Part 16

Before concluding these remarks on the Internal Anatomy and Physiology of Insects, I shall explain to you, as you will probably feel inclined occasionally to pursue the subject, the best mode of _dissecting_ them.--By far the most useful dissecting instruments for this purpose are very fine-pointed and sharp _scissors_, as these will enable you to divide the integument and separate other parts with much less risk of injuring their delicate structure than any knife. These scissors are what Swammerdam chiefly used; and he had some so extremely small and fine, that he was necessitated to employ a lens when he sharpened them. If to these be added a sharp and fine-pointed _knife_ or two, some _needles_ fixed in handles, also fine-pointed--(you will find them more convenient than any other instrument for detaching minute parts and fibres,) a pair of fine and accurately adjusted _pliers_, and an assortment of camel's-hair _brushes_,--you will be nearly set up as an Entomological dissector. You will still, however, require a small dissecting table, with a projecting and moveable arm for lenses of various descriptions, so as to admit both the hands to be employed upon the subject under examination; and for this purpose probably no contrivance can be better adapted than that of Lyonet, of which the figure in Adams _On the Microscope_ will convey a better idea than any description[916].

Previously to dissecting any insect, it must be killed by plunging it into boiling water, which is recommended by Lyonet, or spirits of wine or of turpentine; and it is often useful to let _larvæ_ remain a few days in the latter, by which means the vessels become firmer and stronger. The parts of _pupæ_ become much more distinct if they are boiled for a few minutes: and the same mode may be adopted in the examination of spiders.

The most convenient mode of proceeding, which was that also of Lyonet, is to dissect the insect in water, or, to avoid putridity, in diluted spirits,--if small, upon a concave glass, to which it should be fastened by means of a little melted wax; if larger, in the bottom of a common chip box, surrounded with a border of wax to retain the fluid. The integuments of the insect, being carefully divided longitudinally with scissors, should if flexible be turned back, and fixed by small pins stuck in by a fine pair of pliers, while the skin at the same time is stretched by another. After making such observations as present themselves without further dissection, the viscera must be cautiously extracted, washing away the fat which surrounds them with spirits of turpentine, in which it is soluble, applied by camel's-hair pencils. After separation they may conveniently be examined by putting them into water, and gently shaking them so as to cause the parts to unfold. If endowed with the patience of Swammerdam, you may even arrive at injecting these minute parts with wax or coloured fluids, conveyed by delicate glass tubes having one end as fine as a hair, which he also employed to fill the viscera with air; and afterwards drying them in the shade, and anointing them with oil of spike in which a little resin had been dissolved, he succeeded in preserving them. If it is not convenient to finish the dissection of an insect at once, it should be covered with spirits of wine. Swammerdam found a mixture of spirits and distilled vinegar very useful for keeping caterpillars previously to dissecting them, as it consolidated the parts[917].

* * * * *

And now having brought to a close my long wanderings in this ample and intricate field, and having threaded, as well as my slender powers and limited knowledge enabled me, the infinite turnings and convolutions of this Dædalean labyrinth--the _Anatomy_ and _Physiology_ of insects,--will you not own that the volume of wonders I have laid before you proves irrefragably that, though these minims of nature apparently rank so low in the scale of being, yet in their structure, instead of being, as might be expected, more simple, they are infinitely more complex and highly wrought than those animals that are placed the nearest to ourselves? the CREATOR in the latter doing every thing by a beautiful _simplicity_; while in the former, the more to magnify his power and skill, because they afford no apparent space for it, by a wonderfully curious and intricate _multiplicity_: and whether we study the one or the other, we shall in both trace the footsteps of that adorable LOVE which has shown attention to the comfort and well-being of the lowest insect, as well as of the highest of his creatures.

I am, &c.

FOOTNOTES:

[801] VOL. II. LETTER XXII. VOL. III. LETTERS XXXIV.-XXXVI.

[802] VOL. II. p. 280, 295--, 306, 310--. &c.

[803] _Philos. Trans._ 1818. 174. _t._ viii. _f._ 4-6.

[804] See above, p. 150--.

[805] _Schmetterl._ 105.

[806] _Philos. Trans._ 1819. 172, 174, 187.

[807] _Anat. Comp._ i. 90.

[808] _Philos. Trans._ 1819. 175.

[809] Cuv. _ubi supr._ 90--.

[810] Cuv. _Ibid._ i. 89--.

[811] See above, p. 85.

