Entomology

ENTOMOLOGY. This is the science which treats of the habits, transformations, and physical structure of insects; or the science or natural history, and description of insects. An insect is an articulate animal form, having the body com posed of three distinct parts, the head, corslet or thorax, and abdomen ; the legs are six in num ber with, usually, two or four wings attached to the thorax; and, along the sides of the abdomen minute punctures, called spiracles, by means of which respiration takes place. Formerly spiders, and crnstacea, and even worms and other small animals, were included under the term insect. The term is now restricted to the hexapods, or the six-footed species, known as beetles, bugs, bees, grasshoppers, locusts, fleas, etc., primarily produced from eggs, whatever secondary modes of propagation may take place. The egg state constitutes one of the most important epochs in insect life, since on this depends all the subse quent states and developments affecting devel opment and decay. From the egg the larva is. produced. The late and lamented Dr. Le Baron, has carefully and tersely described the outlines of Entomology, and insects in general, in one of his valuable reports : From it we extract: Insects as a class, and in the wide-t meaning of the word, comprise three divisions, or sub-classes, com monly, known as spiders, insects and millipedes. They may be distinguished by the following characters : 1. Sub-class : Arachnids, including spiders, scorpions and Acari, or mites. Body divided into two parts, the head and thorax being united in one ; legs eight in number : without wings. 2. Sub-class; Insecta, or insects proper. Body divided into three parts, the head, the thorax, and the abdomen ; legs six ; furnished_ with wings, in the perfect or imago state. 3. Subclass: Myriapoda, commonly called milli pedes or centipedes. Body divided into many parts or segments, varying from ten to two hun dred; legs numerous; usually either one or two pairs of legs to each segment of the body; never have wings. The exceptions to these characters are very few. In the Arachnida, some of the most minute (Acaii) have but six legs. Insects. proper are always six-legged in their last or perfect state; and they also generally have six true legs in their larva state; but some larva have no legs, and the larvae of the Lepidoptera, com monly called caterpillars, have, in addition to their six true legs, several pairs of false legs, or pro-legs, which assist in locomotion. There are a few exceptional cases in which insects are des titute of wings. The fleas (Pulice8), the lice (Pediculi), and the little family of insects known as spring-tails (Thy8anoura), never have wings In some rare instances the females are wingless, whilst the males have wings. This is the case with some species of the lightning-beetles (Lam pytidee), and with the canker-worm moth, and the tussock-moth, and a few other species. amongst the Lepidoptera. Similarly exceptional cases are also found in other orders of insects. The present work treats only of insects proper. The nervous system of insects consists of a double cord extending the length of the body, and lying upon the inferior or ventral side of the internal cavity. The two threads which corn• pose this cord do not lie side by side ; but one above the other. The lower thread swells at intervals into little knots of nervous matter, called ganglia. In insects of an elongated form, such as some of the Neuroptera, and the larvte of the Lepidoptera, there is a ganglion at each segment of the body, making thirteen in all ; but in most mature insects the ganglia become more or less consolidated. In the butterfly (Papilio), there are ten ganglia, counting the brain as one: in the bee (Apia), there are eight; in the may beetle (Melolontha), there are five, and in the Cicada there are but two. The upper of the two nervous threads runs nearly in contact with the lower, but is destitute of ganglia. These two threads seem to represent the double and more compact cord which constitutes the spinal mar row of the higher or vertebrated animals. The upper simple thread is supposed to furnish the nerves of motion, and the lower and ganglionic thread, the nerves of sensation. The fibres which compose these cords separate at the anterior extremity of the body, so as to embrace the cesopbagus or gullet, above which they again unite to form the cerebral ganglion or brain, which is somewhat larger than the other gang lia. From the nervous cords, and chiefly from the ganglia, fine lateral threads are emitted, which are distributed to the adjacent parts. The nerves thus far described represent what, in the higher animals, is called the cerebrospinal system of nerves, and are sometimes called the nerves of relation, because they control the sensations and motions which associate the animal with the world around it. But in addition to these, there have been discovered a number of very fine nervous filaments proceeding from the brain, and extending down into the body, and furnished with minute ganglia of their own, which are supposed to represent the sympathetic system of nerves which preside over the internal functions, such as those of digestion and secre tion. The blood of insects is a colorless fluid, which does not circulate in closed vessels or tubes, like that of the higher animals, but permeates the tissues of the body. The only vessel that can be discovered is an oblong, membranous, pulsating sac, situated in the upper or dorsal part of the body, and evidently represents the heart. This is divided into several compartments by cross valves, which are so arranged as to permit the blood to pass only in a forward direction. The heart is prolonged anteriorly into a narrower tube analogous to the aorta. Through this the blood flows first towards the head and thence through the body, returning to the heart, which it enters through openings at its sides. As compared with that of the warm-blooded animals, the blood of insects is not only colorless, but small in