Read Ebook: An Introduction to Entomology: Vol. 3 or Elements of the Natural History of the Insects by Kirby William Spence William

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The sturgeon is said to lay 1,500,000 eggs, and the cod-fish 9,000,000.

Reaum. iv. 392.

De Geer vii. 159.

Ibid. 159. 166.

Ibid. 36--.

De Geer ii. 638.

Gould 36.

Reaum. v. 477.

Ibid. iii. 579. v. 121.

R?sel iii. 152.

De Geer vii. 145.

Ibid. FIG. 3. 4. 7. 9. &c.

Ibid. FIG. 15.

Reaum. ii. 286.

Reaum. iv. 617.

Brahm. 310.

De Geer vii. 195.

Ibid. 196.

Reaum. ii. 167.

LARVA STATE.

Almost all larvae, at their birth, are for a time in a very feeble and languid state, the duration of which differs in different species. In most it continues for a very short time, a few minutes or perhaps hours, after which they revive and betake themselves to their appropriate food. In others, as in the generality of spiders, this debility lasts for seven or eight days, and in some species even a month, during which the young ones remain inactive in the egg-pouch, and it is not till they have cast their first skin that their active state of existence commences.

All larvae may be divided into two great divisions:--

i. Those that resemble the perfect insect, except in the relative proportions and number of some of their parts.

ii. Those which resemble the perfect insect, except that they are apterous, or not yet furnished with organs of flight.

The prolegs with claws may be further divided into four different kinds.

i. With a corneous head of determinate shape .

ii. With a membranaceous head of indeterminate shape .

i. With legs only, and with or without an anal proleg .

ii. Prolegs only .

iii. Both legs and prolegs .

We are equally ignorant of the use of the upright horn found upon the back of the fourth segment in the larva of some moths which is of a construction quite different from that of those last described. It is cylindrical, slightly thinner at the apex, which is obtuse, fleshy, incapable of motion, of a black colour, and about two lines long. On the same segment, also, in the case-worms are three fleshy conical eminences, which the animal can inflate or depress, so that they sometimes totally disappear, and then in an instant swell out again. When retracted, they form a tunnel-shaped cavity, varying in depth. Reaumur conjectured that these eminences were connected with respiration, and one circumstance seems in favour of this conjecture, that this segment has not the respiratory threads observable in the subsequent ones. Latreille mentions certain fleshy naked eminences placed upon the ninth and tenth segments of some hairy caterpillars, which, like those just mentioned, the animal can elevate more or less. They are often little cones; but when it would shorten them, the summit is drawn in, and a tunnel appears where before there was a pyramid.

Another kind of appendage, which is found in some larvae, is the organ employed by them to carry the excrement; with which, instead of letting it fall to the ground, they form a kind of umbrella to shelter and probably conceal them. All the tortoise-beetles have instruments for this purpose, as well as an Indian genus very nearly related to them. This instrument is a kind of fork, half as long as the body, consisting of two branches, growing gradually smaller from the base to the summit, where they terminate in a very fine point, of a substance rather horny, and attached to the body near the anal orifice. They are armed on the outside with short spines, from the base for about a third of their length. When this fork, as it usually is, is laid parallel to the back, with its points towards the head, the anal aperture points the same way. When the animal walks, the fork points the other way, and is in the same line with the body, and the anus assumes a prone position.

In the larvae of the lace-winged flies , and ant-lions , the anus is furnished with a small fleshy retractile cylinder, from which proceeds the silken thread that forms the cocoon inclosing the pupa. Providence has many different ways of performing the same operation. From the structure of the oral organs of these animals, the silk could not conveniently be furnished by the mouth; the Allwise Creator has therefore instructed and fitted them to render it by a spinneret at the other extremity of the body.

The respiratory anal appendages of many Dipterous larvae will be fully described in a subsequent Letter: I shall therefore now only further observe upon this subject, that although there is seldom any alteration in the form of these appendages &c. in the same species, the caterpillars of two moths , however, are exceptions. The former, when young, has two hairy projecting ear-like protuberances, which it entirely loses, as I have myself observed, before it assumes the pupa; and the latter, in like manner, after its third change of skin, is deprived of its bent thorn-like points which attend it when young. It is remarkable that these last larvae, when just excluded from the egg, are also entirely destitute of these appendages; they soon, however, appear, from slight elevations which mark their situation, and rapidly acquire their usual form. Changes of a similar kind, hitherto unobserved, may probably take place in other species.

Instead, therefore, of any further specification of individual forms, I shall now endeavour to give you, as far as my own knowledge of them and the information I can collect from other sources will enable me, a larger and more general view of the kinds of larvae; for analytical inquiries lose half their value and importance unless we proceed to apply them synthetically, by forming, if possible, into groups the objects with which we are individually acquainted.