[812] Lyonet _Anat._ _t._ iv. _f._ 3.

[813] _Ibid._ 93--.

[814] Cuv. _Anat. Comp._ i. 134.

[815] Chabrier _Sur le Vol des Ins._ c. i. 445.

[816] PLATE XXI. FIG. 6. _a._

[817] De Geer iv. _t._ xv. _f._ 11. _m n, o p._

[818] Lyonet _Anat._ 93.

[819] Lyonet _Anat._ _t._ xiii. _f._ 1, 2.

[820] Ramdohr _Anat._ _t._ v. _f._ 1. _e._ _f._ 3.

[821] Chabr. _ubi supr._ 440--.

[822] _Ibid._ 442, &c.

[823] _N. Dict. d'Hist. Nat._ xxii. 80.

[824] VOL. III. p. 663, 670. See above p. 21.

[825] Chabrier _Sur le Vol des Ins._ c. i. 446.

[826] VOL. III. p. 411.

[827] _Ubi supr._ 437, 439.

[828] PLATE XXII. FIG. 11, 12. c. Chabrier _ubi supr._ c. iii. _t._ xi. viii. _f._ 9. S. D. _i, k._ c. i. 440--.

[829] PLATE XXII. FIG. 11, 12. c. Chabrier _Sur le Vol des Ins._ c. iii. _t._ xi. viii. _f._ 9. S. D. _i, k._ c. i. 440--.

[830] Cuv. _Anat. Comp._ i. 94--.

[831] _N. Dict. d'Hist. Nat._ xxii. 80.

[832] _Ubi supr._ 101--.

[833] VOL. I. p. 67.

[834] _Anat. Comp._ i. 432--.

[835] _Anat._ _t._ vii. _f._ 2. left hand.

[836] _Ibid._ right hand.

[837] _Ibid._ 115--.

[838] Cuv. _ubi supr._

[839] VOL. III. p. 135--.

[840] _Anat. Comp._ i. 447.

[841] VOL. III. p. 366. PLATE XXVII. FIG. 1, 4. n´.

[842] Ibid. FIG. 3. n´.

[843] PLATE XXVII. FIG. 1. _a._

[844] VOL. III. p. 367--, 541, 584. PLATE XXII. FIG. 7. Cuv. _ubi supr._ 448.

[845] PLATE XXVII. FIG. 5. _a._

[846] _Anat. Comp._ i. 136.

[847] De Geer iv. _t._ xv. _f._ 11. _o, p._

[848] Marcel de Serres _Comparaison, &c._ 3--.

[849] _Ibid._ 4.

[850] _Ibid._ 5.

[851] PLATE XXII. FIG. 11. _h´_.

[852] VOL. III. p. 579.

[853] PLATE XXII. FIG. 6. VOL. III. p. 585--.

[854] Cuv. _Anat. Comp._ i. 436. PLATE XXI. FIG. 6.

[855] Ibid. _a, b._ Lyonet _Anat._ 37.

[856] Cuv. _ubi supr._ 458--. VOL. III. p. 368, 378, 382.

[857] Cuv. _ubi supr._ 459.

[858] Chabr. _Sur le Vol des Ins._ c. i. 441.

[859] Chabr. _Sur le Vol des Ins._ c. i. 415.

[860] _Ibid._

[861] _Ibid._ c. iii. 344. _t._ viii. _f._ 8, 9.

[862] _Ibid._ c. i. 440.

[863] _Ibid._ 444.

[864] _Ibid._ 445. c. iii. 359.

[865] _Ibid._ c. ii. 332. c. iii. 359.

[866] _Ibid._ c. i. 445.

[867] _Ibid._ c. iv. 78.

[868] Chabr. _Sur le Vol des Ins._ c. i. 415, 442. c. iv. 80.

[869] _Ibid._ c. i. 442.

[870] _Ibid._ 439--.

[871] Chabrier _Analyse_, 28. The latter part of this passage is copied from a MS. note of the author's in my copy.--W. K.

[872] Chabrier _Analyse_, Ibid. _Sur le Vol des Ins._ c. i. 445. VOL. III. p. 617.

[873] _Analyse_ ubi supr.

[874] _Sur le Vol des Ins._ c. i. 448, c. ii. 336.

[875] VOL. III. p. 579.--

[876] Chabr. _Ibid._ c. i. 443. ii. 316, 332.

[877] Chabr. _Sur le Vol des Ins._ c. ii. 333.

