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chevrette was indispensable to the exclusion of the sole frdrti the egg? If the eggs of those that spawned in the vessel remained sterile, while the others produced young, the reason of the difference might have been, either that the males had not fertilised the spawn of the former, and that they had rendered fertile that containing the eggs attached to the chevrette; or perhaps, that these eggs, needing a degree of agitation to make them hatch, the first had not in the vessel, the necessary agitation which they would have received in the sea, while the chevrettes, by their motions, would have procured a sufficient agitation to the others.
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this is so rapid. I should think it useless to observe, that the proverb here mentioned, exceedingly exaggerates the matter, if I did not know, that many people believe it literally true. It is, however, true, that of insects which are not remarkably minute, the generation of fleas, - aphides, and other vermin of that fort, goes on with the greatest rapidity. As to larger infects, a whole year is necessary for their pasting from one generation to another. The species Which multiply twice a year, are in much smaller numbers, as are those which need more than a year to produce their like.
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// is alloioed, that insuBs, &c. This is not an universal opinion. The surest way is, not to decide on a subject wd cannot know. When we take a general view of the operations os insects, the great uniformity, which at once appears in the economy of each species, would make us believe, that they act merely by instinct. But, when we examine theit1 proceedings in detail, and when we fee, that they not only vary their operations, according to the necessity of the cafe; but that, when they are placed in difficult circumstances, in which, according to the ordinary course of things, they should not naturally find themselves, we observe, they do not fail to make the most of their resources, and that they can, with much industry, remedy accidents," and extricate themselves from very embarrassing situations, we are then tempted to allow them a portion of reason.
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It is merely the external form. Although the changes which the external parts of insects undergo, in their different transformations, are the most remarkable, they are not confined to these parts alone. Very considerable changes likewise take place in their internal parts, some of which, are elongated, others contracted; some lose their functions, ethers acquire new ones, and others entirely disappear.
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■Into four different clnjfes. The explanation of the four sorts of changes mentioned in this chapter, is taken from Swammerdam, who expresses himself on the subject, nearly in the same way with our author. Those who are not perfectly versed in the different transformations of insects, will perhaps be at a loss to comprehend what is here related. I shall endeavour, in a few words, to give as _distinct an idea of them as possible.
For this purpose, it is necessary, in the first place, to know, what is properly meant' by the state of nymph and chrysalis, so often mentioned. By these terms is meant, a state of imperfection, attended sometimes with inactivity, inaction, abstinence and weakness, through which the insect passes, after having attained a certain bulk, and in which its body receives the preparatives necessary for its pasting to a state of perfection* All the external parts of the insect are then found enveloped, either with their natural skin, or with 3 fine membrane, or with a hard and crustaceous coat. In the first cafe, the limbs of the insect remain free, it preserves its power of acting, it eats, and its form is little different from what it was before. In the second case, the limbs of the insect are folded over its breast, but separate; it can neither eat, nor act, it retains hardly any traces of its for-* nier figure, and . has only a confused resemblance to that which it is going to assume. In the third cafe, the cover brings all these parts of the animal into one mass; it makes it equally incapable of eating and acting; it has no resemblance, either to what it formerly was, nor to what it is to be. These three sorts of change, are evidently very different, and yet, we have but two words in our language to distinguish them by. We say of the insects in the two first cafes, that they are changed into nymphs, and of those in the last case, that they have assumed the- form of <hrysalid«To these terms, it would be proper to add a third, in order to mark the difference between the first and second cases. It might be done, I think, very conveniently, by allowing the last to retain the name of nymph, and calling those of the first kind iemi-nympli, or demi-nymph; a name which, perhaps, would not be inapplicable to them, considering the small degree of change they have undergone. Grasshoppers, which, instead of the long wings they acquire, have ltill only on their backs, the small cases, in which these wings are formed, are nymphs of this kind; they may properly be railed semi-nymphs. Those who have had an opportunity of examining a bee-hive, cannot fail to have remarked bees, still imperfect in the shut cells; these are nymphs of the second order. The silk-worm furnishes a well-known example of insects under the form of a chrysalis.
Insects, which undergo no other metamorphosis, than; that which has converted them from the soft •substance of an egg, to a wJl-formed and living body, are those which conititutc the first class of transformations spoken of in this chapter. They increase in size; the greater part cast their skin; some of their parts acquire a greater size than the rest, and sometimes take a different colour from what they had before, This is almost the whole change which these undergo.
