_excrements_ of insects; which, strange as the observation may seem,
but it is no less true than strange, are sometimes pleasing to the
eye, from their symmetry, and to the taste, from their sweetness. In
those that masticate their food they are solid, and in those that
take it by suction, fluid or semi-fluid. In the caterpillars of
_Lepidoptera_ they are of the former description, and every grain
wears some resemblance to an insect's egg: as the passage in many
of these consists of _six_ fleshy parts separated by channels,
so the excrement represents six little prisms separated by six
channels[583]. The _Aphides_ all secrete a fluid excrement as sweet
as honey, of which the ants are so fond[584], which is ejected not
only at the anal passage, but, in many, by two little siphonets also
above it[585]. A semi-fluid excrement is produced by some species of
_Chermes_, as that which inhabits the Box, which often comes from the
animal in long convoluted strings resembling vermicelli. Reaumur says
its taste is agreeable, much more so than that of manna[586]. Under
this head should be included the abundant spume with which the larva
of _Cercopis spumaria_ envelopes itself[587].
I am, &c.
FOOTNOTES:
[418] _Anat. Comp._ iv. 129.
[419] Cuv. _Anat. Comp._ iv. 129.
[420] PLATE XXI. FIG. 3. c, d, e, is the intestinal canal of the
larva of the _Cossus_.
[421] Cuv. _Ibid._ 112.
[422] Ramdohr _Anat. der Ins._ 6.
[423] _Ibid._ 25.
[424] _Ibid._ 6.
[425] Cuv. _ubi supr._ 113.
[426] Comp. Ramdohr _Anat._ 7.
[427] PLATE XXI. FIG. 3. _c._
[428] _Tenebrio_ Ramdohr, _ubi supr._ 9. _t._ iv. _f._ 1.
[429] _Agrion._ _Ibid._ _t._ xv. _f._ 4. _a, b._
[430] _Ibid._
[431] Many other insects that live by suction have something similar,
as the honey-bag of butterflies, PLATE XXX. FIG. 10, 11. _a._ Ramdohr
_t._ xviii. _f._ 2. with _t._ xix. _f._ 1-3. and xxi. 1, 3, &c.
[432] Ramdohr _Anat._ 11--.
[433] PLATE XXI. FIG. 3. _d._
[434] Ramdohr _Ibid._ 28--.
[435] Herold (_Schmetterl._ 24) says that Ramdohr is mistaken here,
and denies the existence of this juice in insects; but as Ramdohr's
researches were so widely extended, he is most likely to be right.
[436] Ramdohr _Ibid._ 29.
[437] _Ibid._ 31.
[438] _Ibid._ 28.
[439] _Anat. Comp._ iv. 135. Comp. Dr. Kidd in _Philos. Trans._ 1825.
223. _t._ xv. _f._ 6, 7.
[440] Ramdohr _Anat._ 15.
[441] _Ibid._ 15.
[442] _Ibid._ 18.
[443] _Ibid._
[444] _Ibid._
[445] Swamm. _Bibl. Nat._ i. 94. b. Cuv. _Anat. Comp._ iv. 134.
[446] _Ubi supr._ 18.
[447] _Ibid._ _t._ i. _f._ 1. _e._ 5. _c._ 9. _g, h._
[448] _Ibid._ _t._ xxv. _f._ 4. _bb._
[449] Ramdohr _Anat._ _t._ viii. _f._ 3. _cc._
[450] _Ibid._ _t._ vii. _f._ 2.
[451] _Ibid._ 20.
[452] _Anat. Comp._ iv. 132.
[453] _Ibid._ and 136.
[454] _Ubi supr._ 30.
[455] _Ibid._ 31. _t._ iv. _f._ 2. _c._ _t._ v. _f._ 1. _d. f._ 4. D.
[456] _Ibid._ 32.
[457] _Ibid._ 34.
[458] Ramdohr _Anat._ 35.
