consist of innumerable tracheæ commencing from lateral pores called stigmata, or by anal tubes, which are ramified through and over every tissue and organ of the body. The nervous system is homogangliate; the organs of sense include two jointed antennæ and two compound eyes; the skeleton is principally external, and cut deeply into segments, whence the name of the class Insecta.

Insect

Not every Insect, however, has the power of flight, nor any save in its last and most perfect state; many undergo most remarkable transformations before they acquire their wings, and although some Insects, which ultimately are so endowed, undergo a less amount of change, yet the metamorphoses are always least remarkable in the apterous species.

Of these lowest members of the class of Insects, many have more than three pairs of legs, have sometimes indeed eighty pairs and upwards in their mature state in them metamorphic development exhausts itself, as in the Anellides, in the successive acquisition of new segments

and legs in addition to those which previously or originally existed; these Insects are therefore termed Myriapoda.

True or hexapod Insects have thirteen rings, one for the head, three for the thorax, and nine for the abdomen. Certain flying Insects in their early or larval state present several pairs of rudimental feet, in addition to those attached to the first three segments, succeeding the head, but no true Insect in its mature state has more than the three pairs of articulated limbs just indicated.

Every Insect has a distinct head (fig. 96, a), provided with one

96

h

ab

Locusta.

pair of antennæ (c), and its trunk is divided into two regions called thorax and abdomen (a b).

The thorax is interposed between the head and the abdomen, and so far is analogous to that part in human anatomy; but it has neither the same relation to the contained viscera nor to the locomotive extremities which characterise the thorax in the vertebrate animals. To the Insect's thorax are attached all the locomotive members; both the first pair, which may be compared with the pectoral extremities of the vertebrate animal, and the last pair, which may corre

spond with the pelvic members, as well as the middle pair, to which there is no analogue in the vertebrate series. This centre of the locomotive powers is divided into three segments, which correspond with the three pairs of legs: the first segment is termed the “ prothorax " (d), the second the "meso-thorax (f), and the third, the "meta-thorax" (i). Each of these segments has a dorsal and a sternal piece the dorsal half rings are called respectively "pronotum," "mesonotum," and "metanotum ; " the ventral or sternal arcs bear the corresponding terms, 66 prosternum," " mesosternum," and "meta-sternum." From the inferior arches of the segments, the legs (e, h, k) are developed, or with them they are principally articulated, like the legs of the Crustacea and the ventral oars or seti

gerous prolegs of the Anellides. In the flying insects there are developed from the dorsal arches of the middle and third segments, locomotive appendages which constitute the wings (g,j).

It must not be supposed that the parts of the thorax which have just been described are naturally or uniformly separate, and moveably connected with one another; they are more commonly confluent, but in different degrees in different families; so as more or less to obscure the primitive traces of their original distinctness, which can only be demonstrated, as has been done by Macleay, Audouin, Burmeister, and others, by an extended comparison of the thorax in the whole class of Insects, or by tracing its development and modifications during the various stages of the metamorphoses When the composition of the thorax of an insect is thus studied, it is found to be made up of not less than fifty-two pieces, which have for the most part received, and necessarily, distinct names in Entomology, and many of them, very unnecessarily, more names than one.

The abdomen is usually formed of a greater number of segments, always nine in the larva, which retain a greater degree of mobility upon each other; but it supports no locomotive appendages in the hexapod insects.

The tissue of the external skeleton is of a dense, resisting, but light material; it looks and feels like horn, but it has for its base a peculiar substance called "chitine," which is insoluble in caustic, potash, and retains its form like charcoal when submitted to a red heat. The articulated appendages consist, like the segments of the trunk, of hollow cases or tubes of the same firm and slightly flexible substance; which tubes contain the muscles, nerves, and other soft parts in their interior. The integument is softer and more yielding in larvæ, flies, and most parasitic insects. It consists of three layers, epidermal, pigmental, and dermal, the derm and epiderm more closely resemble each other in physical properties than in other animals: they are separated and cemented together by sometimes two distinct coloured layers of rete mucosum. The hairs, spines and scales are processes of the epiderm, which often include a coloured substance.

