« AnteriorContinuar »
sect scape from intains ca
prick up their ears, and rush to the mountains covered with ice and snow, to escape from the fly, but often in vain; for the insect follows, and generally finds means to lodge its egg in the back of the deer. The worm hatching penetrates the skin, and remains under it in security during the winter; in the year following it falls out, changed to a pupa, and becomes a winged insect. The destrus bovis is an equal terror to oxen; the hippobosca equina to horses; and cestrus ovis to sheep: the latter insinuates its eggs into the head of those useful animals, through the nasal organs.
The nest formed by the female of the hydrophilus piceus, for the preservation of their eggs, is altogether remarkable, and is described with much minuteness by Lyonnet. This nest is whitish, its figure an oblatespheroid, three-fourths of an inch in length, and its breadth two-thirds of its length; on the upper surface it is terminated by a lengthened horn-like process, an inch long, ending in a point, and of a brownish colour. In this nest the eggs are deposited, and left floating on the water till in due time they hatch, and the larvæ desert the little bark contrived for their preservation in the state preceding, committing themselves to the water. These coques generally float among reeds and duck-weed. The purpose of the conical projection is supposed to be that of sustaining the case in an upright position, when assailed by the wind: but this is mere conjecture; we may have yet to learn its actual destination. Another aquatic insect (one of the nepa genus) that inhabits the waters of China, exhibits a far more extraordinary instance of the parental care which the insect race evince for the preservation of their eggs. This diminutive creature, scarcely an inch long, and of a subrotund figure, with the upper and lower surfaces flattened, is seen at particular seasons bearing a large cluster of eggs on its back, which, though disposed as compactly as possible,
by being placed on one end, and having the sides touching each other, cover no inconsiderable portion of the whole surface of the disk. In this manner they are conveyed by the insect, wherever it goes, till the larvæ hatch, and drop instinctively into the water; when the parent insect casts off the exuvia of the nidus, and resumes its former appearance'.
Each, as reflecting on their primal state,
And to their young adapt the future prey. BROOKE'. The different changes of form which many insects undergo, from their first appearance as eggs till they arrive at their perfect and winged state, constitute an important article in their history: these have been termed their metamorphoses, or transformations ; and, from the very language employed to express
· Donovan's Insects of China.
? Of all the productions of nature, insects are supposed to be the most numerous and fertile. With the exception of fishes and crustacea, they are apparently the most prolific. Lyonpet has offered a curious estimate of the increase of insects, taken from their eggs. From a brood of 350 eggs, which he obtained from a single moth, he selected 80. These, when arrived at their perfect state, produced 15 females; and hence he deduces the following conclusion: If 80 eggs give 15 females, the whole breed of 350 would have produced 65. These 65, if equally fertile, would have produced 22,750 caterpillars, among which there would have been 4265 females. These, in the third genération, by the same mode of calculation, must amount to 1,492,750 caterpillars.
them, the false notions which were long entertained, even by naturalists, are still discernible..
A fly, a spider, or an ant, insects of the most different kinds in outward appearance, do not differ more widely than the same insect does from itself, .. under the different forms of a worm, a chrysalis, and a butterfly. For what is at present a worm soon becomes a chrysalis, and this again is as suddenly changed into a winged animal. Changes apparently so instantaneously produced have been compared to the metamorphoses so renowned in antient fable, and probably at first suggested the idea of such transformations. When an insect in so short a space ! appeared under a shape so different from that which it lately exhibited, mankind at first imagined that the change was real: they trusted to appearances, without giving themselves the trouble of reflecting on the improbability of the fact. This point, however, has been successfully laboured by Malpighi and Swammerdam, who, by dissecting insects a short time before the period of their transformation, observed that their first form is owing to an external covering alone, under which their different members are destined to acquire their proper size and firmness: that all the parts of a butterfly, for example, are perfectly distinct under the skin of the worm which invests them; and that under the crustaceous shell of the chrysalis they are still increasing in strength, and fast approaching to that state in which they are destined to appear, when the animal shall arrive at perfection, and be able to propagate its kind. All the parts of the insect in its perfect state are, hence, rather developed than formed by successive creations.
No fictions here to willing fraud invite,
With many a wanton and fantastic dream,
BROOKE. A silk worm, or the worm of a butterfly, which is about to enter into its chrysalis state, is observed for some time antecedently to grow languid, and to cease from gnawing those plants of which it was formerly so voracious: after having retired to a place fit for its purpose, and undergoing a few convulsive struggles, the skin which covered it, and gave it the form of a worm, bursts, and the animal within makes its appearance. At first it is soft and tender, and covered with a viscous fluid which ascends from the body, and which progressively hardens into that crustaceous shell in which all the members are again locked up, till they acquire greater firmness and stability. This viscous fluid, which is generally seen coloured and opaque in its crustaceous state, is at first transparent, and through it the wings, limbs, and antennas of the butterfly, are clearly perceptible. M. de Reaumur collected several hundreds of these worms before their transformation, and placed them together on a table where he had many opportunities of examining them as they passed from one shape to another: it was then that he distinctly perceived all the different members of the butterfly, before the chrysalis had assumed its hard and apparently inanimate state.
After having remained for some time in this torpid state of a nymph, chrysalis, or pupa, the limbs of the animal acquire sufficient strength to perform their functions, and it employs them in breaking open the second prison. On this event all the members are set at liberty, and instantly assume that posture and
arrangement which is best suited to the new functions with which they are now to be charged.
The Son his fost'ring warmth bequeaths,
Who that beholds the summer's glist'ring swarms,
All insects do not undergo the number of transformations, enumerated above, before they arrive at the winged state. Some, immediately on leaving the egg, assume a form pretty nearly resembling that