[459] _Ibid._ _t._ xxiv. _f._ 1. _F._
[460] _Ibid._ 36. _t._ vii. _f._ 2. _kk._ _t._ viii. _f._ 3. _g, hh._
[461] _Ibid._ _t._ xii. _f._ 1. _t._ xvii. _f._ 1. _t._ vii. _f._ 5.
[462] _Ibid._ 37.
[463] _Ibid._ 38.
[464] _Ibid._
[465] Ramdohr _Anat._ 40.
[466] _De Bombyc._ 18--.
[467] _Anat. Comp._ iv. 153.
[468] _Ibid._
[469] _Ibid._
[470] Ramdohr 43. _Cicindela campestris_, _t._ iii. _f._ 1. K.
[471] _Phryganea grandis_, _Ibid._ _t._ xvi. _f._ 2.
[472] _Notonecta glauca_, _Ibid._ _t._ xxiii. _f._ 5.
[473] Of _Musca vomitoria_, _Ibid._ _t._ xix. _f._ 5.
[474] _Ibid._ _t._ viii. _f._ 1. H. and G. _f._ 2.
[475] _Ibid._ 50.
[476] _Ibid._
[477] _Ibid._
[478] _Ibid._ 44. _t._ i. _f._ 9.
[479] _Ibid._
[480] _Ibid._ _t._ vi. _f._ 5. H.
[481] Kidd in _Philos. Trans._ 1825. _t._ xv. _f._ 6.
[482] _Ibid._ _t._ xix. _f._ 1. _N, N, O,_ _f._ 2. _P, P, O._
[483] _Ibid._ _t._ 1. _f._ 1. _kkk._
[484] Ramdohr, _t._ xiii. _f._ 1-3.
[485] _Ibid._ 44.
[486] _Ibid._ 45.
[487] _Ibid._ 45. PLATE XXI. FIG. 3. _f. f._
[488] Rhamdohr, _Ibid._ _t._ iii. _f._ 6. E.
[489] _Ibid._ _t._ i. _f._ 1. 5. 9. _t._ xiv. _f._ 1-3.
[490] _Ibid._ 46. _t._ vi. _f._ 3.
[491] Ramdohr, _t._ vii. _f._ 2.
[492] _Ibid._ _t._ ii. iii. &c. _t._ xx. _f._ 1, 2. 6. _t._ xxii.
_f._ 1-5. &c.
[493] _Ibid._ _t._ xviii. _f._ 1. 5. _t._ iv. _f._ 1. See also _t._
vi. _f._ 1. 3.
[494] _Ibid._ _Anat._ _t._ xvii. _f._ 1, 2. 6.
[495] _Ibid._ _t._ xiv. _f._ 3.
[496] _Ibid._ _t._ xiii. _f._ 4.
[497] _Ibid._ _t._ xv. _f._ 3, 4. _t._ 1. _f._ 1. 5. 9. _t._ xii.
_f._ 4, 5, 6, &c.
[498] _Ibid._ _t._ xi. _f._ 4. _t._ xii. _f._ 4-6. _t._ xiii. _f._
2-4, &c.
[499] _Ibid._ _t._ vii. _f._ 1. _t._ viii. _f._ 1, &c.
[500] Ramdohr _Anat._ _t._ ii. iii. xxv.
[501] _Ibid._ _t._ iii. _f._ 6. _t._ iv. _f._ 2. _t._ v. _f._ 1.
[502] _Ibid._ _f._ l. _e. f._ 3.
[503] _Ibid._ 122.
[504] _Ibid._ 123.
[505] _Ibid._ _t._ v. _f._ 4. B.
[506] _Ibid._ 94.
[507] _Ibid._ 96--.
[508] Ramdohr _t._ x. _f._ 1. 8.
[509] _Ibid._ _f._ 8. _b. c._
[510] _Ibid._ 98. _t._ x. _f._ 2-4. From Ramdohr's figure, compared
with the size of the insect, it appears that the gizzard could
scarcely have been of greater diameter.