quantity, and must circulate very slowly, as is proved by the fact that when their bodies are wounded no blood escapes. Most of the organs of insects, and their functions, have an obvious analogy to those of the higher animals, but their breathing appara tus is constructed upon an entirely different plan. In all the vertebrated animals the blood is carried in vessels to a particular part or organ of the body, for the purpose of being exposed to the life-giving influence of the air. This part in terrestrial ani mals, is the lungs, and in aquatic animals the gills. But in insects the process is reversed, and the air is carried to the blood by being distributed to every part of the body in very delicate pearl white tubes or vessels, which present a beautiful appearance under the microscope. They are called trachea, or air tubes. They admit the air through little openings along the sides of the insect's body, called spiracles. The spiracles or breathing pores can be easily seen along the sides of all caterpillars which are not too densely cov ered with hairs. In the perfect or winged state of insects the branches of the air tubes are ciliated into a great number of little vesicles or air blad ders, which render their bodies lighter, and thus facilitate their flight. In some aquatic larva; the tracheas project from the body in the form of lit tle tufts, analogous to the gills of fishes. The aquatic beetles are under the necessity of rising to the surface, at intervals, for air, in a manner similar to that of the aquatic mammalia, the whales and the dolphins. The digestive appara tus of insects, like that of other animals, consists of an elongated tube called the alimentary canal, extending through the body, and having a num ber of enlargements in its course, and in many insects presents a particular resemblance to the digestive apparatus of birds. First, there is a short, straight oesophagus or gullet; this expands into a much larger cavity, resembling the crop; then follows a smaller muscular part, analogous to the gizzard ; and next, a much larger and longer cavity, which is the true digestive stomach; this becomes contracted into the intestinal canal, which sometimes runs nearly straight through the body, and in other cases is more or less con voluted; the intestine enlarges again before it reaches the end of the body into what is known as the large intestine or colon. As in other ani mals, the alimentary canal is much longer and more capacious in the herbivorous than in the carnivorous kinds. As a rule the canal is more capacious in the larva than in the imago state. The secretory apparatus of insects, though analogous in function, is very different in appearance from that of the higher animals. Instead of solid glands, like the liver or kidney, it has the form of masses of convoluted tubes. The salivary glands, the liver, the kidneys, and the testicles are found represented in insects. The gastric and pancreatic fluids are secreted by little cells or follicles in the coats of the stomach. The muscles of insects, like those of other ani mals, consist of contractile fibres, but in their situation and attachments, as compared with those of the vertebrate animals, they are reversed; that is to say, in the latter, the muscles are situated outside of, and upon the bones, which constitute the supporting part of the body whereas in insects the supporting part is the external crust, and the muscles are attached to its internal surface. The muscles are of a pale yellowish

color, and are usually presented iu the form of thin layers, and sometimes of isolated fibres, and are never united into the rounded compact form which they have in the higher animals. By counting the separate fibres, a very great number of muscles have been enumerated. Lyonet counted nearly four thousand in the larva of Cossus ligniperda, and Newport found an equal number in the larva of Sphinx ligustri. The muscles of insects possess a wonderful contractile power in proportion to their size. A flea can leap two hundred times its own length, and some beetles can raise more than three hundred times their own weight. This remarkable strength may probably be attributed to the abundant supply of oxygen by means of the myriad rami fications of the air tubes. Insects are evidently endowed with the ordinary senses which other animals possess, but no special organs of sense, except those of sight, have been discovered with certainty. The eyes of insects are of two kinds, simple and compound. The simple or single eyes are called ocelli, and may he compared in appearance to minute glass beads. They are usually black, but sometimes red, and are gen erally three in number, and situated in a triangle on the top of the head. In insects with a com plete metamorphosis, these are the only kind of eyes possessed by them in their larva state, and in these they are usually arranged in a curved line, five or six in number, on each side of the head. We have noticed that in some insects which undergo only a partial metamorphosis, as for example the common Squash-bug (Comets tristis), the ocelli are wanting in the larva and pupa states, but become developed in the last or perfect stage. The compound eyes of insects present one of the most complex and beautiful mechanisms in the organic world. They are two in number, but proportionately very large, occupying in many insects nearly the whole of the sides of the head and, in the dipterous order especially, often present across their disks,bands of the richest tints of green, brown and purple. These eyes are found to be composed of a great number of lesser eyes or eyelets, in the form of elongated cones so closely compacted as to form apparently a single organ. The larger ends of these cones point outwards, and by their union form the visible eye. Their smaller extremities point inwards, toward the brain, to which they are connected by means of a large optic nerve. When one of these eyes is examined through a strong magnifying glass, it is seen to be composed of a very great number of