was the natural consequence of his denying them any portion of
intellect; but his opinion cannot I think be consistently assented
to, if it be the fact, as I have just shown[138], that they are not
wholly devoid of the intellectual principle. Whatever is merely
mechanical, must, under similar circumstances, always act precisely
in the same way. An automaton once constructed, whilst its machinery
remains in order, will invariably perform the same actions; and Des
Cartes, when he had constructed his celebrated female automaton,
imagined that he had irrefragably proved his principle, that brutes
are mere machines. But if, instead of losing himself in the wilds
of metaphysical speculation, he had soberly attended to facts, he
would have seen that the instinct of animals can be modified and
counteracted by their intellect, and consequently cannot be regarded
as simply mechanical. Though the instinctive impulse of an empty
stomach powerfully impel a dog to gratify his appetite, yet, if
he be well tutored, the fear of correction will make him abstain
from the most tempting dainties: and in like manner a bee will quit
the nectary of a flower, however amply replenished with sweets, if
alarmed by any interruption. The ants on which Buonaparte amused
himself with experiments at St. Helena, though they stormed his
sugar-basin when defended by a fosse of water, controlled their
instinct and desisted when it was surrounded with vinegar[139]: and
in the remarkable instance communicated to Dr. Leach by Sir Joseph
Banks, the instinct of a crippled spider so completely changed,
that from a sedentary web-weaver it became a hunter[140]. There is
evidently, therefore, no analogy between actions strictly mechanical
and instincts, which, though they may often seem to be excited by
mechanical causes, are liable to be restrained or modified by the
connexion of the instinctive and intellectual faculties[141]; and
while we are ignorant how this connexion takes place, it is obviously
impossible to reason logically on the subject.
In thus denying that any existing _mechanical_ theory of instinct
is satisfactory, I by no means intend to assert that instinct is
purely _intellectual_. I have already given you my opinion[142],
that it is not the effect of any immediate agency of the Deity; nor
am I prepared to assent to the doctrine of a writer, who has in some
respects written ably on the subject in question, who says, that "the
Divine Energy does in reality act not _immediately_, but _mediately_,
or through the medium of _moral_ and _intellectual influences_ upon
the nature or consciousness of the creature, in the production of
the various, and in many instances truly wonderful, actions which
they perform[143]." The same objection applies to this as to so many
other metaphysical theories, that it is not adequately supported by
_facts_; and all theories not so supported are injurious to science
in proportion as their plausibility is greater, by leading the
student to relax in that observation of nature and attentive study
of the instincts of animals, on which alone sound hypothesis on this
subject can be ultimately founded.
I shall conclude these remarks on the nature of instinct with a few
observations as to the circumstances in which insects may be supposed
to be guided by this faculty, and those in which _intellect_ seems
to direct them. The bee, when it takes its flight to a field where
flowers abound, is governed by intellect in the use of its senses;
for these are given to it as _guides_: and when it arrives there,
they direct it to the flowers, and enable it to ascertain which
contains the treasures it is in search of; but having made this
discovery, its instinct teaches it to imbibe the nectar and load its
hind legs with pollen.--Again: its senses, aided by memory, enable
it to retrace its way to the hive, where instinct once more impels
it in its various operations. So that when we ascribe a certain
degree of intellect to these animals, we do not place them upon a
par with man; since all the most wonderful parts of their economy,
and those manipulations that exceed all our powers, we admit not
to be the contrivance of the animals themselves, but the necessary
results of faculties implanted in their constitution at the first
creation by their MAKER. I may further repeat, that the mere fact of
being endowed with the external organs of sense, proves a certain
degree of intellect in insects. For if in all their actions they were
directed merely by their instinct, they might do as well without
sight, hearing, smell, touch, &c. but having these senses and their
organs, it seems to me a necessary consequence, that they must have a
sufficient degree of intellect, memory, and judgement, to enable them
advantageously to employ them.