The transformations of the insects of the other three classes do not terminate here: after having cast off their ikins, for the most part several times, and after having ac» quired their destined bulk, all become either semi-nymphs, nymphs, or chrysalids. They pass a certain time under this form, and upon quitting it, assume that of a perfect insect, capable of generation. It is from the diversity which takes place in these three forts of changes, that the principal characters, which distinguish the insects of the second, from those of the first and third class are taken.
The insects of the second class, are those that pass thro' the state which I have called the state of semi-nymph. They do not undergo ,a transformation which is entirely eompleat, but in their last change, they have generally still all the members they had before, without having acquired any others, except they have got wings; and as we have already remarked, the semi-nymph differs little in form, from the animal which produced it. What always distinguishes it most, is, that there is seen upon its back, at the base of the thorax, the cases in which the wings are formed, which before that, appeared but little, and often not ac all. In other respects, it walks, runs, leaps and swims, as before. The difference between the semUnymph, and the winged insect which it produces, is not always so obscure. In some species, it is even so great, that it is with difficulty we can discover a trace of its first form; but this is not general, and the greater part, in their last hate, differ in no other material part from the nymph, but in the whigs.
The insects of the other two classes, do not enjoy the fame advantage with,the other. They lose the use of all their members when they enter upon their transformation and have no resemblance to what they were before. An animal of these two classes, which before had no legs, or had five, six, seven, eight, nine, ten, or eleven pairs of legs, has now no more nor less than three pairs, which, with the wings and antennæ, are folded under its breast, and there remain immoveable.
What distinguishes these two last classes from each other, is, that the insects of the third class quit their lkin, when, they change into nymphs, or into chrylalids, and that those of the fourth change into nymphs under their skin, which hardens round them, and serves them then for a cafe.
These are the principal differences which Swammerdam and the author find in these four classes. They consist, to express it in two words, in this, that the insects of the first class, after issuing from the egg, undergo no other transformation: that those of the second suffer an incompleat change, and become semi-nymphs, before arriving at their ultimate form $ that those of the third and fourth classes, before arriving at their perfect state, become, the first nymphs or chryfalids, and the others nymphs, by a total change of form, but with this difference, that those of the third class quit their lkin at becoming nymphs Orchtyfalid; and that those; of the fourth become nymphs without quitting their lkin.
M. de Reaumur, to whom Natural History is indebted for so many beautiful discoveries, found, in the transforms tion of insects of the fourth class, a new character, which no one, perhaps, had observed before, and which, 1 think, distinguishes them more essentially from those of the oilier
Uu classes, classes, than the changing into nymphs, without quitting1 their skin. He discovered, that they undergo one transformation more than other infects: that before becoming ry pbs, t'ley assume under their skin, an elliptical form, or that of an elongated spheroid, in which no part of the animal is discernible; that in this state, the head, the thorax, the wings and legs of the nymph are inclosed in the interior cavity of the abdomen, from which they issue successively, by the anterior part, nearly in the same way as the extremity of the finger of a glove, which has been drawn in, is pufli 'd out again. Thus, the insects of this class, are not solely distinguished from others, by their changing into nymphs under their skin: but principally, in that before becoming nymphs, they undergo a double transformation. According to this idea, the differences of the four orders of transformations may be reduced to terms simpler and more easily comprehended, by faying, that insects of the first order, after issuing from the e#g, attain their perfect state, without being previoufly disposed to it, by a change of form; that those of the second class are prepared for it, by-an incompltat change of form; and those of the fourth, by a double change of form.
* Lyonet here gives examples of these four classes of transformations. The first is exemplified by the common earth-worm. The second class, by a dragon-fly, (Libellula puella.) The third, by insects of the three different orders: i. A pseudo-caterpillar, which feeds on the willow, with two and twenty feet, (Tenthredo marginata.) 2. A waterbeetle of the largest size, (Dytiscus piceus.) And 3. A caterpillar with sixteen feet, which lives on the trunks of willows, oaks, &c. (Phalæna Cossus.) The fourth class is illustrated by a white maggot, which proceeds from the eggs deposited by the large blue flies in flesh, when it is about td> turn putrid. (Musca vomitoria.)
The celebrated Bergman, before he betook himself to the illustration of mineralogy, had been fond of the study of infects, and he has left us a classification of larvæ, a concise view of which, it is hoped, it will not be improper to give in this place.
The metamorphosis of larvæ, fays Bergman, consists in the excoriation, or deposition of the external ikin of the infect, joined with a change of form. This, in general, is twofold, to wit, from,th: larva to the puja, and from that.