Part 12

We next come to _fetid_ odours. These in numerous cases are known to be secreted and emitted by appropriate vessels and organs; they are often exhaled from a fluid secretion, of which, in the letter lately referred to, I gave you almost all the known instances. Savi, in his history of _Iulus fœtidissimus_, informs us that it emits a yellow fetid fluid from its supposed spiracles, which if applied in sufficient quantity imparts a red colour to the skin, to be removed neither by friction nor washing, but only disappearing by time; when removed from the black vesicles in which it is stored, it shoots into very transparent octahedral crystals[693].

I have before mentioned the coloured fluid which some insects emit when they are disclosed from the pupa, and that it probably exhales some powerful odour which attracts the males[694].

The great _Hydrophilus_, in its larva state, when first taken into the hand remains without motion; in a minute afterwards it renders itself so flaccid as to appear like a cast skin. Taken by the tail it contracts itself considerably, it then agitates itself briskly, and ejaculates with a slight noise a fetid and blackish fluid[695].

In other cases these odours are produced by _gaseous vapours_. That of the Bombardiers (_Brachinus_) is the most celebrated and remarkable. It is whitish, of a powerful and stimulating odour, very like that exhaled by nitrous acid. It is caustic, producing upon the skin the sensation of burning, and forming instantly upon it red spots which soon turn brown, and which, in spite of frequent lotions, remain several days. It turns blue paper red[696]. That amiable, intelligent, and unfortunate traveller Mr. Ritchie,--whose premature death, when attempting to penetrate to the interior of Africa, all lovers of Natural History so deeply lamented, and whose ardour in the pursuit of that science I had an opportunity of witnessing, when, in company with him, Messrs. Savigny, Du Fresne, and W. S. MacLeay in 1817, I visited the forest of Fontainebleau,--in a letter to the last-mentioned gentleman[697], relates that his companion M. Dupont, near Tripoli took a nest consisting of more than a thousand of a species of this genus. "I am making a few experiments," says he, "on the substance which they emit when they crepitate, but do not know whether I can collect enough to arrive at any conclusion. It made Dupont's fingers entirely black when he took them. It is neither alkaline nor acid, and it is soluble in water and in alcohol." From this we may conjecture that it formed crystals.

xi. _Phosphorus._ On this remarkable secretion I have so fully enlarged on a former occasion[698], that here I shall merely add a few observations which Mr. Murray obligingly communicated to me. He remarks that in a box in which glow-worms were kept--five luminous specks were found secreted by the animal, which seemed to glow and were of a different tinge of light. One put into olive oil at eleven P. M. continued to yield a steady and uninterrupted light until five o'clock the following morning, and then seemed, like the stars, to be only absorbed by superior effulgence. The luminous spherical matter of the glow-worm is evidently enveloped in a sac or capsule perfectly diaphanous, which when ruptured discloses it in a liquid form, of the consistency of cream. M. Macaire, he observes, in the _Bibliothèque Universelle_, draws the following conclusions from experiments made on the luminous matter of this animal;--that a _certain_ degree of heat is necessary to their voluntary phosphorescence--that it is excited by a degree of heat superior to the first, and inevitably destroyed by a higher--that bodies which coagulate albumen take away the power--that phosphorescence cannot take place but in a gas containing no oxygen--that it is not excited by common electricity, but is so by the Voltaic pile--and lastly, that the matter is chiefly composed of _albumen_.

xii. _Fat._ There is one product found in the body of insects most copiously in their larva state, but more or less also in the imago, which may be called their _fat_. In the former it is a many-lobed mass, occupying the whole of the interior, except the space that is required for the muscles and the internal organs, which it wraps round and protects. It is contained in floating membranes, very numerous, which fill all the interstices, and assume the appearance sometimes of small globules, and sometimes of a thickish mucilage, which easily melts and inflames; in colour it is most commonly white, but sometimes yellow or green. It is imagined to be a kind of _epiploon_ or _caul_, and is accumulated in the _larva_ as a store of nutriment for the growth and development of the organs of the perfect insect while in the _pupa_ state[699]. The blood in which the different organs float that is not required for their nutriment, is supposed to be expended in the formation of this substance. Marcel de Serres is of opinion that it is secreted from the chyle by passing through the pores of the dorsal vessel, formerly called the heart of insects[700].

