網頁圖片
PDF
ePub 版

ther to eat. So strict; a fast is undoubtedly necessary to prepare them for the change they are about to undergo: and this seems the more probable as they discharge all the fæces their intestines are filled with,. before they attempt the change.

The metamorphoses of all insects do not resemble one another, but are generally divided into four different classes. The first comprehends those insects which after being formed in their egg, without the aid of food, and which after having taken, by the evaporation of the superabundant humidity, the necessary consistence, quit that state and issue from the shell under the form they are to retain during life, without undergoing any other transformation. To this class belong spiders, lice, fleas, the onisci, the IuIi, kc. The transformation of the second class consists in this, that the insect which was enclosed under a disguised form in an egg, and without food, after having been fortified by the evaporation of the superabundant humours, leaves the shell and appears under the form of an Infect without wings with all its other appropriate members; which in this state eats and grows till having entred a second time into what is called the jNymph state, it issues from that state with wings and is capable of propagating its species. I include in this division, ants, dragon flies, grasshoppers crickets, the mole-cricket, cimices, aquatic flies, &c. In the third transformation, the animal, after having issued from the egg where it also lay in a disguised stiape, and without food, appears under that of an insect which eats and grows, while the members of the animal into which it is to change, are formed under its skin, which it at last quits, and becomes a nymph or chrysalis; and then after the evaporation of the superfluous humidity, is transformed into its last state, which is that of a perfect insect. This class includes bees of all sot ts, gnats, beetles, butterflies, and moths. In the last fort of transformation, the insect, after having arrived at the nymph state, like those before mentioned, does not divest itself of its skin in order to enter into that state, but assumes, the form of a nymph under its skin, where it continues stiut up, till quitting two skins at once, it comes forth in its perfect state: This is the metamorphosis which flies, Ichneumons, undergo.

Besides these changes; insects several times , cast their skins; but this does not happen to them all at the fame time or in the fame way. Some, as spiders* change their skins only once a-year; others as grasshoppers, and the cabbage .caterpillars change it four times ; and others even six times. The greater part quit it entirely, but some retain it attached to the extremity of the abdomen, and carry it ovet their head to defend them from bad weather, or other insects their enemies. This is the case, as Jrisch observes, with the Cassida nebulosa. The manner in which they divest themselves of it, varies also according to the diversity of species. In some it is rent near the top of the head, and the insect puts it off as one draws off a stocking. In others it is first torn under the belly, and they throw it oyer their head as one does a shirt. The spoils of many insects preserve exactly the figure of the animal, which is particularly remarkable in those of spiders. Some ot these cast skins, are lined intefnally with a white membrane much more delicate than the outer one. When thrown off, they are sometimes so contracted as not to retain a third of their former length; at other times they appear swollen, and discover nothing but the hole through which the insect made its exit.

"When the insect has thrown off its last skin it appears in the state of a nymph or chrysalis : these

I are are only covers under which the animal is- forraed^ and which it preserves till it has taken its natural shape. These nymphs are soft at first,and contain some liquid matter which in time evaporates, and they acquire more consilience y but in general they are very thin and fragile. ■

Infects in this state may be divided into two classes. There are some of a conicas shape, almost of the figure of a date; others of an angulatedform, with sharp corners at the rings and anterior part, and little elevations at these corners. There is a great diversity in their figure. Without mentioning those that are of the form of a date, some have the appearance of a child in swaddling cloaths laid in a cradle; others have the face of a man. Others resemble the head of a dog, a cat, a bird, a mouse with its tail, and even ot the Infect itself which is to proceed from it.

The limbs of insects are not folded with less art in chrysalids and nymphs than they are in the egg. If is wonderful to fee the artifice with which they are disposed, and the wisdom- which has- contrived to lodge in so small a space, so many different member* without hurting them, or producing the least confusion. In some we may perceive externally all the limbs of the animal it encloses; others are so transparent that the insect is easily discoverable upon looking through it: but others must be opened before we can judge of the insect they contain.

Great variety is remarkable in the colour of chrysalids. fkown, yellow, red, green, white, violet and black are the chief; but it must bs observed that these colours appear in various degrees, and that ail the different shades of them may be perceived in different forts of chrysalids, and in some mingled with so much art, that the eye is astonished in viewing them. . The ancients imagined that the beauty of

th e

i

the c&iouTS in a chsyfilis was a proof of the beauty pf the insect it contained ; but nothing is more deceitful; we might as well fay that the beauty of a era*die was an indication of the beauty of the child that slept in it. Besides, experience teaches us that a plain insect very often issues from a gilded chrysalis, JKfhile another with a less specious appearance often1 produces a very gaudy one.

No motion is perceived in some of those infects while they continue in this stage of transformation; but as this incapacity of moving themselves might expose them to be devoured by their enemies, they shelter themselves ynder a stone, a root, or a piece of wood. This is not all, they make that side which is exposed, so round ^nd tense, that worms cannot lay hold of them. They are not however all motionless. Some spontaneously agitate themselves, and others do not move except they are touched. These last, on such occasions move their abdomen and (hake their heads as if to defend themselves, and intimidate their enemy. There are likewise some (as the Dytifcus latissimus) which when turned upon their back, instantry replace themselves, in their first situation. Others, (as the Coccineila septempunctata,) turn themselves round for some time. Others though they are in general motionless, if they are taken into the hand, seem to be revived by the heat which puts their humours in circulation, and makes them perform various contorsions. But neither the one nor the other take the least nourishment during the time they continue in the pupa state.

The precaution of .choosing a convenient place in order to preserve themselves from injuries, does not always appear to them sufficient; they fortify the place they have chosen with a kind of entrenchjnent against attacks from without. The method

I 2 of pf some is to suspend themselves by the tail tq threads which they draw from their own bodies, and thus they are safe from the attacks of creeping infects, and they hold so firmly by these threads that they are not easily detached. Qthers weave around them a web with large meflies, nearly resembling a fisher's net: this keeps at a distance from the centre such insects as might injure them, and prevents their being hurt by the fall of any body. These two precautions are peculiar tq those only which have stuns sufficiently thick to resist the inclemencies of the weather. Thole which have not the fame advantage pover themselves with a particular fort of web. Some spin cones of silk, others discharge from the pores pf their body a fort of long wool which covers them during ah" the time they remain in the nymph state. Many fortify these cones with their hairs, which they clivest themselves of, (as the Phalæna Caja ;) and those that have no hairs, and also want silk, gnaw pieces of wood, and employ the sirjall bits in fortifying the inside and outside of their mansion, as is the practice of jhe Phalæna aceris. Some of these, cones are so hard and so well constructed that they are with difficulty torn; they may be properly compared to parchment. To connect the threads together, they moisten them with a sort of gum which issues from their bodies, and which gives their work the necessary degree of hardness. The cones are not all of the fame figure. The greater part are oval or spheroidal, but there are some that represent an egg cut longitudinally. They are attached by the flat side to something solid which may contribute tq their safe'y. The precautions of some are not confined! solely to this external bne: in order the better to shelter themselves against the inclemency of the weather, they cover it with a leaf, or many leaves gathered togejher: others enter, into the earth and hide themselves $eFe> bHL %*e$r of its falling, they either plaster the

« 上一頁繼續 »