There is this difference between intellect in man, and the rest of
the animal creation. Their intellect teaches them to follow the lead
of their senses, and make such use of the external world as their
appetites or instincts incline them to,--and _this is their wisdom_;
while the intellect of man, being associated with an immortal
principle, and being in connexion with a world above that which his
senses reveal to him, can, by aid derived from heaven, control those
senses, and bring under his instinctive appetites, so as to render
them obedient to the το ἡγεμονικον, or governing power of his nature:
AND THIS IS HIS WISDOM.
I am, &c.
FOOTNOTES:
[1] Το Ἡγεμονικον.
[2] See Hooper's _Medical Dictionary_, under _Nervous Fluid_, and Mr.
Sandwith's useful _Introduction to Anatomy and Physiology_, 83.
[3] _N. Dict. d'Hist. Nat._ xvi. 305--.
[4] Cuv. _Anat. Comp._ ii. 362. Compare MacLeay _Hor. Entomolog._ 215--.
[5] _N. Dict. d'Hist. Nat._ _ubi. supr._
[6] Cuv. _Anat. Comp._ ii. 360. MacLeay _Hor. Ent._ 201.
[7] _N. Dict. d'Hist. Nat._ xvi. 306. MacLeay _Hor. Ent._ 200--.
[8] _Ibid._ 307. The great sympathetic nerves in _fishes_ are said to
have no ganglions. Cuv. _ubi. supr._ 297.
[9] They are called _trisplanchnic_ because they render to the
_three_ cavities of the _viscera_:--viz. the thoracic, the abdominal
and the pelvic. _N. Dict. d'Hist. Nat._ xxii. 524. 527.
[10] In _Hemiplegia_, &c.
[11] _N. Dict. d'Hist. Nat._ xvi. 307.
[12] Thus in the _Molluscæ_ there must be a great difference in this
respect, since in some of these the brain or cerebral ganglion has
been cut off with the head, and another reproduced. _Ibid._ xvi. 306.
Comp. v. 391.
[13] VOL. III. p. 29.
[14] Comp. PLATE XXX. FIG. 1. and 6. and Carus. _Introd. to Comp.
Anat._ i. 64.
[15] Lyonet _Anatom._ 100.
[16] _Ibid._ 101.
[17] Lyonet _Anatom._ 100. In man and the vertebrate animals, the
medullary pulp is every where homogeneous; under the microscope it
appears to consist of a number of minute conglomerated globules.
M. Vauquelin has analysed it, and found it to contain, of water 80
parts; of albumen in a state of demicoagulation 7·0; of phosphorus
1·50; of osmazone 1·12; of a white and transparent oily matter 4·53;
of a similar red do. 0·75; of a little sulphur and some salts 5·15.
_N. Dict. d'Hist. Nat._ xxii. 531--.
[18] _Anat._ 99.
[19] Malpigh. _de Bombyc._ 20. Swamm. _Bibl. Nat._ i. 224. a.
[20] _Anat. Comp._ ii. 348.
[21] Lyonet _Anat._ 100. _t._ iv. _f._ 6. Sandwith _Introd._ 59--.
[22] PLATE XXI. FIG. 1. 7. 8. _a._
[23] _N. Dict. d'Hist. Nat._ xxii. 527.
[24] _Ibid._ v. 591.
[25] Cuv. _Anat. Comp._ ii. 318. Swamm. _Bibl. Nat._ _t._ xxix. _f._
7. Herold _Schmetterl._ _t._ ii. _f._ 1-10. _a._
[26] Cuv. _Ibid._ 322. 337.
[27] Cuv. _Anat. Comp._ 324.
[28] _Arachnid._ _t._ i. _f._ 13. _m.m._
[29] Cuv. _ubi supr._ 343. 346. Treviranus _Arachnid._ _t._ v. _f._
45. _a._ PLATE XXI. FIG. 8. _a._
[30] Ibid. FIG. 1. _b.b._
[31] Cuv. _ubi supr._ 337.