Under this head I may mention what little is known with regard to the _perspiration_ of these animals[701]. That a considerable quantity of fluid passes off from them when in the pupa state, is sufficiently proved by the loss of weight which they undergo, and by the experiments of Reaumur, who collected the fluid in closed glass tubes; and that in their perfect state they are constantly passing off perspirable matter by the pores of their skin or crust, is not only rendered probable by the succulent nature of their food and the absence of any urinary discharge, but is proved by what takes place in a swarm of bees. These insects, when crowded together in hot weather in a large mass, become heated to such a degree, and perspire so copiously, that those near the bottom are quite drenched with the moisture it produces, which so relaxes their wings that they are unable to fly[702].

I am, &c.

FOOTNOTES:

[588] Cuv. _Anat. Comp._ iv. 163--.

[589] VOL. III. p. 124--.

[590] Malpigh. _De Bombyc._ _t._ v. _f._ 2. Swamm. _t._ xxxiv. _f._ 5. Lyonet, _t._ v. _f._ 1.

[591] _Anat. der Ins._ 59.

[592] _Ibid._ 60. Malpigh. 20.

[593] Lyonet _Anat._ 111.

[594] _N. Dict. d'Hist. Nat._ xv. 483.

[595] _Anat. Comp._ v. 198.

[596] Ramdohr, 60. _t._ xvii. _f._ 1. _f, g, h, r._

[597] VOL. I. p. 403--. Treviran. _Arachnid._ 42.

[598] Treviran. _Arachnid._ 43. _t._ iv. _f._ 42. _o. p._ 9.

[599] _Ibid._ α, y.

[600] Swamm. ii. 21. a. _t._ xxxvi. _f._ 1. _abcd._ Ramdohr 58.

[601] _Schmet._ _t._ iii. _f._ 1.

[602] Lyonet--. 112. _t._ v. _f._ 1. _P, Q, R, S._

[603] Ramdohr _Anat._ _t._ xviii. _f._ 1. _M._ _f._ 5. _F._

[604] _Ibid._ _t._ x. _f._ 1. _m._

[605] _Ibid._ _t._ xxii. _f._ 3. _M L._ Ramdohr regards the double one as a pair; but as they terminate in a single tube, they ought to be reckoned as one.

[606] _Ibid._ _f._ 4.

[607] _Ibid._ _f._ 2. _K, L, M, N._ _t._ xxiii. _f._ 6.

[608] _Ibid._ 177. _t._ xxi. _f._ 3. _F. F._

[609] _Ibid._ _f._ 2. _G, H._

[610] _Ibid._ _t._ xxii. _f._ 2. _L._

[611] _Ibid._ _t._ xxi. _f._ 1. _O._ _t._ xvii. _f._ 6. _n._

[612] Ramdohr _Anat._ _t._ xx. _f._ 6. _D._

[613] _Ibid._ _t._ xxii. _f._ 1. _K, L._ _f._ 2. _I, K, L._

[614] _Ibid._ _f._ 3, 4, 5.

[615] _Ibid._ 57--.

[616] Reaum. ii. 81. Herold _Expl. of Plates_, x. Malpigh. _De Bombyc._ 37. PLATE XXX. FIG. 12. _c._

[617] Herold _Ibid._ x. _t._ iv. _f._ 1. _p, u, y._ Marcel de Serres _Mem. du Mus._ 1819. 141.

[618] Gaede _Anat._ _t._ i. _f._ 3. _d._

[619] _Ibid._ 17. _t._ i. _f._ 4.

[620] _Bibl. Nat._ _t._ xix. _f._ 3. β.

[621] Reaum. v. 377. _t._ xxix. _f._ 7. _s._

[622] _N. Dict. d'Hist. Nat._ xxx. 388.

[623] _Ibid._ 427--.

