Part 13

The insects of the _Hymenoptera_ Order have long been celebrated for the organs we are describing, whether used as _saws_, _augers_, or _darts_. I formerly gave you a very _general_ account of the _saws_,--I shall now give you a very interesting one in _detail_ copied from an admirable little essay of Professor Peck. "This instrument," says he, "is a very curious object; and in order to describe it it will be proper to compare it with the _tenon-saw_ used by cabinet-makers, which being made of a very thin plate of steel, is fitted with a back to prevent its bending. The back is a piece of iron, in which a narrow and deep groove is cut to receive the plate, which is fixed: the saw of the _Tenthredo_ is also furnished with a back, but the groove is in the plate, and receives a prominent ridge of the back, which is not fixed, but permits the saw to slide forward and backward as it is thrown out or retracted. The saw of artificers is _single_, but that of the _Tenthredo_ is _double_, and consists of two distinct saws with their backs: the insect in using them, first throws out one, and while it is returning pushes forward the other; and this alternate motion is continued till the incision is effected, when the two saws receding from each other, conduct the egg between them into its place. In the artificial saw the teeth are alternately bent toward the sides, or out of the right line, in order that the fissure or kerf may be made sufficiently wide for the blade to move easily. To answer this purpose in some measure, in that of the _Tenthredo_ the teeth are a little twisted, so as to stand obliquely with respect to the right line, and their point of course projects a little beyond the plane of the blade, without being laterally bent; and all those in each blade thus project a little outwards: but the kerf is more effectually made, and a free range procured for the saws, by small teeth placed on the outer side of each; so that while their vertical effect is that of a _saw_, their lateral effect is that of a _rasp_. In the artificial saw the teeth all point outward (_towards the end_) and are simple; but in the saw of the _Tenthredo_ they point inward, or toward the handle, and their outer edge is beset with smaller teeth which point outwards (_towards the end_)[757]." Valisnieri, Reaumur, and De Geer describe the groove as being in the back; but in Mr. Peck's insect, if there is no error in his account, it is, as in the _Cicadæ_, in the saw itself[758]. In the genus _Cimbex_, belonging to the same tribe, the saw differs in shape, being somewhat _sigmoidal_ or resembling the letter S, while in that of other saw-flies it is _cultriform_ with a concave edge: other minor differences distinguish them, which need not be particularized.

A similar structure, with regard to the organ in question, obtains in the rest of the _Hymenoptera_, even those that use it as a weapon of offence; but the backs of the saws in them, composed of a single piece, become a sheath for the darts. The valves, however, vary. In most of those with an exerted sting, as _Pimpla_, they are linear, exerted, and as long as the aculeus itself[759]. In _Proctotrupes_ they appear to be united so as to form a tube for the ovipositor, and are produced by a prolongation of the last abdominal segment. The darts usually run in two grooves of the sheath, and at their apex are retroserrulate[760]. In some cases the sheath itself is serrated[761]. The shanks of the darts are connected with the valves; so that when these open they are pushed out: sometimes on their outer side they have a triangular plate towards the base, which prevents their being pushed out too far[762].

In _Sirex_ and many ichneumons, in which the ovipositor is too long to be withdrawn within the abdomen, it remains always exerted; but in general it is retracted within that part when unemployed. In the gall-fly (_Cynips_) this instrument is really as long as in _Pimpla_, &c.; but as it is infinitely more slender, when in repose it is rolled up spirally and concealed within the abdomen. It is the puncture of this minute organ that produces the curious galls formerly described to you[763]. But the most anomalous ovipositor in this Order appears to be that of _Chrysis_ (_C. ignita_, &c.), which is covered by several demi-tubes or scales enveloping and sliding over each other: when these scales are removed, the true ovipositor appears, which is of a structure similar to that of the rest of the Order, but the valves are long and slender with their summit generally visible without the anus[764].

Though the ovipositor of the majority of _Dipterous_ insects is a tube with retractile joints[765], in the crane-flies this organ is different, and, like that of _Acrida_ above described, consists of what at first sight appear two valves, but each of which is formed of two pieces, the upper ones sharp and longer, and the lower pair blunt. The upper pair forms the auger that bores a hole in the ground, and the lower conducts the eggs into it after it is bored[766].