The system here stated, of naming and characterizing larvae from the resemblance and analogy, in many cases very striking, that they bear to the apterous tribes, is a very happy and original one, and does its author great credit; yet I think in some instances, as I shall soon have occasion to point out to you, the application of it is not so happy as the first idea. But this is always the case when a new law of nature is discovered; the proper application of it is gradually developed, and it does not at all detract from the merit of the first discoverer, that all the bearings of such law do not strike him as it were intuitively.

I shall begin by drawing up for you a list of the Primary forms that I seem to have observed, and their characters; and then going through the orders, shall give you the examples of each, with such observations upon them as the case may require.

APTERA. ARACHNIDA. CRUSTACEA.

ANOPLURIFORM. ARANEIDIFORM. ISOPODIFORM. THYSANURIFORM. ONISCIFORM. CHILOPODIFORM. IDOTEIFORM. CHILOGNATHIFORM. AMPHIPODIFORM. STOMAPODIFORM. MOLLUSCA. ANNELIDA. DECAPODIFORM. BRANCHIOPODIFORM. LIMACIFORM. VERMIFORM.

If you feel disposed to investigate the reasons of that law of the Creator which has ordained that the skins of the higher animals shall be daily, and imperceptibly, and as it were piece by piece renewed, while those of insects are cast periodically and simultaneously,--the proximate cause must be sought for in the nature of the two kinds of skin, the one being more pliable and admitting a greater degree of tension than the other, and being so constructed as to scale off more readily. If, ascending higher, you wish to know why the skins of insects are so differently circumstanced from our own, the most apparent reason is, to accommodate the skin to the very rapid growth of these animals, which a gradual and slower change would have impeded too much, or the skin have suffered constant dilapidation and injury; therefore their Beneficent Creator has furnished them with one which will stretch to a certain point, and during a certain period, and then yield to the efforts of the inclosed animal, and be thrown aside as a garment that no longer fits the wearer.

There is not always that proportion between the size of larvae and of the insects that proceed from them that might have been supposed, some small larvae often producing perfect insects larger than some of those proceeding from such as are of greater size.

But a still larger tribe, those which feed on leaves, animals, &c. act as if more sensible of the insecurity of this to them important epoch. They are about to exchange their state of vigour and activity for a long period of deathlike sleep. The vigilant caution which was wont to guard them from the attack of their enemies will be henceforward of no avail. Destitute of all the means of active defence, their only chance of safety during their often protracted night of torpor must arise from the privacy of their place of repose. About this, therefore, they exhibit the greatest anxiety. Many, after wandering about as if bewildered, retire to any small hole on the surface of the earth, covering themselves with dead leaves, moss, or the like, or to the chinks of trees, or niches in walls and other buildings, or similar hiding-places. Many penetrate to the depth of several inches under ground, and there form an appropriate cavern by pushing away the surrounding earth; to which they often give consistence by wetting it with a viscid fluid poured from the mouth. The larvae of other insects undertake long and arduous journeys in search of appropriate places of shelter. Those of flesh-flies, now satiated with the mass of putridity in which they have wallowed, leave it, and conceal themselves in any adjoining heap of dust. The grubs of the gad-fly creep some of them out of the backs of cattle, in tumours of which they have resided, and suffer themselves to fall to the earth; while others, which have fed in the stomach of horses, quit their hold, and by a still more extraordinary and perilous route are carried through the intestines the whole length of their numerous circumvolutions, and are discharged at the anus. And without enumerating other instances, various aquatic larvae, as that of a common fly , &c. leave the water, now no longer their proper element, and betake themselves to the shore, there to undergo their metamorphosis.

In about thirty hours after the larvae which girth themselves have finished their operations, the skin splits, and the pupa disengages itself from it by those contractions and dilatations of its segments which have been before described, pushing the exuviae in folds to the tail, by different motions of which it generally succeeds in detaching them. One would have thought there would be considerable difficulty in slipping the skin past the girth; but this, according to Reaumur, seems to be easily effected.

I must observe here, that although the vertical and horizontal are the two principal positions in which caterpillars suspend themselves, yet that others are inclined at various angles; and some are attached with less art, appearing only to be fastened by some part of their abdomen to the body upon which they are fixed.

It is a general rule, that those larvae which spin cocoons, never in ordinary circumstances become pupae without having thus inclosed themselves. An exception, however, is met with in the larva of a species of ant noticed by De Geer , some of the individuals of which inclose themselves in cocoons; while others neglect this precaution, and undergo their metamorphosis uncovered. R?sel also made nearly the same observation on the larva of the flea.

These, and the other larvae mentioned above, commonly form their cocoons of the substances I have indicated; but when by any cause they are prevented from access to them, they often substitute such other materials as are at hand. Reaumur fed a larva that formed its cocoon of minute fragments of paper, which with its mandibles it had cut from the piece that covered the glass vessel that contained it: and the same circumstance happened to Bonnet.

FOOTNOTES:

De Geer vii. 197.

De Geer vii. 197.

Ibid. 85.

De Geer vii. 576.

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