STRUCTURE.
The imago or perfect insect is encased in a more or less perfect horny integument composed of a substance called chiten, which forms in many a regular box containing all the vital organs. Every insect can be divided into three primary divisions: first, the head, to which is attached the mouth parts, antennae, and eyes; second, the thorax or chest, sometimes forming a solid mass, but properly composed of three segments, namely, the prothorax, mesothorax, and metathorax, to which are attached three pairs of legs, and two pairs of wings (there are, however, many exceptions to the last, as some have only one pair, and others are wingless); third, the abdomen or body, consisting of a number of segments variously estimated from five to eleven, the normal number being ten, which enclose the digestive, breathing, and reproductive organs.
Every insect in the first instance comes from an egg or living larva produced by the female, and though, even to the naturalist, it seems very hard to account for the countless millions of some of the smaller insects such as aphids and scale insects which suddenly swarm as if by magic over plants, there is no such thing as spontaneous generation; insects cannot come out of the ground from nothing, or be produced from the crystalline dew upon the foliage as we have sometimes found stated in newspapers. Again, a grain weevil cannot change into a flour moth, or vice versa, as many of our farmers will say in all good faith. In some groups the insects are produced as living larvae, and commence to feed at once; but in the majority, eggs are deposited in or upon the food, in which state they may remain without hatching but a few days, or more than a year. As soon as the little caterpillar or grub emerges, it starts upon its food, spending the whole of this stage of its existence in eating and growing, moulting at intervals by casting off the outer skin to accommodate its increasing bulk; when full-grown and ready for the final moult, it stops eating, crawls away into a suitable place, and forms a cocoon, cell, or shell, in which it pupates. It is now a chrysalis or pupa, quiescent, without any movement except a slight twitching of the tip of the abdomen when disturbed. Under the pupal covering the different organs of the perfect insect become gradually defined, until one bright day the last evolution is completed, and with a few convulsive movements the perfect insect bursts out of its enveloping swaddling clothes and appears in all its beauty and perfection.
Some groups, however, undergo a much more simple or incomplete metamorphosis; emerging from the egg a baby insect ready to eat, (like the grasshopper,) the same food as its mother; it undergoes a series of moults, and after casting its skin every time, becomes more nearly perfect without any true pupal stage, and finally after the last moult comes forth with well-developed wings, a perfect insect.
The typical insect is furnished with a large compound eye on either side of the head composed of a number of small sections called facets, varying in number from sixteen to several thousands in some of the more highly developed families, and two or three simple eyes forming bright shining dots between the compound pair called ocelli. In some groups these ocelli are wanting; in others both eyes and ocelli, so the insects are therefore blind.
The mouth parts are composed of several hard plates; in chewing or biting (mandibulate) insects, they consist of a pair of stout jaws, in front of which lies the labrum, and behind the maxillae; again behind the maxillae follows a second pair fused together to form the labium. Both labium and maxillae are provided with a pair of slender jointed appendages known as labial and maxillary palpi; these are used as fingers to assist in drawing food into the mouth. In those groups with sucking (haustellate) mouths these various parts are coalesced into a simple sucking tube ending in a sharp style-like tip, which is buried in the tissue of the plant when the insect is feeding. The antennae when well developed consist of a number of distinct rings or segments, standing out on either side of the head, and generally attached to the front of the head between the eyes; they serve as organs of touch, smell, and probably hearing. The legs contain five distinct joints; first, the coxae or hips; next, the trochanters, small joints with a ball and socket-like action from which the femora or thighs move backward and forward; to these are attached the tibiae or shanks terminating in the tarsi or feet at the extremity. Most insects are also furnished with a pair of tarsal claws, between which may be a small pad, called the pulvillus or empodium.
The wings of insects vary considerably. Some are membranous and smooth; others are covered with down or scales; while in many the fore pair are solid chitinous wing-cases, useless for flight, and chiefly acting as protective covers to the hind pair, which, when the insect is at rest, lie folded up beneath them. The flying wings are traversed with branching tubes called nervures, which, while strengthening them, also perform an important function in the breathing of the insect, and are pierced with small openings; these openings are very distinct in some of the hymenoptera.
Insects breathe by means of openings situated along the sides of the thoracic and abdominal segments called spiracles, opening out into branching air vessels called trachea, which pass into the interior, ramifying throughout the body and extending into every part and appendage, even to the tips of the antennae. The nervous system, the life and movement of the insect, consists of a double chain of ganglia, (ganglion, a knot,) nerve cells, which are connected with finer encircling nerve tissues, that radiate in all directions, returning to the ganglia, the latter regulating the nerve sensation. Therefore, as their perceptions are so much less confined to the brain than in vertebrate animals, they cannot feel to the same extent. Thus, you can frequently find a locust, beetle, or ant that has escaped from a bird, minus its abdomen, still crawling about, quite able to move all its remaining organs. You can even remove the long slender body of a dragon-fly, and carefully insert a bit of grass stalk of the same length and weight to balance the wings, and it will fly off quite readily; but of course they will not live long after such injuries.
We know that many insects must have very keen perceptions of sound, or the movement of the air around them, for they will drop to the ground at the least alarm, before the bush upon which they are resting has been touched. Very little is understood about the organs of hearing, except in the case of grasshoppers and locusts where the ears have been located at the base of the abdomen or on the front leg; these in some species can be detected with an ordinary lens. It is considered by some writers that the hairs and spiracles upon the different parts of the body may transmit sound and act as ears. The organs of sound are very interesting, but can be better treated when dealing with the different groups.
Usually, there are only two sexes of insects, males and females; but among those living in social communities, like the bees, ants, wasps, and termites, the majority of the inhabitants are neuters. These neuters are usually aborted females, which do all the work in the construction of the nest and look after the food supply of the rest of the community.
FOSSIL INSECTS
In comparison with other countries, fossil remains of insects are scarce; only ten species have been described and named. My information on this subject is obtained from Messrs. Etheridge and Olliff’s Memoir of the Geological Survey, (Palaeontology No. 7,); “The Mesozoic and Tertiary Insects of New South Wales,” 1890. The first record of fossil insects was made by Moore in the Quarterly Journal of the Geological Society, 1870, entitled “Note on a plant and insect bed on the Rocky River, N.S.W.” “These insects were obtained from a chocolate-coloured micaceous laminated marl, forming a bed ten feet thick, at a depth of about one hundred feet from the surface, and forming a portion of the Tertiary drift worked at the above locality. The latter are probably of Pliocene age.” (E. & O.)
Jack obtained the wing of a dragon fly in the Cretaceous beds of the Flinders River, N. Queensland, which was described and figured by Woodward under the name of Aeschna flindersensis in the Geological Magazine, 1884. It was entombed in a dark chocolate limestone.
The insects described and figured by Messrs. Etheridge and Olliff consist of a cicada (C. lowei) from the “Taeniopteris-bearing beds of the Talbragar River in New South Wales, and of Lower Mesozoic age”: a fly, Chironomus venerabilis: and a mayfly, Ephemera culleni: and a beetle larva belonging to the Lampyridae, under the name of Palaeolycus problematicus, from the Tertiary beds at Emmaville, New England.
From the Ipswich Coal Measures of Queensland comes the fossil wing of a Buprestid beetle, allied to existing Stigmodera, which they called Mesostigmodera typica. Among the insect remains from this locality the authors note several wings that appear to belong to weevils and other beetles allied to existing species.
Mr. W. S. Dun informs me that insect remains have been found at Narellan N.S.W. in Wianamatta shales, and also in the brick pits at St. Peters near Sydney.