In the _Aptera_ and _Arachnida_ in general there seems no remarkable instrument of this kind; but Treviranus has described one in spiders for extruding the eggs of a singular construction. It is an oval plate lying between the external genitals and spinning organs, and is composed of a number of small screw-shaped cartilages, connected together in the most wonderful manner. There are few organs, he observes, in the animal kingdom which for their artificial mechanism can be compared with this. Each cartilage inosculates very closely in the adjoining one, and all are besides bound together by a strong skin[767].

The manner in which the eggs of insects are _fecundated_ by the male sperm is one of those mysteries of Nature that are not yet fully elucidated and understood. We can readily conceive that all the eggs may be fertilized by a single intercourse in the case of insects which, like the _Ephemeræ_ and _Trichoptera_, exclude the whole mass at once; or like many moths and butterflies, in a very short time afterwards; but the subject becomes much more difficult to explain when we advert to the female of the hive-bee, the whole number of whose eggs, deposited in _two years_, are, as Huber has demonstrated, in like manner fertilized by a single act[768]:--if you bear in mind, however, what I have lately observed with regard to Malpighi's discovery of a sperm-reservoir in insects, you will more readily comprehend how in this case a _gradual_ fecundation may take place. The principal objection to this solution of the difficulty in the case before us, is derived from the very small size of the organ supposed to be destined for this purpose--it being scarcely bigger than the head of a pin[769]: it seems therefore incredible that it should retain any portion of an extraneous fluid at the end of twelve or eighteen months, and still more unlikely that the fluid should in the interval have sufficed for the slightest moistening of not fewer than 30,000 or 40,000 eggs. The only hypothesis that seems at all to square with this fact, is that of Dr. Haighton,--that impregnation is the result not of any actual contact of the sperm with the eggs, but of some unknown sympathetic influence[770], or rather perhaps of some penetrating effluvia or _aura seminalis_, which, though small in quantity, it may retain the power of emitting for a long period.

Certain female moths, of the species of that family which, from the remarkable cases or sacs the larvæ inhabit, the Germans call _sack--träger_, before noticed[771], have been supposed to have the faculty of producing fertile eggs without any sexual intercourse; and various observers, after taking great pains, appeared to have satisfactorily proved the fact; so that some doubted whether these insects produced any males at all[772]. The enigma was at length explained by the accurate Von Scheven. At first his experiments were attended with the same result as those of his predecessors; but upon making them more carefully, and separating what he conceived to be the female from the male pupæ, he ascertained not only the existence of a _female_ in the species he examined (_Psyche vestita_), but that when thus secluded she laid _barren_ eggs; evidently proving that in the contrary instances above alluded to, an unperceived sexual intercourse must have taken place[773]. Though he thus ascertained that these insects do not in this respect deviate from the general rule, he remarked or confirmed several facts in their economy sufficiently anomalous and striking;--as that the female is not only without wings, but with scarcely any feature of a _moth_, much more closely resembling a _caterpillar_; and that in ordinary circumstances she never attempts to leave the pupa-case in which she has been disclosed, but that being there impregnated by the male, she there also, apparently after the manner of the female _Cocci_, deposits her eggs, which hatching produce young larvæ that make their way out of the case, and thus seem to originate without maternal interference[774].