[511] _Ibid._ _f._ 2.
[512] See W. Curtis in _Linn. Trans._ i. 88.
[513] Ramdohr _t._ x. _f._ 1. _d._
[514] _Ibid._ _l l._
[515] _Ibid._ _t._ ix. _f._ 1, 2. _t._ xi. _f._ 3. _t._ xxiv. _f._ 1, 2.
[516] Ramdohr 103.
[517] _Ibid._ 104. _t._ vi. _f._ 4. D.
[518] _Ibid._ _f._ 2. B.
[519] _Ibid._ _t._ vi. _f._ 3. E.
[520] _Ibid._ 101.
[521] _Ibid._ _t._ i. _f._ 1. 5. 9.
[522] _Ibid._ _f._ 2, 3, 4. 7, 8. 12.
[523] _Ibid._ _f._ 1. _e,_ _f._ 5. _c._ _f._ 9. _g h._
[524] _Ibid._ _f._ 1. 9. _k._
[525] _Ibid._ _t._ xv. _f._ 3, 4. _t._ xvii. _f._ 2. 6.
[526] _Ibid._ _t._ xv. _f._ 3, 4, _f._
[527] _Ibid._ _t._ xvii. _f._ 2. _c._ _f._ 6. _d._
[528] _Ibid._ _f._ 2. _b._ _f._ 6. _c._
[529] Ramdohr _t._ xii. _f._ 6. _H._ _t._ xiii. _f._ 1. _f._
[530] _Ibid._ _t._ xiv. _f._ 2, 3, _C._
[531] _Ibid._ _t._ xii. _f._ 6. _D._ _t._ xiii. _f._ 1. _b._
[532] _Ibid._ 133. _t._ xii. _f._ 1-3.
[533] _Ibid._ _f._ 4.
[534] Comp. Ramdohr _t._ xxii. _f._ 3. _M._ FIG. 4. 3. with _t._ xxi.
_f._ 1. _I._
[535] Ramdohr _t._ xxii. _f._ 1. c. _f._ 3, 4. _B_--.
[536] _Ibid._ _f._ 1. _D E._ _f._ 3. _C D._
[537] _Ibid._ _t._ xxii. _f._ 1. _D, E._ _f._ 3. _C, D._ _f._ 4. _C._
[538] _Ibid._ 198.
[539] _Ibid._ _t._ xxvi. _f._ 2. 4.
[540] _Ibid._ _t._ xxxiii. _f._ 3.
[541] Ramdohr _t._ xviii. _f._ 1. _F, G._
[542] _Ibid._ _L, K._
[543] PLATE XXX. FIG. 7.
[544] Ibid. FIG. 8.
[545] Ibid. FIG. 9.
[546] PLATE XXX. FIG. 10.
[547] Ibid. FIG. 11. _a._
[548] Ibid. _e._
[549] Ibid. _d._
[550] Ramdohr, _Ibid._ _t._ xx. _f._ 1. _E._ _f._ 6. _C._
[551] _Ibid._ _t._ xix. _f._ 2. _C._ _f._ 3. _CCD._ _t._ xx. _f._ 2.
_E._
[552] _Ibid._ _t._ xix. _f._ 2. _D._
[553] _Ibid._ _t._ xx. _f._ 2. _FF._ _f._ 6. _DD._ 184. 180.--
[554] _Ibid._ _t._ xix. _f._ 1. _ON._ _f._ 2. _OP._ _f._ 3. _F._ _t._
xxviii. _f._ 1, 2. _p. q._
[555] Ramdohr, _Ibid._ _t._ xx. _f._ 1. _G._ _f._ 2, 3. _L._
[556] _Ibid._ _t._ xxi. _f._ 1. _D._
[557] _Ibid._ 172.
[558] _Ibid._ _t._ xix. _f._ 2. _K L._ This organ seems analogous to
that with four retractile fleshy horns, observed by Reaumur and De
Geer in other species of _Muscidæ_. Reaum. iv. _t._ xxviii. _f._ 13.