[32] PLATE XXI. FIG. 8. Swamm. _Bibl. Nat._ i. 36. b.
[33] _Arachnid._ _t._ v. _f._ 45.
[34] Swamm. _ubi supr._ _t._ xliii. _f._ 7.
[35] _Ibid._ 112. a.
[36] Cuv. _Anat. Comp._ ii. 337. 343--.
[37] _Ibid._ 336.
[38] Herold _Schmetterl._ _t._ ii. _f._ 1.
[39] Lyonet _Anat._ 98.
[40] Cuv. _ubi supr._ 342. Gaede N. _Act. Acad. Cæs._ XL. ii. 323.
Cuv. _Ibid._ 351.
[41] Cuv. _ubi. supr._ 348.
[42] Treviranus _Arachnid._ _t._ v. _f._ 45.
[43] PLATE XXI. FIG. 7. 8. Swamm. _Bibl. Nat._ _t._ xliii. _f._ 7.
[44] PLATE XXI. FIG. 7. 8. _c._
[45] Lyonet _Anat._ 100.
[46] _N. Dict. d'Hist. Nat._ xxii. 522--.
[47] Lyonet _ubi supr._ _t._ ix. _f._ 1-4.
[48] Cuv. _Anat. Comp._ ii. 339. 343.
[49] PLATE XXI. FIG. 7.
[50] Swamm. _ubi supr._ _t._ xl. _f._ 5. Cuvier (ii. 332.) accuses
Swammerdam of representing the spinal marrow in this grub as
producing nerves only on _one_ side; whereas he expressly states
(ii. 50. b.) that a considerable number spring on each side from the
eleven ganglions, but that to avoid confusion he had omitted some.
[51] Cuv. _ubi supr._ 325.
[52] Swamm. _Bibl. Nat._ _t._ xv. _f._ 6.
[53] Treviran. _Arachnid._ _t._ l. _f._ 13. 1-4.
[54] Swamm. _ubi supr._ _t._ xxii. _f._ 7.
[55] Treviran. _ubi supr._ _t._ v. _f._ 45.
[56] PLATE XXI. FIG. 7.
[57] Cuv. _Anat. Comp._ ii. 346.
[58] PLATE XXI. FIG. 8.
[59] Cuv. _ubi supr._ 337.
[60] _Ibid._ 335--.
[61] Cuv. _ubi supr._ 348.
[62] _Ibid._ 320--.
[63] _Ibid._ 340--.
[64] _Ibid._ 338--.
[65] Gaede _ubi supr._
[66] Cuv. _ubi supr._ 323--. 327--. Mr. Bauer (_Phil. Trans._ 1824.
_t._ ii. _f._ 1.) has figured only _seven_, excluding the brain, in
that of the silk-worm, and Malpighi (_De Bombyc._ _t._ vi. _f._ 2.)
_ten_,--Swammerdam (_Bibl._ Nat. _t._ xxviii. _f._ 3.) however has
_twelve_.
[67] _Ibid._ 326.
[68] _Ibid._ 352.
[69] _Ibid._ 343--.
[70] _Ibid._ 345.
[71] _Ibid._ 325--.
[72] _Ibid._ 351.
[73] Cuv. _ubi supr._ 339.
[74] _Ibid._ 335--.
[75] Lyonet _Anat._ 190.
[76] Cuv. _ubi supr._ ii. 340. Malpigh. _de Bombyc._ _t._ vi. _f._ 2.
[77] Cuv. _Ibid._ 348.
[78] Swamm. _Bibl. Nat._ _t._ xlviii. _f._ 7.
[79] Cuv. _Ibid._ 319.
[80] _N. Dict. d'Hist. Nat._ xxx. 420.
[81] Treviran, _Arachnid._ _t._ v. _f._ 45. _m._
[82] PLATE XXI. FIG. 1. 7. 8. _d._
[83] Lyonet _ubi supr._ _t._ x. _f._ 5. 6.