But the most remarkable fact bearing upon this head, though as relating to a _viviparous_ insect it does not strictly belong to it, is the impregnation of the female _Aphides_, or plant-lice, before alluded to[775]. If you take a young female _Aphis_ at the moment of its birth, and rigorously seclude it from all intercourse with its kind, only providing it with proper food, it will produce a brood of young ones: and not only this; but if one of these be treated in the same way, a similar result will ensue, and so on, at least to the _fifth_ generation!! to which period Bonnet, who first made an accurate series of observations on this almost miraculous fact, successfully carried his experiments, till the approach of winter and the want of proper food forced him to desist[776]; and Lyonet extended it still further[777]. It is now generally admitted as an incontestible fact, that female _Aphides_ have the faculty of giving birth to young ones without having had any intercourse with the other sex. How are we to explain this most extraordinary fact? Are we to suppose with Bonnet that these insects are truly androgynous, as strictly uniting both sexes in one? This supposition, however, is completely overturned by the circumstance, that there are actually _male_ as well as _female Aphides_, and that these, as was first observed by Lyonet, are united towards the close of the summer in the usual manner[778]. The most likely supposition therefore is, that one conjunction of the sexes suffices for the impregnation of all the females that in a succession of generations spring from that union. It is true that at the first view this supposition appears incredible, contradicting the general laws and course of nature in the production of animals. But the case of the hive-bee, stated above, in which a single intercourse with the male fertilizes all the eggs that are laid for the space of _two_ years, and in the case of a common spider mentioned by Audebert[779], for _many_ years, shows that the sperm preserves its vivifying powers unimpaired for a long period, indeed a longer period than is requisite for the impregnation of all the broods that a female Aphis can produce; and if immediate contact with the fluid be not necessary, who can say that this is impossible? It is, however, one of those mysteries of the CREATOR that human intellect cannot fully penetrate. But this anomaly in nature is not wholly confined to the Aphides; since Jurine has ascertained that the same thing takes place with _Daphnia pennata_ Müll (_Monoculus Pulex_ L.), one of Branchiopod _Crustacea_[780]. It is worth observing whether the female _Aphides_ in their natural state, I mean those of the summer or viviparous broods, have intercourse with the male. I think I have noticed males amongst them; but they seem to become most numerous in the autumn, preparatory to the impregnation of the oviparous females. The object of this law of the CREATOR is probably the more ready multiplication of the species[781].

* * * * *

As to the period of _gestation_, most insects begin to lay their eggs soon after fecundation has taken place: but in some _Arachnida_, as the Scorpion, which seems to be both oviparous and ovo-viviparous, nearly a year intervenes, and the eggs increase to _four_ times the size which they had attained at that period, before they are extruded[782]. The time that is required to lay the whole they are to produce, varies also in _insects_. In this respect they may be divided into two great classes:--those namely which deposit the whole at _once_, as _Ephemerina_, _Trichoptera_, &c., and those which deposit them in _succession_, occupying in this operation a longer or shorter period. Many in the _first_ class, as the _Trichoptera_ or caseworm-flies, envelope their eggs in a gelatinous substance[783], which renders their extrusion in a mass more easy. Of the _second_ class, which includes by far the greater proportion of insects, some exclude the whole number in a very short period, others require two or three days or a week, as the cockroach[784]; and others, as the queen-bee, not less than two years. The eggs in the ovaries of the last vary infinitely in size; those that have entered the oviduct have arrived at maturity, while the rest grow gradually smaller as they approach the capillary extremity of the tubes, where they become at length invisible to the highest magnifier[785]. In many insects the eggs seem nearly to have reached their full growth previously to the exclusion of the female from the pupa; and this exclusion and the impregnation and laying of the eggs rapidly succeed each other. One moth (_Hypogymna dispar_), which is remarkable for the number of eggs she contains, sometimes deposits them, even before they are fecundated, in the pupa-case[786]. But in other cases the sexual union is not so immediate, and some time, longer or shorter, is requisite for the due expansion of the eggs; and the ovaries of the animal swell so much, as often to enlarge the abdomen to an extraordinary bulk: this is seen in a very common beetle (_Chrysomela Polygoni_) that feeds upon the knot-grass; but in no insect is it so striking as in the female of the white ants, whose wonderful increase of size after impregnation I have related to you on a former occasion[787].

* * * * *

I shall conclude this subject with a few observations upon _ovo-viviparous insects_; _supposed neuters_, and _hybrids_, which, though they do not fall in regularly under any of the foregoing heads, may very well have a place in this letter.

1. It has already been observed that there are a few _ovo-viviparous_ insects[788], the young of which exist in the ovaries at first as eggs, but are hatched within the body of the mother, and come forth in the living form of a larva and sometimes even of a pupa. Of the first description are certain _Diptera_, the _Aphides_, and the _Scorpion_.

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.

FOOTNOTES:

[703] Herold _Schmetterl._ tab. expl. vii.

[704] Herold _Schmetterl._ _t._ iv. _f._ 1. _x._ &c. PLATE XXX. FIG. 12. _d._

[705] _De Bombyc._ 36.

[706] _Ibid._ _t._ xii. _f._ 1. I. and, _f._ 2. O. M.

[707] _Philos. Trans._ 1792. 186.