[624] _Arachnid._ 31. _t._ ii. _f._ 21. _p._ 9.

[625] _N. Dict. d'Hist. Nat._ xxii. 114. 117. comp. VOL. I. p. 127.

[626] _Ibid._ xxviii. 6.

[627] _Osservazioni_, &c. 13--.

[628] VOL. II. p. 243--. _N. Dict. d'Hist. Nat._ iv. 308.

[629] _Ibid._ iv. 309.

[630] _Ibid._ v. 252.

[631] De Geer iv. 358. _t._ xiii. _f._ 9. _m._

[632] VOL. II. p. 241--. III. p. 147--.

[633] De Geer iii. 41.

[634] VOL. I. p. 451, where by mistake it is represented as the work of _Aphis Abietis_.

[635] De Geer iii. 111.

[636] Reaum. iii. _t._ xxvi. _f._ 4-6.

[637] _N. Dict. d'Hist. Nat._ vi. 305.

[638] VOL. III. p. 221.

[639] Treviran. _Arachnid._ 44. In Paraguay a spider is found which makes spherical cocoons of yellow silk, which are spun because of the permanence of the colour. This operation occasions a flow of water from the eyes and nose of the spinners. Azara _Voyag._ 212. See also Murray in _Werner. Trans._ 1823. 8--.

[640] Reaum. v. 24.

[641] VOL. II. p. 244--.

[642] Ramdohr _Anat._ _t._ ii.-vi.

[643] _Ibid._ 20. See above, p. 107. As some of the _Sialisteria_ render to the _stomach_ (see above, p. 131), there seems no small affinity between these shags and those organs.

[644] Cuv. _Anat. Comp._ iv. 132, 136.

[645] Reaum. vi. Pref. xxviii. 177--.

[646] _Ibid._ 253--.

[647] _Ibid._ iii. 375.

[648] _Anat._ _t._ xii. _f._ 6.

[649] _Ibid._ xxi. _f._ 3. _I I._

[650] Reaum. iii. 230.

[651] VOL. III. p. 79--.

[652] Reaum. iii. 215. Bonnet ix. 182.

[653] VOL. III. p. 68--.

[654] Marcel de Serres _Mem. du Mus._ 1819. 133, 141.

[655] De Geer, v. 6.

[656] Rai. _Hist. Ins._ 62.

[657] VOL. II. p. 242--. 248. Rai. _Hist. Ins._ 94, 382.

[658] Reaum. v. 448.

[659] _Ibid._ v. 722.

[660] VOL. I. p. 196. II. p. 176.

[661] _Encyclop. Britan._ viii. 205. from _Journ. de Phys._

[662] VOL. II. p. 174.

[663] Reaum. iii. 318--. _t._ xxvi. _f._ 1-6.

[664] _Ibid._ 396--. _t._ xxxi. _f._ 20-29.

[665] _Insect. Suec._ i. 257.

[666] VOL. I. p. 327.

[667] _N. Dict. d'Hist. Nat._ xvii. 189.

[668] Nicholson's _Journ._ i. 298--.

[669] VOL. III. 281.

[670] _Philos. Trans._ 1670.

[671] _Philos. Trans._ _Ibid._ Ray's Lett. 74.

[672] Amoreux _Ins. Venim._ 236--.

[673] _N. Dict. d'Hist. Nat._ xii. 94.

[674] Southey's _Brazil_, i. 645.

[675] _N. Dict. d'Hist. Nat._ ubi supr.

[676] VOL. II. p. 67.

[677] _Syst. of Chemist._ 533.

[678] Germar _Mag. der Ent._ iii. 445--.

[679] _Mem. Dijon_ 1783. ii. 70.

[680] Reaum. v. 354.

[681] _On Poisons_, i. 265--.

[682] _Ibid._ 269.

[683] Reaum. _ubi supr._

[684] VOL. I. p. 124. III. p. 716--.

[685] _N. Dict. d'Hist. Nat._ xxx. 427.

[686] I use the term _odorous_, not in the same sense as _odoriferous_, but to include both sweet and fetid scents.