[878] _Ibid._ 332. PLATE XXII. FIG. 11, 12. c. A cupuliform process is also observable at the side of the metaphragm. Ibid. FIG. 10. a.

[879] Chabr. _Ibid._ c. iv. _t._ xi.-4. _f._ 14.

[880] _Ibid._ c. i. 445. xi.-8. _f._ 8, 9.

[881] Chabr. _Sur le Vol des Ins._ c. ii. 336. note 1. VOL. III. p. 292--.

[882] Chabr. _Ibid._ c. i. 447.

[883] See above, p. 66--.

[884] See above, p. 73--.

[885] Chabrier _Sur le Vol des Ins._ c. i. _Addend._ 298.

[886] See above, p. 178--.

[887] VOL. III. p. 700--.

[888] Chabr. _ubi supr._ c. i. 422.

[889] Cuv. _Anat. Comp._ i. 451.

[890] Chabr. _Analyse_ 25. _Sur le Vol des Ins._ c. i. 423, 452. _Addend._ 301.

[891] See above, p. 83.

[892] Lyonet _Anat._ _t._ xiii. _f._ 1, 2.

[893] Lyonet _Anat._ _t._ xiii. 188--, 584.

[894] _Ibid._ 189.

[895] _N. Dict. d'Hist. Nat._ xxx. 421.

[896] _Arachnid._ 9. _t._ i. _f._ 7. _r._

[897] _Ibid._ _o._

[898] _Ibid._ 10.

[899] _Arachnid._ 45. _t._ iii. _f._ 31. _m, n, q, r, t._

[900] VOL. II. p. 309--.

[901] Mouffet _Theatr._ 275.

[902] _N. Dict. d'Hist. Nat._ xxviii. 249.

[903] _Phil. Acc. of Works of Nat._ 144.

[904] Clark in _Linn. Trans._ iii. 309.

[905] _Fn. Suec._ 1799.

[906] _Anatomy of Expression in Painting_, 170.

[907] Bonnet _Œuvr._ ii. 124.

[908] _N. Dict. d'Hist. Nat._ xxii. 81.

[909] 1 Cor. xv. 50--.

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

[911] Swamm. _Bibl. Nat._ _t._ xviii. _f._ 2. _l, m, n, o._ Reaum. v. _t._ xxix. _f._ 7. _m, n, o, p, q._

[912] VOL. II. LETTER XXVI.

[913] _De Bombyc._ 5.

[914] Reaum. ii. 185--.

[915] VOL. II. p. 186.

[916] _t._ vi. _f._ 3.

[917] These directions for dissecting are chiefly taken from Swammerdam, _Life_ xiv.-- and Lyonet _Anat._ 7--.

LETTER XLIV.

_DISEASES OF INSECTS._

Having laid before you what observations I thought might sufficiently explain all the principal features of the Anatomy of insects both external and internal, you will next expect to be informed whether, like the higher animals, they are subject to have the admirable order observable in their frame interrupted by _Disease_; and you will perhaps imagine, from the multiplicity of their organs and vessels, that they must be peculiarly exposed to derangements of the vital and other functions. That they have their diseases is certain; but, except in the case of their appropriate parasitic assailants, which is a part of their economy, it does not appear that their maladies are more numerous and frequent than those of other animals. The same ALMIGHTY POWER which endowed them with so complex a structure, generally upholds them in health during their destined career, until they have fulfilled the purpose of their creation, when _they die and return again to their dust_[918].

But perhaps I may seem to you as making too great a parade about these little insignificant creatures if I assign a separate letter to the consideration of their _diseases_: but when you recollect that Aristotle has a chapter on this subject[919], and that the learned Willdenow has devoted a distinct portion of his excellent introductory work on Botany to the diseases of Plants[920],--you will perhaps be of a different mind: indeed, some facts I shall have to communicate are so remarkable and interesting, that I am sure, when you have read this letter, you will not think the subject one that deserves to be slighted.

Insect diseases may, I think, be divided into two great classes; those resulting, namely, from some accidental _external_ injury or _internal_ derangement, and those produced by _parasitic_ assailants.