[84] _Ibid._ 192.
[85] Cuv. _ubi supr._ 323. 335.
[86] _Ibid._ ii. 339.
[87] _Ibid._ 342.
[88] Swamm. _Bibl. Nat._ _t._ xxii. _f._ 6. _m.m._
[89] Cuv. _ubi supr._ 350.
[90] _Ibid._ 335.
[91] VOL. III. p. 495--. Lyonet. _Anat._ 581.
[92] Cuv. _ubi supr._ 337.
[93] Cuv. _ubi supr._ 351.
[94] _Ibid._ 352.
[95] Cuvier (_Ibid._ 319.) seems not to have been aware that
Swammerdam was the first discoverer of these nerves, since he
attributes their name to Lyonet.
[96] _Bibl. Nat._ i. 138. b. _t._ xxviii. _f._ 2. _a_, _b_, _c_. _f._
3. _g_.
[97] _Ubi supr._ 578.
[98] _Ubi supr._ 320. 339, &c.
[99] Cuv. _ubi supr._ 349.
[100] Lyonet _Anat._ _t._ ix. x.
[101] PLATE XXI. FIG. 8. Swamm. _Bibl. Nat._ _t._ xxii. _f._ 6.
[102] _Ibid._ _t._ xv. _f._ 6.
[103] PLATE XXI. FIG. 7.
[104] Swamm. _ubi supr._ _t._ xliii. _f._ 7. _h_, _h_.
[105] PLATE XXI. FIG. 8.
[106] In Mr. Bauer's figure (_Philos. Trans._ 1824. _t._ ii. _f._ 1.)
no less than _eighteen_ pairs of nerves are represented as issuing
from the internodes; but it should seem as if in the specimen from
which his figure was taken, several of the ganglions, perhaps from
some injury received in the dissection, had become obliterated, while
their nerves remained: yet still, even making allowance for these,
many pairs will appear to take their origin from the spinal chord.
[107] Comp. Cuv. _Anat. Comp._ ii. 102-123.; with Swamm. Expl. of
PLATES XXXII. _t._ xxviii. _f._ 3. _k._
[108] Malpighi seems, however, to agree with him. _De Bombyc._ _t._
vi. _f._ 1.
[109] Lyonet _ubi supr._ 201. _t._ ix. _f._ 1, 2. n. 1, 2. &c.
[110] Swamm. _ubi supr._ 1. 139. a. _t._ xxviii. _f._ 3. _s_, _s_.
[111] In Lesser _Insecto-theol._ ii. 84. note *.
[112] _Linn. Trans._ ii. 8. Aristotle had observed this vitality of
insects, and that that of the myriapods is greatest. _Hist. Animal._
_l._ iv. _c._ 7. _De Respiratione_, _c._ 3. _Reptiles_ have also this
faculty. _N. Dict. d'Hist. Nat._ xxix. 161.
[113] Cuv. _Anat. Comp._ ii. 283--. These are named "the upper and
lower cervical ganglions."
[114] Lyonet _Anat._ _t._ ix. x. PLATE XXI. FIG. 1. _a._ _b._
[115] VOL. III. p. 663. 670.
[116] _N. Dict. d'Hist. Nat._ ii. 47--. v. 592. xvi. 308--.
[117] VOL. II. p. 519--. 507--.
[118] Huber _Fourmis_, 260--. Reaum. vi. 172--.
[119] VOL. II. p. 204.
[120] _N. Dict. d'Hist. Nat._ ii. 279--.
[121] Cuv. _Anat. Comp._ ii. 319. 337.
[122] Ibid. ii. 322, 323--; 338. 339--.
[123] PLATE XXX. FIG. 1.
[124] Ibid. FIG. 2.
[125] PLATE XXX. FIG. 3.
[126] Herold _Schmett._ _t._ ii. _f._ 6.
