Winged insects, many branchiopod Crustacea, and the Batracian reptiles, have been observed by Dr. Virey to bear some analogy to the mammalia, aves, &c. in another respect. In leaving their egg, they only quit their first integument, answering to the chorion or external envelope of the human fœtus; they therefore still continue a kind of fœtus, so to speak, more or less enveloped under other tunics, and principally in their amnios, or the covering in which the fœtus floats in the liquor amnii[78]. This the butterfly does in the pupa case; and its birth from this, under this view, will be the true birth of the animal. In the human subject, the ova upon impregnation are said to pass from the ovary through the Fallopian tube into the uterus. In the insect world, upon impregnation, the eggs pass first from the ovaries into the oviduct, answering to the Fallopian tube, which in them terminates in the ovipositor, or the instrument by which the parent animal conveys the eggs to their proper station: there is, therefore, nothing properly analogous to the uterus in the insect, and the substance upon which the larva feeds upon exclusion answers the purpose of a placenta.
After this general view of the most modern theories with regard to the metamorphosis of insects, I shall in the present and some following letters, treat separately of the different states through which these little beings successively pass.
The first of these is the Egg state, the whole class of insects being strictly oviparous. Some few tribes indeed bring into the world living young ones, and have on that account been considered as viviparous, but incorrectly, for the embryos of none of these are nourished, as in the true viviparous animals, within a uterus by means of a placenta, but receive their development within true eggs which are hatched in the body of the mother. This is proved by the observations of Leeuwenhoek, who found eggs in the abdomen of a female scorpion[79]; and of Reaumur, with regard to the flesh-fly (Musca carnaria) and other viviparous flies as they have been called[80]. A similar mode of production takes place in vipers and some other reptiles, which have hence been denominated ovo-viviparous, to distinguish them from the true viviparous animals—the class Mammalia. By far the larger portion of insects is oviparous in the ordinary acceptation of the term. The ovo-viviparous tribes at present known are scorpions; the flesh-fly and several other flies; a minute gnat belonging to Latreille's family of Tipulariæ[81]; some species of Coccus; some bugs (Cimicidæ)[82]; and most Aphides, which last also exhibit the singular fact of individuals of the same species being some oviparous and others ovo-viviparous, the former being longer in proportion than the latter.—Bonnet, however, is of opinion that the eggs of the first are not perfect eggs, but a kind of cocoon, which defends the larva, already formed in some degree, from the cold of winter[83].
When excluded from the body of the mother, or from the egg, as has been before observed, some insects appear nearly in the form of their parents, which, with a very slight alteration, they always retain; others, and the greater number, assume an appearance totally different from that of their parents, which they acquire only after passing through various changes. It is to these last, which have chiefly engaged the attention of Entomologists, that the title of metamorphoses has been often restricted. As, however, those insects which undergo the slightest change of form, as spiders do, undergo some change, and almost all insects cast their skins several times[84] before they attain maturity, Linné and most Entomologists, till very recently, have regarded the whole class as undergoing metamorphoses, and as passing through four different states, viz. the Egg—the Larva—the Pupa—and the Imago.
It is obvious, however, that in ovo-viviparous species three states of their existence only come under our cognizance, as these, being hatched in the body of the mother, come forth first under the form of larvæ. There is even one tribe of insects which presents the strange anomaly of being born in the pupa state. This is the Linnean genus Hippobosca (Pupipara fam. Latr.), to which our forest-fly belongs, the females of which lay bodies so much resembling eggs, that they were long considered as such until their true nature was ascertained by Reaumur (most of whose observations were confirmed by De Geer), who, from their size, which nearly equals that of the parent fly—from their slight motion when first extruded—from spiraculiform points which run down each side of them—and lastly, from their producing not a larva, as all other insects' eggs do, but perfect flies in the winged state—inferred, and doubtless with reason, that they are not real eggs, but pupæ, or larvæ just ready to assume the pupa state, which, however strange it may seem, have passed the egg and larva states in the body of the mother[85].
Insects, therefore, as to their mode of birth, may be divided into—
I. Ovo-viviparous, subdivided into—
1. Larviparous, coming forth from the matrix of the mother in the state of larvæ, as the Scorpion (Scorpio), the Flesh-fly (Musca), the Plant-louse (Aphis), &c.
2. Pupiparous, continuing in the matrix of the mother during the larva state, and coming forth in that of pupa, as the Forest-fly (Hippobosca equina), the Sheep-louse (Melophagus ovinus), the Bat-louse (Nycteribia Vespertilionis), &c.