I may now proceed to a more particular description of the jointed and aliform appendages of the skeleton. The first pair inserted into the front or upper part of the head, are the antennæ, which present a vast variety of shapes and sizes in different insects, but seem in all to have most intimate relation to the senses of touch and hearing. Their precise function has not, however, yet been well defined. The Entomologist avails himself of their various conformation to obtain characters for the distinction of families, of genera, or

of species of insects; and a considerable section of the glossology of this extensive department of Natural History is devoted to the technical terms required to express the antennal characters. To the head likewise belong more or less complicated oral instruments, called " trophi," or "instrumenta cibaria," modified in some insects to serve for suction, in others, for mastication: they properly fall under the demonstration of the digestive system.

The jointed legs attached, as before stated, to the three thoracic segments, consist each of a hip, a thigh (k), a leg (1), and a foot (m), commonly called the tarsus; but which are not to be taken as rigorously answerable to the parts so termed in Human Anatomy. The hip, for example, consists of two joints, usually the shortest of the whole leg: the foot or tarsus includes from two to five joints, and is terminated by a pair of diverging hooks or claws. The peculiar powers of moving upon land or in water depend upon the modifications of the forms or proportions of these extremities. In water insects the tarsi are usually flattened, fringed with hair, and stretched out in the same plane with the trunk, like oars. In leaping insects the hinder limbs present as disproportionate a development as the legs of the kangaroo. In burrowing insects, the anterior limbs are distinguished by short, broad, and massive proportions, with a strong and flattened hand, like that of the mole, as in this best of insect burrowers *, which has been called the mole-cricket. Most insects are able to crawl up vertical walls, and some along glass and the ceilings of rooms, against gravity: the house-fly achieves this by virtue of the development of little suckers upon the under surface of certain expanded joints of the tarsus.

The wings of insects are essentially flattened vesicles, sustained by slender but firm hollow tubes called "nervures," along which branches of the trachea, and channels of the circulation, are continued. The wings never exceed two pairs, which are developed from the mesonotum and metanotum. Sometimes one or other of these pairs is wanting. The wings present many varieties in their shape, their consistence, and their teguments. When they subserve flight, they are thin and transparent; or if opake, are rendered so by an imbricated clothing of most delicate scales, which, when detached, resemble the pollen of flowers. In certain insects, especially those that burrow, the first pair of wings become thick, hard, and opake, forming a kind of shield to the back; they are called "elytra," and cover the posterior pair of membranous wings when these are not expanded for flight. Sometimes the anterior wings are membranous

* Prep. No. 463, A.

at their extremities, hard and opake at their base, when they are called "hemelytra." When the hinder pair of wings is wanting, it is replaced by a pair of rudimental appendages called balancers: other modifications of, or appendages to, the wings have been called "alulæ" and "patagia."

The muscular system is, as may be supposed, developed in relation to the several kinds and powers of locomotion indicated by the modifications of the extremities. As a necessary corollary of the cylindrical form and external position of the principal parts of the skeleton, the joints are for the most part ginglymoid, and restricted to movements in one plane. The muscles of the legs are consequently simply flexors and extensors. The coxæ have a round head inserted into cup, and the movements of the hip-joint are rotatory; the head is usually connected with the thorax by a similar joint, which, from the greater freedom of the movements, may be termed arthrodial. In insects of flight, the cavity of the thorax is almost entirely occupied by the muscles of the wings. The muscular fibre is transversely striated, and is also characterised by a second series of transverse indentations at regular but wider intervals.

The Orders of Insects are founded upon the modifications of the wings; those in which the first pair serve as sheaths, and the second alone are used for flight, and are folded transversely when at rest, constitute the order Coleoptera: they undergo complete metamorphosis, and are subdivided according to the number of joints of the tarsi. Beetles and most burrowing Insects belong to this order.

Those insects in which the anterior pair of wings are converted into elytra, of less density than in the Coleoptera, and in which the posterior wings are folded longitudinally when at rest, constitute the order Orthoptera: they are said to undergo a semi-metamorphosis, the chief change being the acquisition of wings. This order includes the most voracious and destructive Insects, as the Locust, Cockroach, &c.

Those Insects which have both pair of wings membranous, transparent, strengthened by numerous nervures, and finely reticulated, form the order Neuroptera, which includes the highest organised insects, as the predatory dragon-flies.

The Insects which have four membranous wings simply veined, and crossing each other horizontally when at rest, form the order Hymenoptera: they undergo a complete metamorphosis, and include the most useful of insects, as the bee.

The Insects with four wings, more or less clothed with minute scales, are called Lepidoptera: they undergo complete metamorphosis, and include the most beautiful species of the class, as the butterflies: in one family of this order the wings are divided lengthwise into a