[127] Ibid. _t._ ii. _f._ 7.
[128] PLATE XXX. FIG. 4.
[129] Ibid. FIG. 5.
[130] Ibid. FIG. 6.
[131] _Anat. Comp._ ii. 348.
[132] _N. Dict. d'Hist. Nat._ xvi. 313. Comp. i. 420.
[133] See above, p. 23.
[134] VOL. I. p. 217--.
[135] VOL. II. p. 461.
[136] VOL. II. p. 493.
[137] Ibid. p. 503.
[138] See above, p. 21.
[139] Antommarchi's _Last Days of Napoleon_.
[140] _Linn. Trans._ xi. 393.
[141] VOL. II. p. 509.
[142] VOL. II. p. 463, 5.
[143] _Zoological Journal_, n^o. i. 5.
LETTER XXXVIII.
_INTERNAL ANATOMY AND PHYSIOLOGY
OF INSECTS CONTINUED._
RESPIRATION.
"Life and flame have this in common," says Cuvier, "that neither
the one nor the other can subsist without _air_; all living beings,
from man to the most minute vegetable, perish when they are utterly
deprived of that fluid[144]." The ancients, however, not perceiving
insects to be furnished with any thing resembling _lungs_, took it for
granted that they did not _breathe_; though Pliny seems to hesitate on
the subject[145]. But the microscopic and anatomical observations of
Malpighi, Swammerdam and Lyonet, and the experiments of more modern
physiologists, have incontestably proved that insects are provided with
respiratory organs, and that the respiration of air is as necessary
to them as to other animals. They can exist indeed for a time in
irrespirable air; and immersion in hydrogen or carbonic acid gases is
not, as I have often ascertained, so instantly fatal to them as it
would be to vertebrate animals; but like them, they speedily perish in
air altogether deprived of its oxygen, or placed in situations to which
all access to this essential element is excluded. Their respiration
too of atmospheric air produces the same change in it with that of the
vertebrate animals, the oxygen disappearing, and carbonic acid gas
being produced in its place. Boyle had long since ascertained, that
when bees, flies, and other insects were placed under an exhausted
receiver, they often perished[146]: and the same effect was even
observed by the ancients to ensue, when their bodies were by any means
covered with oil or grease, which necessarily closed the orifices of
their respiratory organs[147].
But for the first series of experiments ascertaining the necessity of
a supply of air to insects, and their conversion of it into carbonic
acid, we are indebted to the illustrious Scheele[148]; and his
experiments have been repeated and confirmed by Spallanzani, Vauquelin,
and other chemists. The former found, that when caterpillars and
maggots were confined in vessels containing only about eleven cubic
inches of atmospheric air, though furnished with sufficient food,
they soon died, and sooner when the space was more confined[149]. He
ascertained too, that a larva weighing only a few grains consumed, in
a given time, as much oxygen as an amphibious animal a thousand times
as voluminous[150]. A male grasshopper (_Acrida viridissima_) in six
cubic inches of oxygen lived but eighteen hours, and the female placed
in eight cubic inches of atmospheric air, only thirty-six hours. The
usual tests in both instances detected the conversion of the oxygen
present into carbonic acid[151]. Precisely the same result was obtained
by Sorg and Ellis, who, having placed a number of flies in nine cubic
inches of atmospheric air, found them all dead by the third day, the
oxygen intirely vanished, and a quantity of carbonic acid nearly equal
in bulk produced[152].
It is ascertained too, that insects like other animals require in
the process of respiration not merely oxygen, but such a mixture of
it with nitrogen or azote as composes atmospheric air: for Vauquelin
found that a grasshopper placed in six cubic inches of oxygen lived
only half as long (eighteen hours) as another placed in eight
inches of atmospheric air; its breathing was much more laborious,
and it died when not more than one-twentieth of the oxygen had
been converted into carbonic acid[153]. That a large quantity of
_oxygen_ penetrates all parts of insects, is evident also from the
_acid_ prevalent in the fluids of most of them, as likewise from the
wonderful power of their muscles. That _azote_ is also received,
seems probable from the _ammonia_ which has been extracted from the
fluids of many, and from the rapid putrescence of these animals[154].