_a, s._ De Geer vi. _t._ iii. _f._ 18. _c, d._
[559] Ramdohr _t._ xxi. _f._ 6.
[560] Ramdohr _t._ xxix. _f._ 1 *. _A._
[561] _Ibid._ and _f._ 3. _B, D._
[562] _Ibid._ _f._ 2, 3. 5. &c.
[563] See above, p. 99--.
[564] Treviranus and Ramdohr are of the former opinion; and Meckel,
Cuvier, Marcel de Serres, and Leon du Four, of the latter.
[565] Treviran _Arachnid._ _t._ 1. _f._ 6. _v._
[566] _Ibid._ _n._
[567] _Ibid._ _t._ ii. _f._ 24. β.
[568] Treviran _Arachnid._ _f._ 6. _B B._
[569] _N. Dict. d'Hist. Nat._ xxx. 423--. Comp. Treviranus,
_Arachnid._ _t._ i. _f._ 6.
[570] Treviranus, _Ibid._ _v._
[571] _N. Dict. d'Hist. Nat._ xxx. 421--. Comp. Treviran. _Ibid._
[572] _N. Dict. d'Hist. Nat._ _Ibid._
[573] Treviran. _Ibid._ _t._ i. _f._ 6. _i i, c c._
[574] _N. Dict. d'Hist. Nat._ _Ibid._
[575] Treviran. _Ibid._ _t._ ii. _f._ 24. _a._
[576] _Ibid._ _v, b._
[577] _Ibid._ _c, d, f._
[578] _Ibid._ _g, n._
[579] _N. Dict. d'Hist. Nat._ _Ibid._
[580] Treviran. _Ibid._ 28.
[581] _Ibid._ _t._ ii. _f._ 24. β.
[582] Ramdohr, _t._ xix. _f._ 1.
[583] Reaum. i. 143. _t._ v. _f._ 9.
[584] VOL. II. p. 88--.
[585] De Geer iii. 26.
[586] Reaum. iii. 357. _t._ xxix. _f._ 6-10.
[587] VOL. II. p. 225.
LETTER XLI.
_INTERNAL ANATOMY AND PHYSIOLOGY
OF INSECTS, CONTINUED._
SECRETION.
Having given you so full an account of the system of _digestion_ in
insects, I am now to say something concerning their _secretions_, and
the organs by which they are elaborated. Though no individual amongst
them perhaps secretes so many different substances as the warm-blooded
animals; yet in general the Class abounds in secretions perhaps as
numerous and extraordinary as in the last-mentioned tribes, to some
of which a few of them are analogous, while others are altogether
peculiar. We know little or nothing of the mode in which the process
of secretion in insects is accomplished; in most cases we cannot even
discover, except in general, whence the secreted substance originates;
and in others, though we are able to trace the vessels that contain
it, we are often in the dark as to their structure.--Cuvier, as has
been before hinted, from not being able to detect any thing in them
like _glands_, and from their being constantly bathed in the _blood_ or
nutritive fluid, conceives that they separate the peculiar substances
they contain, by imbibition or infiltration, through the pores of
the skin[588]; a circumstance which seems to indicate a certain
conformation of the pores both as to size and figure, so as to enable
them to admit only one peculiar product.
In treating on this subject, I shall first consider the _organs_ of
secretion, and next their _products_.