[687] VOL. II. p. 238--. III. p. 147--.

[688] A Brazilian wood so called, but differing from the common cedar.

[689] Dotharding _Insect. Coleopt. Danic._

[690] Sturm _Deutsch. Fn._ i. 27.

[691] Reaum. iii. 494.

[692] _Mon. Ap. Angl._ i. 136.

[693] _Osservaz. sullo Iulus, &c._ 14.

[694] VOL. III. p. 297--.

[695] _N. Dict. d'Hist. Nat._ xv. 487.

[696] _N. Dict. d'Hist. Nat._ iv. 308.

[697] Dated Tripoli in the West, January 21, 1819.

[698] VOL. II. p. 418--.

[699] Reaum. i. 145. Lyonet _Anat._ 106--. _N. Dict. d'Hist. Nat._ xvi. 224. PLATE XXI. FIG. 5. _a._

[700] See above, p. 90. note^a.

[701] See above, p. 78.

[702] Huber i. 273.

LETTER XLII.

_INTERNAL ANATOMY AND PHYSIOLOGY OF INSECTS, CONTINUED._

REPRODUCTION.

The reproductive organs of insects in their _general_ denominations and functions correspond with those of the higher classes of animals; but as to _number_, _proportions_, and other _particular_ details of their structure, they differ from them very considerably. I shall not now, however, enter at large upon this subject, but confine myself principally to the consideration of those organs in the female which are appropriated to the formation, fecundation, maturation, exclusion and deposition of their eggs, and other circumstances relating to that subject. The organs connected with this function are the _Sperm-reservoir_; the _Oviduct_; the _Ovaries_; and the _Ovipositor_.

I. The Sperm-reservoir (_Spermatheca_) is an organ connecting the vagina with the oviduct, which, according to Herold, receives the male sperm as into a reservoir[703], and fecundates the eggs in their transit through that passage. This vessel, which consists of a double tunic, in the cabbage-butterfly terminates the vagina, and is connected with the oviduct by a lateral undulating tube: in shape it is a rather irregular oblong, and is surmounted by a small orbicular vesicle, connected by a short tubular footstalk with the main reservoir[704]. A similar organ was discovered by Malpighi in the imago of the silkworm, who denominates it the _uterus_; to which indeed it seems analogous, and which he also regards as a reservoir for the sperm for the gradual fecundation of the eggs[705]. But in that fly the organ is of a rather different shape, and the interior vessel terminates in several spherical vesicles[706]. John Hunter by the most decisive experiments, such as covering the eggs of the unimpregnated moth, after exclusion, with the liquor taken from the spermatheca in those which had been impregnated, and rendering them fertile, he demonstrated that this organ was a reservoir for the spermatic fluid, to impregnate the eggs as they were ready for exclusion, and that coition and impregnation were not simultaneous[707]. It is not improbable that in all insects whose eggs are gradually laid, this provision for their gradual fecundation, if carefully sought for, might be detected[708]. Rifferschweils is of opinion, that in these cases the eggs are fertilized in their transit through the oviduct by sperm adhering to the folds of the _cloacæ_[709]: but this opinion seems less analogous to what takes place in other cases, with regard to the due preparation of the eggs for a safe and effectual transit[710].

II. The Oviduct (_Oviductus_) is the canal, always separate from the vagina, which receives the eggs from the ovary, transmitting them, often by a peculiar and complex instrument in which it terminates, to their proper station. This canal sometimes opens into the anal passage or _cloaca_, and at others, as in the cabbage-butterfly[711], is distinct, and lies between the sexual organ and the anus. In the _Arachnida_ there are _two_ oviducts[712].

