Reaumur has described two modes in which the larvæ of the first are arranged in the matrix of the mother. In some they are heaped together without much appearance of order, being placed merely parallel to each other[789]; but in others they are arranged in a kind of riband—the length of the little animals, which are also parallel, forming its thickness—rolled up like the mainspring of a watch[790]. These larvæ in general are not divided into two masses corresponding with the pair of ovaries in other insects, but form only a single one[791]. You must not suppose that these little fetuses lie naked in the womb of the mother; each has its own envelope formed of the finest membrane, which, however, is not entirely divided from that of those adjoining to it, but appears to be one tube, which becomes extremely slender between each individual, so as when drawn out to look like a chain[792]. Reaumur seems to have thought that in these flies the larvæ were never confined in any other case or egg[793]; but De Geer sometimes found eggs in the body of Sarcophaga carnaria, though most generally larvæ, from which he conjectures that it is really ovo-viviparous, the eggs being hatched in the body of the mother[794]. As these flies are all carnivorous, and their office is to remove putrescent flesh, you may see at one glance the object of Providence in this law of nature—that no time may be lost, and the animal exercise its function as soon as it is disclosed from the matrix.
The Aphides, so fruitful in singular anomalies, are ovo-viviparous, as I have before hinted[795], at one period of the year, that is during the summer, but strictly oviparous at its close. From the experiments of De Geer, however, upon Aphis Rosæ, it would appear that this faculty is not conferred upon the same individuals, but only upon those of different generations of the same species; all the generations being ovo-viviparous except the last, which is oviparous[796]: nor does it appear, as has been sometimes imagined, that it is common to the whole genus. De Geer observed a species in the fir, which makes curious galls resembling a fir cone (Aphis Abietis), which appeared never to be ovo-viviparous[797].
With regard to scorpions, it does not seem clear that they are always ovo-viviparous: M. Dufour twice found in the midst of the eggs nearly mature, a young scorpion which appeared to him at large in the cavity of the abdomen; it was so large that it was difficult to comprehend how it could possibly be excluded from the animal, without an extraordinary operation[798]. The pupiparous insects (Hippobosca, &c.) have been sufficiently noticed before[799].
2. I have already in several of my former letters stated to you what the modern doctrine of physiologists is with respect to certain individuals, usually forming the most numerous part of the community with insects living in society, that were formerly supposed to be neuters, or as to their sex neither male nor female—that they are in almost every instance a kind of abortive females, fed with a different and less stimulating food than that appropriated to those whose ovaries are to be developed, and in consequence in most instances incapable of conception[800]. Upon these sterile females, you also heard, devolve in general the principal labours of their respective colonies, showing the beneficent design of Providence in exempting them from sexual cares and desires, and meriting for them the more appropriate name, now generally used, of workers. The differences in the structure of the female bee and the workers were also then accounted for; and similar reasoning may be had recourse to with regard to those of ants, in which the worker and the female differ still more materially. My reason for introducing this subject here, is to observe to you that I have some grounds for thinking that this system extends further than is usually supposed, and that to each species in some Coleopterous and other genera there are certain individuals intermediate between the male and female; this I seem to have observed more especially in Copris and Onthophagus. For in almost every British species in my cabinet of these genera I possess such an individual, distinguished particularly by having a horn on the head longer than that of the female, but much shorter than that of the male. I once observed a pair of Pentatoma oleracea, a very pretty bug, in coitu, both sexes being ornamented with white spots, and by them stood a third distinguished from them by red ones. I do not, however, build on this circumstance, though singular; but mention it merely that you may keep it in your eye. It would be curious should it turn up, that, to answer some particular end of Providence, in some tribes of insects there are two kinds of males, as in the gregarious ones two descriptions of females.
I am, &c.