[708] Swammerdam, in dissecting the female of _Oryctes nasicornis_, discovered a blind vessel opening into the vagina, and at the other or inner extremity not terminated by any secretory tube, containing a yellowish matter, that seems analogous to the organ mentioned in the text; and in the hive-bee he found a similar organ covered with air-vessels, which he supposes to be connected with the _Colleterium_ (see above, p. 132.), and which he states to contain a slimy matter. _Bibl. Nat._ i. 151. b. _t._ xxx. _f._ 10. _g._ 204. b. _t._ xxix. _f._ 3. _t._ Perhaps likewise the organ discovered by M. L. Dufour in _Scolia_,--which he imagines to belong to the poison-secretor, and which he describes as a sac consisting of a double tunic, the exterior one muscular and the interior membranous, and filled with a blueish-green gelatinous matter (_N. Dict. d'Hist. Nat._ xxx. 388.)--may be a _spermatheca_.

[709] _De Insector. Genital._ 17.

[710] I allude to those organs above described (p. 132.) for the secretion of matter for varnishing the eggs or lubricating the oviduct. It seems most probable, if the fecundation of the eggs takes place gradually, that upon their passing into the oviduct, a special reservoir should be appropriated to the reception of the male sperm, adapted to maintaining in due activity the vivifying principle, or _aura seminalis_.

[711] Herold _Schmett._ _t._ iv. _f._ 2. _m n._

[712] Treviran. _Arachnid._ 36. _t._ iv. _f._ 32. _aa._ Marcel de Serres in _Mém. du Mus._ 1819. 89.

[713] Marcel de Serres, _Mém. du Mus._ 1819. 115.

[714] Rifferschw. _De Genital. Ins._ 11.

[715] Marcel de Serres in _Mém. du Mus._ 1819. 109. PLATE XXX. FIG. 12. _a._

[716] Rifferschw. _ubi supr._ 23--. Swamm. _Bibl. Nat._ _t._ xlii. _f._ 8. _a, f, g, h._

[717] _Ibid._ i. 104. _t._ xv. _f._ 3. ii. 62. _t._ xii. _f._ 8. Treviran. _Arachnid._ _t._ iv. _f._ 32.

[718] Reaum. iv. 391.

[719] Posselt _Anat. der Ins._ _t._ i. _f._ 28, 29.

[720] _N. Dict, d'Hist. Nat._ xxx. 387--. Swamm. _ubi supr._ ii. 23. _t._ xxxv. _f._ 3.

[721] _Ibid._ i. 203.

[722] PLATE XXII. FIG. 2.

[723] Swamm. _ubi supr._ i. 151. Gaede _Anat. der Ins._ _t._ ii. _f._ 3.

[724] Swamm. i. 203.

[725] Gaede _Anat. der Ins._ 20. _t._ i. _f._ 9.

[726] _Ibid._ 25, 28. _t._ ii. _f._ 10.

[727] _Ibid._ 32.

[728] Swamm. ii. 74.

[729] _Ibid._ 203. _t._ xix. _f._ 3.

[730] Reaum. iv. 391--.

[731] Swamm. _t._ xliii. _f._ 19.

[732] Gaede 22.

[733] Swamm. _Bibl. Nat._ i. 203.

[734] _Ibid._

[735] Rifferschw. 11--.

[736] Swamm. _t._ xlii. _f._ 8. Gaede, _t._ i. _f._ 3. _cc._

[737] Herold _Schmett._ _t._ v. _f._ 10. 12.

[738] PLATE XXX. FIG. 12.

[739] PLATE XXII. _f._ 2. _b._

[740] Swamm. _t._ xix. _f._ 4. _b._

[741] _Ibid._ _f._ 3.

[742] VOL. I. p. 355--.

[743] De Geer iv. 127. _t._ iv. _f._ 17.

[744] De Geer iv. 143. _t._ v. _f._ 15.

[745] VOL. I. p. 357.

[746] De Geer. v. 62. _t._ iii. _f._ 12.

[747] PLATE XV. FIG. 18.

[748] Stoll _Sauterel._ _t._ xxii. b. _f._ 87, &c.

[749] De Geer iii. 418. _t._ xxi. _f._ 10, 11. Latr. _Gen. Crust. et Ins._ iii. 98.

[750] Stoll _ubi supr._ _t._ xiii. a. _f._ 51.