The mode, however, in which the respiration of insects is carried on,
differs greatly from that which obtains in the higher animals. They
have no lungs, no organs confined to a particular part of the body,
by means of which the whole of the blood is regularly exposed to the
action of the inspired air. They do not breathe through the _mouth_,
but through numerous orifices called _spiracles_, and the respiratory
vessels connected with these are conducted to every part of the
body. In some indeed, that we have included under the denomination
of insects, as the _Arachnida_, an approach is made to the branchial
respiration of fishes.
The respiratory apparatus of insects may be considered under
_two_ principal heads:--viz. the orifices or spiracles, and other
_external_ organs by which the air is alternately received and
expelled; and the _internal_ ones, by which it is distributed. Each
of these is well worthy of your attention.
I. The _external_ respiratory organs of insects may be divided into
_three_ kinds. _Spiracles_; _Respiratory plates_; and _branchiform_
and _other pneumatic appendages_.
i. _Spiracles_[155] (_Spiracula_), or breathing pores, are small
orifices in the trunk or abdomen of insects, opening into the
_tracheæ_, by which the air enters the body, or is expelled from
it[156]. They may be considered principally as to their _composition_
and _substance_; _shape_; _colour_; _magnitude_; _situation_; and
_number_.
1. _Composition_ and _substance_. Perhaps you may not be aware that
the structure of these minute apertures is not so simple as at the
first view it may seem; but when you recollect that by them the insect
_breathes_, you will suspect that provision may be made for their
opening and shutting. A spiracle therefore, speaking analogically,
may be regarded in numerous cases as a _mouth_ closed by _lips_. In
caterpillars and many other insects, the substance of the crust where
it surrounds the spiracle, is elevated so as to form a ring round it.
The lips, properly speaking, are formed of a single cartilaginous
piece or platform, with a central longitudinal cleft or opening, when
closed often extending the whole length of the piece[157]; but in some
appearing always open and circular: of the former description are those
covered by the elytra in the common cockchafer; and of the latter,
those that are not so covered: in some, as in the antepectoral pair of
the mole-cricket, there appear to be no lips, the orifice being merely
closed with hairs[158]. Though the aperture is usually in the middle
of the platform, in the female of _Dytiscus marginalis_, it is nearer
the posterior side, the anterior or upper lip being the longest. In the
majority, the mouth or cleft is nearly as long as the spiracle; yet in
the puss-moth (_Cerura Vinula_) it is shorter[159]. Some spiracles,
however, are unilabiate, or have only _one_ lip. This is the case with
_Gonyleptes_ and perhaps others[160]. The lips are usually horizontal,
but sometimes they dip so as to make the spiracle appear open.
With regard to the _substance_ of these organs, it is more or less
cartilaginous, and probably elastic; the surface frequently appears
to be corrugate or plaited; this is very distinctly seen in the
stag-beetle and the cockchafer: in the last insect, under a powerful
magnifier, we are told that the lips appear to consist of parallel
cartilaginous processes, separated by a cellular web[161]. In some
species of _Copris_ the corrugations form a perplexed labyrinth;
in the caterpillar of the puss-moth the plaits are so narrow as
to look like rays[162]; and in some _Dynastidæ_ the lips approach
to a lamellated structure. Again, in _Hydrophilus caraboides_ the
_upper_ lip, and in _Dytiscus circumflexus_, _both_ lips seem formed
of elegant plumes[163]: a similar ornament distinguishes the inner
edge of the lips in the caterpillar of the great goat-moth (_Cossus
ligniperda_) and others[164]. In the grub of the rhinoceros-beetle
(_Oryctes nasicornis_) the margin of the lower or inner lip is
decorated by pinnated rays, which enter the cellular membrane that
covers the upper lip[165]: in this larva, and that likewise of the
cockchafer, the two lips are formed of different substances; in
the last the upper or outer one consists of a perforated cellular
membrane, through which the air can pass, while the lower or inner
one is a cartilaginous valve that closes the orifice[166]: in
the former this valve is surmounted by a boss[167]. In the pupa
of _Smerinthus Populi_, a hawk-moth not uncommon, and of some
dragon-flies (_Libellula depressa_), the margin of the two lips is
crenated, probably with notches which alternate, that the mouth
of the spiracle may shut more accurately[168]. The substance is
unusually thick in the spinose caterpillars of butterflies; and in
the pupa of one, _Uria Proteus_, it is villose.