I. Under the _first_ head we may begin with _wounds_, _fractures_, _mutilations_, and other _extraneous_ causes of disease. To these--insects are peculiarly subject; and though they are not, like the _Crustacea_ and _Arachnida_[921] and some other invertebrate animals, endowed with the power of _reproducing_ a mutilated limb, yet their wounds appear to heal very rapidly, and at the time they are inflicted to produce little pain[922]. But if those important members, their _antennæ_, are mutilated, insects seem to suffer a kind of derangement; the great organ of their communication with each other, and in various respects with the external world, being removed, all their instincts at once fail them. I formerly related how the amputation of these affects the _queen-bee_[923]. A similar result, as Huber tells us[924], follows, when the same experiment is repeated on the _workers_ or _drones_: they immediately become unable to take any further part in the labours of the hive; they can no longer guide themselves except in the light; if they petition one of their fellow-citizens for honey, they are unable to direct their tongue to its mouth to receive it; they remain near the entrance of the hive, and when the light is intercepted they rush out of it to return no more.

Insects occasionally are subject to _tumours_ or a preternatural enlargement of their parts and organs. The antennæ of bees sometimes swell at their extremity so as to resemble the bud of a flower ready to open, becoming at the same time very yellow, as does the fore part of the head[925]. I once saw a specimen of a _Hydrobius_--agreeing with _H. fuscipes_ in every other respect even to the most minute punctum--which had a large tumour on each side of the _prothorax_, evidently accidental, occasioned probably by the stoppage of the pores by which the superfluous moisture and air escape when it undergoes its last change. The converse of this I have observed to take place sometimes in the same part of _Geotrupes foveatus_, the ordinary lateral _foveæ_ becoming very considerably enlarged;--this was the case with the specimen from which Mr. Marsham made his description of that insect. The species is, however, very distinct in other respects, and may always be known by its small size. It happens now and then also, that these tumours represent _blisters_. I saw one once on one elytrum of a beetle and not on the other. Those of _Serropalpus_ (as Mr. MacLeay, on the authority of M. Clairville, informs me) are particularly subject to this disease. But, of all the organs, the wings are most exposed to derangements of this kind. De Geer, in a specimen of _Pieris Cratægi_ just excluded from the chrysalis, observed that one of these was distended by a considerable quantity of extravasated green fluid--two or three large drops following an incision. This disease appeared to arise from the lower membrane not adhering to the upper; so that the nervures--which are rather longitudinal channels, being open below, than tubes--were not closed to confine the fluid to its proper course. The malady, which might be called a dropsy of the wing, carried off the insect the day after its exclusion[926]. Reaumur observed that the wings of some flies were affected by an _air_-dropsy, as he calls it, which appeared to arise from the air escaping from its natural channels, and thus separating, the two membranes that form the wing, and filling the cavity produced by their separation[927].

Sometimes also _monstrosities_ are to be met with in these animals, or variations from a symmetrical structure in organs that are pairs. I have a beetle in which the terminal joint of one of the maxillary palpi is short, ovate, and acute; and that of the other, long, semiovate, and rather obtuse. A specimen of _Blaps mortisaga_ in my cabinet, taken by Mr. Denny, besides the terminal mucro of the _elytra_, has a long diverging lateral one. Goeze had the larva of a _Semblis_ brought to him in which one of the two fore-legs, though perfect in all its parts, was only half the length of the other[928]; which he regarded as a reproduction, but it seems rather a malformation. Müller mentions a most extraordinary fact of one of the _Noctuidæ_, which when disclosed from the pupa retained the head of the larva[929]. One of the most remarkable instances of this kind that have fallen under my own observation, may be seen in a specimen of _Chrysomela hæmoptera_ in the cabinet of our friend Curtis; in which one of the thighs produces a double tibia, but only one of these is furnished with a tarsus.

The diseases of insects which arise from some _internal_ cause are not very numerous. The first that I shall mention is a kind of _vertigo_. "Ants have also their maladies," says M. P. Huber: "I have noticed one extremely singular; the individuals attacked by it lose their power of guiding themselves in a straight line, they can walk only by turning round in a circle of small diameter and always in the same direction. A virgin female shut up in one of my glasses was seized on a sudden with this distemper; she described a circle of an inch in diameter, and made about a thousand turns in an hour, or not quite seventeen in a minute. She continued constantly turning round for seven days, and when I visited her in the night I found her still in motion. I gave her honey--and I think that she ate some of it." He observed that some workers were attacked by a similar disease: one of these, however, had the power of walking from time to time in a straight line; when placed upon its head it continued its gyrations[930]. Similar motions of a little moth, mentioned on a former occasion[931], may perhaps have been produced by the same cause. Bees are also subject to vertigo, which has been attributed to their eating poisonous honey[932]--but may not this disease in all these cases arise from some derangement of the nervous system? One of the ants which was so affected had lost one of its antennæ; but as this was not the case with the others, no great stress is to be laid upon the circumstance. Huber does not inform us whether those attacked by this disease recovered or not.