little facets, sometimes square, but usually six sided, each one of which represents the outer and larger extremity of one of the component parts. These facets vary greatly in number in the eyes of different kinds of insects. In the ants there are about fifty in each eye, in the Sphinx moths, about 1 300; in the house fly, 4,000; in the butterfly, upwards of 17,000; and in some of the small beetles of the genus Mordella, it is said that more than 25,000 facets have been enumerated in one compound eye; so that if we suppose that each of these component parts possesses the power of separate vision, one of these insects must have more than 50,000 eyes. How vision is effected, or how a unity of impression can be produced by so com plex an organ, we are unable to conceive. Insects are evidently affected by loud noises, and moreover, as many insects have the power of producing voluntary sounds, it is reasonable to suppose that they possess the sense of hearing. No organ, however, which has been generally admitted to be an organ of hearing, has been discovered. It is the most common opinion of entomologists that the antennae are instrumental in receiving the impressions of sound, and that the sense of hearing is located at or near their place of attachment to the head. and this view is much strengthened by the fact that in some of the larger crustaceans, such as the lobster and crab, a distinct organ of hearing is found located at the base of the antenna;. That insects are endowed with the sense of smell, is proved by the fact that the carrion-fly, and other insects which feed upon, or deposit their eggs upon, putrescent matter, detect such substances at a distance, however completely they may be hidden from the sight. The bee also discovers honey under similar circumstances, and it is therefore fair to presume that insects discover flowers, more by their perfume than by their visible characters. But no organ of smelling has been discovered, and this sense is supposed, from analogy, to be located in the lining membranes of the spiracles. It is impossible to determine, but there is no reason to doubt, that insects, like other animals, taste and enjoy food of which they partake ; and the manner in which they frequently touch their food, and the surfaces over which they walk, with the tips of their palpi, which, indeed, have received the common name of feelers, renders it probable that these organs are endowed with a special sense of The songs of birds, and the noises made by other animals, are produced by the forcible passage of air through the glottis, which is the narrow opening at the top of the wind pipe, aided by the vibration of certain muscular folds near the outlet, called the vocal chords. But we have seen that insects never breathe through their mouths, and, therefore, they never make any oral sounds. But the bumming of bees and flies is produced in an analogous manner, by the expul sion of air through the thoracic spiracles, and the vibration of a delicate valve-like fold, just within the opening. But besides this, insects make a variety of noises, which are produced in different ways. The singing of the Cicada, which is the loudest noise made by any insect, is pro duced by the expulsion of air from the first abdominal spiracle, striking upon a large trans parent drum-like apparatus, situated at the base. of the abdomen. The chirping of crickets is produced by rubbing together their parchment like wing covers. The well-known noise of the katydid is produced in the same way, but here the sound is intensified by a thin talc-like plate set into the base of each wing-cover. The stridulation of grasshoppers is caused by the friction of their spined shanks across the edge of their wing covers. The fainter, squeaking sounds, made by many insects when captured, are produced simply by the rapid friction of one part of their bodies upon another; in certain. Hemiptera, by the friction of the head upon the pro-thorax; in the Capricorn beetles, by the friction of the pro-thorax upon the meso-thorax; and in some of the Lamellicorn beetles, by the friction of the abdomen against the wing The more complex and special apparatuses of insects for the production of sounds, are pos sessed exclusively by the males, and are supposed. to be exercised by them as calls to the opposite sex; but the simpler squeaking sounds are emitted by both sexes, and appear to be mere notes of alarm. Nothing in the history of insects is more remarkable than the striking changes of form which many or them undergo, in the course of their development. Whilst other animals progress from infancy to maturity, simply by a process of growth, and by such gradual and imperceptible changes only as their growth necessitates, many insects assume totally different forms in the course of their develop ment, so that they could never be recognized as the same individuals, if this development had not been actually traced from one stage to another. These changes are called the meta morphoses or transformations of insects. All insects, in their growth, pass through four stages, designated as the egg state; the larva, or caterpillar state; the pupa, or chrysalis state; and the imago, or perfect and winged state. The metamorphoses of insects are of two prin cipal kinds, complete and incomplete. In the complete metamorphosis the larva bears no resemblance to the imago, and the insect, in the intermediate or pupa state, is motionless, and, takes no food. This kind of metamorphosis presents two principal varieties. In some (Lepi doptera and many Diptera,) the legs and wings are completely inclosed in the pupa case. In others, (Coleoptera, Hymenoptera, and some others,) the legs of the pupa, though useless, are free, and the rudimental wings lie loosely upon the sides. Moreover, in some (the nocturnal Lepidoptera, and many Hymenoptera,) the pupa. is inclosed in a separate covering or cocoon, whereas the majority of insects have no such covering. Pupae thus inclosed are calledfolliculate. The term chrysalis, from a Greek word meaning.