The Insect World: Being a Popular Account of the Orders of Insects

INTRODUCTION.

above the water, certain insects employ their elytra* as a sort of reservoir; others make use of their antennae, the hairs of which retain the globules of air. In this case it is brought under the thorax, whence a groove carries it to the spiracles. Sometimes the same result is obtained by a more complicated arrangement, consisting of respiratory tubes which can be thrust into the air, which it is their function to introduce into the organisation.

Insects which breathe in the water without rising to the surface are provided with gills — organs which, though variable in form, generally consist of foliaceous or fringed expansions, in the midst of which the tracheae ramify in considerable numbers. These vessels are filled with air, but it does not disseminate itself in them directly, and it is only through the walls of these tubes that the contained

gas is exchanged for the air held in suspension by the surrounding water. The oxygen contained in the water passes through certain very permeable membranes of the gill, and penetrates the tracheae, which discharge, in exchange, carbonic acid, which is the gaseous product of respiration.

Fig. 1 4 represents the gills or breathing apparatus in an aquatic insect. We take as an example Ep/iemera.\ It may be observed that the gills or foliaceous laminae are placed at the circumference

Fig. 14. of the body, and at its smallest parts.

We have now seen that the respiratory Branchiae, - or {EpJienierd).gills, of an aquatic larva apparatus is considerably developed in A, foliaceous laminae, or gills. insects; it is, therefore, easy to foresee that those functions are most actively employed by them. In fact,

• The horny upper wings with which some insects are provided are called elytra.— Ed. t May -ily family. — Ed.

tS THE INSECT WORLD.

if one compares the oxygen they imbibe with the heavy organic matter of which their body is composed, the amount is enormous.

Before finishing this rapid examination of the body of an insect, we shall have to say a few words on the nervous system.

This system is chiefly composed of a double series of ganglions, or collections of nerves, which are united together by longitudinal cords. The number of these ganglions corresponds with that of the segments. Sometimes they are at equal distances, and extend in a chain from one end of the body to the other; at others they are many of them close together, so as to form a single mass.

The cephalic ganglions are two in number; they have been described by anatomists under the name of brain. " This expression," says M, Lacordaire, "would be apt to mislead the reader, as it would induce him to suppose the existence of a concentration of faculties to control the feelings and excite the movements, which is

not the case.""' The same naturalist observes, "All the ganglions of the ventral chain are endowed with nearly the same properties, and represent each other uniformly."

The ganglion situated above the oesophagus gives rise to the optic nerves, which are the most considerable of all those of the body, and to the nerves of the antennae. The ganglion beneath the oesophagus provides the nerves of the mandibles, of the jaws, and of the lower lip. The three pairs of ganglions which follow those placed immediately below the oesophagus, belong to the three segments of the thorax, and give rise to the nerves of the feet and wings. They are in general more voluminous than the following pairs, which occupy the abdomen.

Fig. 15 represents the nervous system of the Carabus auratus: A is the cephalic ganglion; b, the sub-oesophagian ganglion; c, the prothoracic ganglion; d and e are the ganglions of the mesothorax and metathorax. The remainder, f f, are the abdominal gangHons.

Before finishing these preliminary observations, it is necessary to say that the preceding remarks only apply absolutely to insects arrived at the perfect state. It is important to make this remark, as insects, before arriving at that state, pass through various other stages. These stages are often so difl^erent from each other, that it would be difiicult to imagine that they are only modifications of the same animal; one would suppose that they were as many different kinds of animals, if there was not abundant proof of the contrary.

• ** Introduction a I'Entomologie," tome ii. p. 192. 8vo. Paris. 1838.

The successive stages through which an insect passes are four

Fig. 15.— Nervous system of Carabus auratus.

in number:— the egg; the larva; the pupa, nymph, or chrysalis | and the perfect insect, or imago.

The egg state, which is common to them, as to all other articulate animals, it is unnecessary to explain. Nearly all insects lay eggs, though some few are viviparous. There often exists in the extremity of the abdomen of the female a peculiar organ, called the ovipositor, which is destined to make holes for the reception of the eggs. By a wonderful instinct the mother always lays her eggs in a place where her young, on being hatched, can find an abundance of nutritious substances. It will not be needless to observe that in most cases, these aliments are quite different to those which the mother seeks for herself

In the second stage, that is to say, on leaving the egg — the larva period — the insect presents itself in a soft state, without wings, and resembles a worm. In ordinary language, it is nearly always called a worm, or grub, and in certain cases, a caterpillar.