Under the present head I may observe, that in some cases, as in the
puss-moth, and the larva of the common water-beetle (_Dytiscus
marginalis_), the spiracles are closed by a semifluid substance,
which however, according to Sprengel, is permeable to the air[169].
The animal, where these organs are furnished with lips, has
doubtless, by means of a muscular apparatus, the power of _opening_
and _shutting_ them: this is done, we are told, by elevating and
depressing, or rather by contracting and relaxing them. Sorg counted
in one case (_Oryctes nasicornis_) _twenty_, and in another (_Acrida
viridissima_) _fifty_, of these motions to take place in little
more than _two_ minutes[170]: but the quickness and force of this
motion is not always uniform; for the same physiologist observed,
that in _Carabus auratus_, when feeding or moving its body rapidly,
the contraction of the spiracles took place at very short intervals;
but when it was fasting, and its motions were slow, the intervals
were longer[171]: it is probable also, that the temperature may
accelerate or retard the motion. In the summer I examined a specimen
of _Phyllopertha horticola_, that had indeed been somewhat injured,
with this view: the pulses of the abdomen, which alternately rose
and fell, were at about the rate of the pulse of a man in health,
sixty in a minute, and the spiracles appeared to me to keep pace with
this motion: later in the year, when the temperature was lower, as
I was walking, I took a specimen of some grasshopper (_Locusta_).
Upon viewing it under a lens, I observed one of the convex pectoral
spiracles open and shut, and the interval between two breathings
appeared nearly half a minute.
2. With regard to their _shape_, spiracles vary considerably. In
general we may observe that the abdominal ones are usually flat,
while those of the trunk are often convex[172]. Sometimes they are
very narrow and nearly linear, as in many pupæ of _Lepidoptera_,
and those in the _metathorax_ of the sand-wasps (_Ammophila_) and
affinities; at others they are wider and nearly elliptical, as in
_Lucanus_ and many Lamellicorn beetles: again, in _Copris_ they are
circular; in _Cordylia Palmarum_ ovate; in _Dytiscus_ oblong[173]; in
_Goerius olens_ lunulate; in _Gonyleptes_ nearly of the shape of a
horse-shoe[174]; and probably many other forms might be traced, if a
thorough investigation with this view were undertaken.
3. The _colour_ of spiracles will not detain us long. In the
caterpillars of _Lepidoptera_ this is often so contrasted with that
of the rest of the body, as to produce a striking and pleasing
effect. Thus when the body is of a _dark_ colour, they are usually of
a _pale_ one[175]; or if the body is _pale_, they are _dark_[176],
or surrounded with a dark ring[177]. This contrast is often rendered
more striking by their position with regard to the partial colours
that often ornament caterpillars: in those whose sides are decorated
by a longitudinal stripe, the spiracles are often planted in it[178];
or just above it[179]; or between two[180]: in some hawkmoths the
intermediate ones are set in white or pale spots, which gives great
life to the appearance of the animal. In general, in perfect insects
the most prevalent colour is buff, or reddish-yellow. In the larva
of the great water-beetle these organs resemble the iris of the eye,
being circular with concentric rings alternately pale and dark[181].