III. The Ovaries (_Ovaria_) in insects are the viscera in which the eggs are generated and grow till they arrive at maturity, when they pass through the oviduct, and are extruded or deposited in their appropriate station. They vary considerably in their structure. In all however, except the _Iulidæ_, in which there is only a _single_ ovary[713], the oviduct at its upper or inner extremity terminates in _two_ branches, usually further subdivided into a number of smaller conical ones, which several ramifications constitute the _ovaries_, or egg-tubes as they are sometimes called: these tubes generally consist of a single membrane, and are joined to the oviduct by membranous rugose _cloacæ_[714]: in the _Phalangia_, however, there are _two_ tunics; the outer one of a cellular substance, and the inner one consisting of spiral fibres like _tracheæ_--a kind of structure which renders them capable of great extension[715]. Rifferschweils considers the ovaries as formed upon _two_ primary types.--_First_, _flagelliform_ ovaries, consisting of conical tubes equal in length, and inserted at the same place at the end of the primary branches as in the _Lepidoptera_, the Bee, &c. _Secondly_, _racemose_ ovaries, consisting of short conical tubes, so proceeding from the primary branches as to render the ovary racemose or pinnated, as in certain _Neuroptera_, _Coleoptera_, and _Diptera_[716]: but perhaps their structure will be better understood if they are divided into _agglomerate_ ovaries and _branching_ ovaries: in the _first_ the egg-tubes form _two_ bundles, in which the branches are not discernible, as in the _Ephemera_, the chamæleon-fly, and spiders[717]: and in the _second_ the branches are distinct, as in the _Lepidoptera_ and the majority of insects.

The number of branches varies in different genera and species. In _Echinomyia grossa_, a large fly, there are only the _two_ primary branches[718]; in the common dung-beetle (_Geotrupes stercorarius_) these appear divided at their apex into fingers[719]: in _Scolia_, a Hymenopterous genus, and the butterfly of the nettle, there are _three_ secondary branches on each side[720]: in many other _Lepidoptera_ and the humble-bee there are _four_[721]; in the common louse there are _five_[722]; in the rhinoceros-beetle and the cockchafer, _six_[723]; in the wasp, _seven_[724]; _eight_ in the cockroach[725]; _twelve_ in the _Carabi_ and the mealworm-beetle[726]; _thirty_ in the large green grasshopper (_Acrida viridissima_[727]); _thirty-two_ in the cheese-maggot-fly[728]; and in the hive-bee more than _a hundred and fifty_[729].

The number of _eggs_ also contained in the ovaries varies. In _Echinomyia grossa_ there is only _one_ egg in each, and only _two_ at once in the matrix[730]: in another fly produced by the cheese-maggot there are _four_[731]; in the louse there are _five_; in the cockchafer _six_[732]; in the hive-bee _sixteen_ or _seventeen_ are visible at the same time[733]; and in the silkworm-moth _sixty_ or _seventy_[734]. Besides the eggs, the tubes contain a pellucid mucus, and at their upper extremity the eggs are lost in a granular mucous mass, in which, however, they may still be discovered with a microscope[735]. With regard to the termination of the ovaries or egg-tubes internally,--in those that have agglomerated ones it is not to be traced, the whole appearing like an oblong obtuse or acute body[736]: but in the branching ones it is more easily traced; at first they converge in most cases to a point; this is seen to advantage in the caterpillar of some butterflies, when near assuming the pupa, in which they are readily discovered, and represent with great truth and elegance the bud of some blossom[737]; but in time they diverge, and sometimes become convoluted[738]; they generally terminate in a slender simple filament, but in the louse in a fork[739]; they are sometimes extremely long, as in the wasp and _Lepidoptera_[740]; in the hive-bee they appear to be shorter[741].

IV. We are next to consider the _Ovipositor_, or instrument by which numerous insects are enabled to introduce their eggs into their appropriate situations, and where the new-born larva may immediately meet with its destined food. As this instrument is one of the most striking peculiarities with which the wisdom of the CREATOR has gifted these little animals, and in many cases is extremely curious and wonderful, both in its structure and modes of operation--though on a former occasion I gave you a brief account of several kinds of them[742], I shall now enter more at large into the subject, and describe these often complex machines, as they are exhibited in most of the different Orders of insects.