I.innasus was the first to use the term '' larva "' — taken from the Latin word larva ^ " a mask " — as he considered that, in this form, the insect was as it were masked. During this period of its life the insect eats voraciously, and often changes its skin. At a certain period it ceases to eat, retires to some hidden spot, and, after changing its skin for the last time, enters the third stage of its existence, and becomes a chrysalis. In this state it resembles a mummy enveloped in bandages, or a child in its swaddling clothes. It is generally incapable of either moving or nourishing itself It continues so for days, weeks, months, and sometimes even for years.

While the insect is thus apparently dead, a slow but certain change is going on in the interior of its body. A marvellous work, though not visible outside, is being eftected, for the different organs of the insect are developing by degrees under the covering which surrounds them. When their formation is complete, the insect disengages itself from the narrow prison in which it was enclosed, and makes its appearance, provided with wings, and capable of propagating its kind in short, of enjoying all the faculties which Nature has accorded to its species. It has thrown off the mask; the larva and pupa has disappeared, and given place to the perfect insect.

To show the reader the four states through which the insect passes in succession, in Fig. i6 is represented the insect known as the Hydi'Ophilus* firstly, in the egg state; secondly, as the larva, or caterpillar; thirdly in the pupa; and fourthly as the perfect insect or imago. The different degrees of transformation and evolution which we have just described, are those which take place either completely

■* A kind of water-beetle. — Ed.

INTRODUCTION.

2!

ir incompletely in all insects. Their metamorphoses are then at an nd. There are certain insects, however, that show no difference in heir various stages, except by absence of wings in the larva; and in hese the chrysalis is only characterised by the growth of the wings, shich, at first folded back and hidden under the skin, afterwards )ecome free, but are not wholly developed till the last skin is cast. rhese insects are said to undergo incomplete metamorphoses, the

Fig. i6. — Hydrophilus in its four states. A, eggs; B, larva; C, pupa; D, imago, or perfect insect.

former complete metamorphoses. Some never possess wings; indeed, there are others which undergo no metamorphosis, and are born possessed of all the organs with which it is necessary they should be provided. Certain curious researches have been made on the strength of insects. M. Felix Plateau, of Brussels, has published some observations on this point, which we think of sufficient interest to reproduce here.

In order to measure the muscular strength of man, or of animals-as the horse, for instance — many different dynamometric apparatuses have been invented, composed of springs, or systems of unequal levers. The Turks' heads which are seen at fairs, or in the Champs Elysees, at Paris, and on which the person who wishes to try his strength gives a strong blow with his fist, represent a dynamometer oi this kind. The one which Buffon had constructed by Regnier the

mechanician, and which is known by the name of Regnier' s Dynamometer, ismuch more precise. It consists of an oval spring, of which the two ends approach each other; when they are pulled in opposite directions, a needle, which works on a dial marked with figure?, indicates the force exercised on the spring. It has been proved, witi this instrument, that the muscular effort of a man pulling with bot; hands is about 124 lbs., and that of a woman only 74 lbs. Thi ordinary effort of strength of a man in lifting a weight is 292 lbs.; | and a horse, in pulling, shows a strength of 675 lbs.; a man, under ' the same circumstances, exhibiting a strength of 90 lbs. \

Physiologists have not has yet given their attention to the strength ( of invertebrate animals. It is, relatively speaking, immense. Many people have observed how out of proportion a jump of a flea is to its size. A flea is not more than an eighth of an inch in length, and it jumps a yard; in proportion, a Hon ought to jump one-third of a mile. Pliny shows, in his ' Natural History," that the weights carried by ants appear exceedingly great when they are compared with the size of these indefatigable labourers. The strength of these insects is still more striking, when one considers the edifices they are able to construct, and the devastations they occasion. The Terrnes, or White Ant, constructs habitations many yards in height, which are so firmly and solidly built, that the buffaloes are able to mount them, and use them as observatories; they are made of particles of wood joined together by a gummy substance, and are able to resist even the force of a hurricane. There is another circumstance which is worth being noted. Man is proud of his works; but what are they, after all, in comparison with those of the ant, taking the relative heights into consideration? The largest pyramid in Egypt is only 146 yards high, that is, about ninety times the average height of man; whereas, the nests of the Temiites are a thousand times the height of the insects which construct them. Their habitations are thus twelve times higher than the largest specimen of architecture raised by human hands. We are, therefore,

• A neuropterous insect, not a true ant. — Ep,

far beneath these little insects, as far as strength and the spirit of working go. The destructive powers of these creatures, so insignificant in ai)pearance, are still more surprising. During the spring of a single year they can effect the ruin of a house by destroying the beams and l)lanks. The town of La Rochelle, to which the Termites were imported by an American ship, is menaced with being eventually suspended on catacombs, like the town of Valencia in New Granada. It is well known what destruction is caused when a swarm of locusts alight in a cultivated field; and it is certain that even their larvae do as severe injury as the perfect insect. All this sufficiently proves the destructive capabilities of these little animals, which we are accustomed to despise.