Fig. 384.—Vertical section of cornea of Eye of Fly.
The principal use of these antennæ is that of organs of touch, but it is quite probable that they may subserve other functions, as of taste or even hearing. The eyes of insects consist of either a pair of ocelli, or of a great number, when they are termed compound eyes, formed of an aggregation of external hexagonal facets and lenses, and nerve filaments, all of which have a distinct connection with the mass of ganglia recognised as the brain, as will be seen in [Fig. 384], a section of the eye of a fly. The number of facets varies very greatly in these compound eyes; ants, for example, have fifty facets, flies two thousand or more, and butterflies as many. Dr. Hooke counted seven thousand, and Leuwenhoeck as many as twelve thousand in the eye of a dragon fly. The eyes of some insects are supported on short stalks or pedicles, but these are never movable, as, for example, in Stalk-eyed crustaceans.
The organs of the mouth in insects present a striking homology or similarity in their fundamental structure. Two chief types of mouth are found. The biting or masticatory, as in beetles, includes a labium or upper lip, a pair of mandibles or lower jaws, a pair of lesser jaws or maxillæ, which bear one or two pairs of palpi, and a lower lip or labium, also with palpi. This latter and primitive condition of the labium is seen in Orthropterous insects and some Neuroptera. Other structures occurring in those of the mouth are the ligula, this being sometimes divided, as in bees, into three lobes, of which the two outer are the paraglossæ and the middle process the lingua or tongue. There is a second form of mouth, termed the suctorial. This is seen in Lepidoptera (butterflies), and is adapted for extracting the pollen and juices of flowers, and in which the palpi are greatly developed, and form two hairy pads or cushions, between which the proboscis is coiled up when at rest. Thus we find in the Lepidoptera the same fundamental condition of mouth as in some Coleoptera. In Hymenoptera (wasps and bees), a variety of mouth is found which presents a combination of the masticatory with the suctorial types. The labium and mandibles exist as in the beetle, the maxillæ being developed to form long sheaths protecting the labium, which now takes the form of a tongue. In Hemiptera (bugs and their allies), the mandibles and maxillæ exist as sharp lancets, while the labium forms a protective sheath. In the Diptera (flies, gnats, &c.), the labium undergoes a great development, and forms a very prominent tongue, the other parts of the mouth being developed simply as sheaths to the labium. See Figs. 389 and 390.
The thorax or chest of insects consists of three segments, named from before backwards: the prothorax, mesothorax, and metathorax. The first bears the anterior pair of legs; the mesothorax, the second pair of legs and the first pair of wings; and the metathorax, the third pair of legs and second pair of wings. The last joints of the leg constitute the tarsus or foot-claws. The nervures of the wings are in reality hollow tubes, and are extensions of the spiracles, or respiratory apertures.
The muscles of insects lie concealed beneath the integument; they are not gathered into distinct bundles as in the higher animals, although they exhibit in many cases a striated or striped structure. This is well seen in some of the beetle tribe, the water-beetle in particular. In certain larvæ the muscles are exceedingly complicated. Lyonnet found in the larva of the goat-moth, two hundred and twenty-eight muscles in the head alone, and in the whole body no less than three thousand nine hundred and ninety-three. The muscular power of insects is, relatively to the size of the body, very great. The flea, for instance, leaps two hundred times its own height. There are beetles weighing a few grammes that will escape from a pressure of from twenty to thirty ounces.
Professor Schäfer infers that the structure of the wing-muscles of insects furnishes the key to the comprehension of the more intricate muscular structure of vertebrates. The sarcode element, however, is not made up of a bundle of rods, but of a continuous sarcous element, readily made out by staining with hæmatoxylin. This substance is then seen to be pierced by minute tubular canals, and the longitudinal striation of muscle is due to this canalisation. The whole is connected and enclosed by a membrane of extreme delicacy.
The digestive system of insects varies with their habits and food. In Stylops, bee-parasites, and in young bees living on fluids, the intestine ends in a blind sac. There are three coats of structure throughout the digestive system. The œsophagus or gullet is provided with a crop in flies, bees, and butterflies; a true analogue of the gizzard in birds. There is in some respects a curious likeness between the conformation of the digestive organs of birds and that of insects. No true liver, but salivary glands in the mouth have been made out; the heart lies dorsally, and consists of a pulsating sac divided into compartments, and the fluid flows through it towards the head, whence it circulates freely to other parts of the body. Each trachea is an elastic tube formed of two delicate membranes, between which the spiral filament is coiled up, and is of sufficient density to prevent the collapse of the tube by the movements of the body. These tracheæ are distributed throughout the muscular tissue and the whole of the body. Thus the insect, like the bird, may be said to breathe in every part of the body, and is in this way rendered light and buoyant for flight. The air is admitted to the tracheæ by apertures termed spiracles, which the insect can close at will, and these are distributed to the number of eleven on each side of the body. The nervous system consists of a chain of ganglia or nerve-knots, which unite towards the head to form a single cord, as seen in the section made through the spider ([Fig. 409]).
The reproduction of all insects takes place by ova, and they are diæceous—that is, have two distinct sexes. In some few instances, as that of Aphides, or plant-lice, we have the peculiar phenomenon of parthenogenesis, the process of reproduction being performed by imperfect wingless females. These bring forth living young ones, which begin to feed the moment they are born, and constitute a viviparous brood; in other cases females lay eggs, and the process proceeds in the ordinary way, and nearly all the year round. The former is provided with a lancet-like beak for piercing and sucking the juices of the leaf, and a pair of curious honey-tubes. Insects generally undergo a transformation or metamorphosis in passing from the egg to the adult stage. While within the egg the body may be seen to become segmented, and in the course of time—in such insects as flies, bees, beetles, and butterflies—issue forth from the egg as larvæ, or caterpillars. This worm-like creature makes for itself an investing case or cocoon, in which it passes into the pupa stage of its existence. Within the pupa case a wonderful transformation takes place; the larval body being literally broken down by the process of histolysis, while its elements are rebuilt and transformed into that of the imago, or perfect insect. In grasshoppers, crickets, dragon-flies, bugs, &c., the metamorphosis is incomplete (hemimetabolic). Some few lower insect forms (lice, spring-tails, &c.) undergo no change of the kind, and in no way differ from the adult except in size. These are termed ametabolic insects. Others again, as the cockchafer and gold beetle, pass three years in the larval stage. Development in all cases is arrested or retarded by cold. Reaumur kept a butterfly pupa for two years in an ice-house, and it exhibited no tendency towards a change until removed to a warm temperature.
From the short natural history of insect life I have endeavoured to sketch out, it will have been surmised that insects offer a wide field of research, and an almost endless number of objects of interest for the microscope. The variety of material is great, and the structure and adaptation of means to an end is of the most fascinating kind. Most cabinets abound in preparations gathered together with some care and mounted with all the skill at the command of the collector, affording, as a rule, as endless an amount of pleasure to the tyro as to the more practised entomologist. It may be surmised, then, that to enter fully into a description of the several parts of insect structure would require a volume[81] of very large bulk, and occupy months and years. I will, therefore, take some points of interest in the structural characteristics of insects, and take them in the order in which they have already been brought to notice. The head, eyes, and other appendages of these insects we are more or less acquainted with.