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Show 238 INSBCTA. pupre or nymphs, and in the third perfect insects. It is only in the last state that they are capable of reproduction. All Insects do not pass through these three states. Those which are apterous issue from the ovum with the form they are always to preserve( I): they are said to be without a metamorphosis. Ofthose that have wings, many experience no other change than that of receiving them: these are said to undergo a demi-metamorplwsis. Their larva resembles the perfect insect, with the single exception of the wings, which are totally wanting. The nymph only differs from the larva in possessing stumps or rudiments of wings, which are developed at its final change of tegument, and render the ani. mal perfect. Such are the Cymeces, Grylli, &c. Finally, the remaining Insects provided with wings that are said to undergo a complete metamorphosis, are at first larvm, resem· bling caterpillars or Worms, and then become motionless nymphs, but presenting in that state all the parts of the per· feet insect contracted, and as if wrapped in a bandage. In the nymph of the Coleoptera, N europtera, Hyme~op· tera, &c., these parts, though closely approximated and in contact with the body, are free; but they are not so in that of the Lepidoptera and of many Diptera. An elastic or solid skin is moulded over the body and its external parts, forming a kind of case for it. That of the chrysalides of the Lepidoptera merely cons~t· ing of a simple pellicle applied to the external organs, follow· ing their contour in every direction, and forming, for each of them, so many moulds, like the envelope of a mummy, allows us to recognize and distinguish them(2); but those of Flies and Syrphi, formed of the dried skin of the larva, resem· ble an egg-like shell. It is a species of capsule or case in which the animal is shut up(3). Many larvre, before they pass into their pupa state, pre· (1) The Pulex, the female Mutilke, the Working .llnt8, and some few other In· sects excepted. (2) Pupa obtecta, L. (3) Pupa coarctata, L. INSECTA. 239 are a cocoon in which they enclose themselves, either with ;;lk which they draw from the interior of their bodies by means of the ~pinning apparatus of their lip, or other materials which they collect. The perfect Insect issues from the nymph through a fissure or slit ~hich opens on the back of the thora~. In the pupre of Fhes one of the extremities is detached, like a cap, to allow the egress of the animal. The ]arvre and pupre of those Insects which experience a demi-metamorphosis only differ from the same in a perfect state, in the absence of wings. The other external organs are precisely alike. · But in such as undergo a complete metamorphosis, the form of the body of the larva has no constant relation with that it is to posse~s in its perfect state. It is usually more elongated; the head is frequently very different, as well in its consistence as in its figure, having mere rudiments of antennre or perhaps none at all; there are never any compound eyes. There is also a great disparity in the organs of manducation, as may be, easily s~en by comparing the mouth of a caterpillar with that of the Butterfly, or the mouth of the larva of a Fly . with that of the perfect Insect. Several of these larvm are destitute of feet; others, such as the caterpillars, have many, all, the six first excepted, membranous, and without terminal hooks. Some Insects, such as the Ephemerre, exhibit a singular anomaly in their metamorphosi~ the animal arrived at its perfect state undergoes another change of tegument(!). The Insects which constitute our three first orders preserve for life their natal form. The Myriapoda, however, exhibit a kind of metamorphosis. At first they have but six legs, or, according to Savi, are altogether destitute of them; (1) "~e depouillent encore de leurs ailes," is the unguarded expression of our author. It is not the wings alone, but the entire animal, after attaining its perfect condition, that is thus divested of its external pellicle, even to the slender, setaceous appendages which terminate the posterior extremity of the body. It is the common May.fl.y of our country. .11m. Ed. |