I have observed more than once, that the _flesh-fly_ and some others of the same tribe are subject in particular seasons to a kind of _convulsions_. When thus attacked, they kick and struggle, and seem unable to fly. Sometimes they lie upon their backs without motion, but if a finger be placed near them their convulsive motions are renewed. When thrown into the air, instead of flying, they fall to the ground. Had this distemper occurred earlier or later in the year I should have attributed it to the benumbing effects of cold; but as my observations were made one year (1816) in _May_, and in another (1811) in the latter end of _June_, this could scarcely be the case. In the year last mentioned I observed that many flies died under its influence. In wet seasons this tribe is subject to another disease, which proves fatal to many of them, and indeed to other _Diptera_. A white crust appears to be formed upon the abdomen both above and below, of a granular appearance, much resembling fine moist sugar. On the back of that part this crust does not cover the margins of the segments, which gives it the appearance of white bands; so that deceived by it, I have often at first flattered myself that I had met with some new species. The under-side of the abdomen is wholly covered by it, divided in the middle into two longitudinal masses, the anal segment being bare. De Geer has noticed this or a similar disease, which, when flies are attacked by it, causes the abdomen to swell so as even to burst, and the segments become dislocated. Upon opening the abdomen it is found filled with a white unctuous substance, which often accumulates (as above described) on its external surface[933]. Dr. Host says that in this disease when the animal is dead, the wings, which were before incumbent, become extended, and its almost invisible pubescence grows into long hairs[934]. De Geer seems to think that these flies are thus affected in consequence of having eaten some poisonous food[935]; but I rather suspect, as I have observed it become prevalent chiefly in wet seasons, that it arises from a superabundance of the nutritive fluid, or of the fat, so that it seems to be a kind of _plethora_. I once observed a fly fixed to a pane of glass, round which was a semicircle of what appeared to be merely vapour, whose radius was nearly three-fourths of an inch. Taking it for an aqueous fluid that had transpired from the dead animal, I paid no further attention to it at that time: but observing from day to day that the moisture did not evaporate, after two or three months had elapsed, I had the curiosity to examine it more closely, and, upon scraping some of it off with a penknife, I found it was a white substance of a fatty nature. In this case, then, the fat must have exploded on all sides with considerable violence from half the body or the abdomen. Probably this was a more intense degree of plethora. When I examined this appearance the fly had fallen off, and I could not find it.

Mr. Sheppard once brought me a panicle of grass, the glumes of which were rough with hairs, or small bristles, to which several specimens of a fly related to _Xylota pipiens_ adhered by their proboscis. At first I thought that having been entrapped by the bristles, and unable to extricate themselves, they had perished from want of food; but since when touched they readily dropped from the glumes, some other cause, perhaps disease, probably occasioned this singular suspension of themselves.

The maladies to which _bees_ and _silkworms_ are subject are more interesting to us than those of _flies_, on account of their utility as _cultivated_ insects. One of the worst distempers which attacks the first of these animals is a kind of _looseness_ or _dysentery_: this happens early in the year, when they are fed with too much honey without any portion of bee-bread[936], and sometimes destroys whole hives. Their excrements, instead of a yellowish red, then become black, and the odour they emit is insupportable; the bees no longer observe their usual neatness, inducing them to leave the hive when they void their excrements, but they defile it, their cells, and each other. Several remedies have been prescribed for this disease. To prevent it, a syrup made by an equal mixture of good wine and honey is recommended; and as a cure, to place in the hive combs containing cells filled with bee-bread[937]. But one of the worst maladies to which these useful animals are subject, is that called by Schirach _Faux Couvain_. It originates with the larvæ; and is caused either by their being fed with unwholesome food, or when the queen, as sometimes happens, lays her eggs so that the head of the grub is not in a proper position for emerging from the cell when the period for its disclosure is arrived:--the consequence is, that in both cases it dies and becomes putrid, which sometimes produces a real pestilence in a hive. The remedy for this evil is to cut away the infected combs, and to make the bees undergo a fast of two days[938]. The hive should be cleaned and fumigated, by burning under it aromatic plants.