With regard to the _Coleoptera_ Order, there are doubtless numerous variations in the structure of this organ; but very few have been noticed, and those chiefly belong to insects whose grubs feed on timber. In these it is usually retractile one part within another, like the pieces of a telescope: in _Buprestis_ it consists of three long and sharp _laminæ_, the two lateral ones forming a sheath to the intermediate one, which probably conveys the eggs[743]: in _Elater_ it is a cylindrical organ, terminating in a pair of conical joints, which seem to form a forceps, and including a tube probably conveying the egg to the forceps, which perhaps introduces it[744]. The Ovipositor of _Prionus coriarius_ differs from that of _Callidium violaceum_, and many Capricorns before described[745]: it consists merely of a long bivalve piece ending in a kind of forceps, and hollowed above into a channel for the passage of the eggs[746].

In the _Orthoptera_ the instrument of oviposition is more simple; in _Locusta_ consisting merely of four robust three-sided pieces, two above and two below, the former pair at the end curving upwards and the latter downwards[747], these pieces seem calculated when they have entered the earth to enlarge the burrow, and the animal appears able to separate them very widely from each other[748]. The ovipositor of _Acrida viridissima_, which like that of many Hymenopterous insects forms a kind of appendage or tail to the body, has been described both by De Geer and Latreille as consisting of _two_ valves only[749]; but in reality it consists of _six_, two upper and four lower, as you may ascertain by means of a pin or the point of a penknife, which will readily separate them. This is confirmed by a figure of Stoll's of a species which seems to connect _Conocephalus_ with _Gryllus_. In this the ovipositor is considerably longer than the body of the animal, and is composed of _six_ distinct pieces; viz. _two_ external ones stouter than the rest, and within these _four_ others finer than a hair and convolute at the apex[750]. There is a considerable variety in the shape of the ovipositors of the _Acridæ_ and the cognate genera:--thus in _A. viridissima_ this organ is straight, in _A. verrucivora_ bent like a sabre, and in _Pterophylla citrifolia_ and some others, the whole machine is short and boat-shaped; in _Scaphura Vigorsii_ it is also rough with sharp little tubercles[751]. I had an opportunity of observing, with respect to the first of these insects, that in boring, as is the case with the _Cicadæ_ and saw-flies, the motion of the valves was alternately backwards and forwards. It appeared also to me that the two outer pieces of each of the apparent valves were fixed in a groove in the margin of the intermediate one. I saw this clearly with respect to the _upper_ pieces, and it is most probable that the lower are similarly circumstanced. In the cricket tribe (_Gryllus_) the ovipositor is as long as the abdomen, very slender, terminating in a knob[752]. It is _apparently_ bivalve like that of _Acrida_, but I believe is resolvable into the same number of pieces.

In the _Homopterous Hemiptera_ there seems to be more than one type on which the ovipositor is constructed. In an insect very common with us, the froth froghopper (_Cercopis spumaria_), some approach is made to the ovipositors last described, at least the number of pieces is the same--for it has a pair of external valves forming a sheath, which includes three sharp _laminæ_ resembling the blades of a lancet, the middle one of which can be separated into two; this instrument De Geer had reason to think was scored transversely like a file[753]. In the insects of this Order so noted for their song[754] (_Cicada_), there are only _five_ pieces; namely, two valves forming the sheath, two augers or borers, and an intermediate piece upon which they slide, each being furnished with an internal groove for that purpose, and the middle piece with a ridge to fit; a contrivance of Divine Wisdom, to prevent their dislocation when employed in boring; the augers terminate in a knob which is externally toothed[755]. This structure approaches that of the _Hymenoptera_, especially the saw-flies. With regard to the _Heteropterous_ section of this Order--as they usually do not introduce their eggs _into_ any substance, they have no call for any remarkable ovipositor, and therefore are not so furnished. A remark which will also apply to the _Lepidoptera_ Order.

In the _Libellulina_ amongst the _Neuroptera_, an organ of this kind is sometimes discoverable. In _Agrion_, Reaumur noticed a part which he conjectured to be an _ovipositor_; it consists of four _laminæ_ or lancets, the interior pair slender, the exterior wider, and all externally serrated[756].