I. _Organs of Secretion._ In general, these are membranous vessels
that float in the blood or nutritive fluid, and secrete from it
a peculiar substance. They may be denominated according to their
products--_Silk-secretors_, _Saliva-secretors_, _Varnish-secretor_,
_Jelly_ or _Gluten-secretor_, _Poison-secretor_, and _Scent-secretors_.
i. _Silk-Secretors_ (_Sericteria_). These organs are most remarkable in
the caterpillars of the _nocturnal Lepidoptera_ or moths, especially
in that tribe called _Bombyces_, to which the silkworm belongs: but
this faculty is not confined to these insects, but is shared by many
other _larvæ_ in different Orders; and in one instance at least, by
the _imago_. In general, the outlet of the silk-secretors is at the
_mouth_; sometimes, however, as in the larva of _Myrmeleon_ and the
imago of _Hydrophilus_, its exit is at the _anus_. The first is the
organ which in the silk-worm provides for us that beautiful substance
from which the animal takes its name. There are always _two_ of these
vessels, which are long floating tubes, growing slender towards the
head of the insect, where they unite to form the spinneret (_fusulus_)
before described[589], which renders the silk. Their lower extremity
also is commonly more slender than the middle, and is closed at the
end. These organs are usually very much convoluted and twisted[590].
According to Ramdohr[591], they consist of two transparent membranes,
between which is found a yellow or transparent jelly. The greater the
quantity of silk employed by the caterpillar in the construction of its
cocoon, &c., the longer are the silk-secretors. Those of the silkworm
are a _foot_ long[592], while those of the larva of the goat-moth are
little more than _three inches_[593].
Other insects spin silk with the _posterior_ extremity of their
body. In the great water-beetle (_Hydrophilus piceus_) the
anus is furnished with two spinnerets, with which it spins its
egg-pouch[594]; these are in connexion, probably, with the five long
and large vessels containing a green fluid, described by Cuvier[595],
which surround the base of each branch of the ovaries. The larva
of _Myrmeleon_, which also spins a cocoon with its anus, differs
remarkably in this respect from other insects, since its reservoir
for the matter of silk is the _rectum_; this is connected with a
horny tube, which the animal can protrude, and thus agglutinate the
silk and grains of sand that compose its cocoon[596].
The _web_ of spiders is also a kind of _silk_ remarkable for its
lightness and extreme tenuity. It is spun from four anal spinnerets,
which never vary in number; two longer organs peculiar to some species
have been mistaken for additional ones, but Treviranus affirms that
they are merely a kind of anal _feeler_. Their structure, as far
as known, has been before described[597]. The web is secreted in
vessels varying in form. In some (_Clubiona atrox_) they consist of
two larger and two smaller ones, at the base of which lie many still
more minute[598]. The four larger vessels are wide in the middle,
branching at top, and below terminating in a narrow canal leading to
the spinnerets[599]. Treviranus thinks the fluid contained in the lower
minute vessels different from that furnished by the larger ones--but
for what purpose it is employed has not been ascertained.
ii. _Saliva-secretors_ (_Sialisteria_). These are organs, rendering
a fluid to the mouth or stomach, that are found in many insects,
especially those that take their food by _suction_, as the
_Hemiptera_, _Lepidoptera_, and _Diptera_, though they are not
confined to the perfect insect, being also in some cases visible in
the larva. Swammerdam was one of the first that discovered them,
and he suspects that they may be _salival_ vessels; though he, as
well as Ramdohr, thinks they are the same with the _silk_ vessels
of the caterpillar[600]; an opinion which Herold has sufficiently
disproved, by showing that at one period of the insect's life they
co-exist[601], and Lyonet discovered a very conspicuous pair in the
caterpillar of the Cossus, co-existent with the silk-secretors[602].
But the physiologist who has given the fullest account of these
organs is Ramdohr:--I shall therefore extract chiefly from him what I
have further to communicate with respect to them.
They are variously constructed blind vessels, that are present in
almost all insects that take their food by _suction_, but are mostly
wanting in those that _masticate_ it. They have been found, however, in
_Cryptorhynchus Lapathi_, _Chrysopa Perla_, and _Iulus terrestris_.
The most usual number of the saliva-secretors is _two_[603]; but
sometimes, as in the first of the last-named insects, there is only
_one_[604]; in others (_Pentatoma Baccarum_) there are _three_, the
exterior one consisting of a pair of reservoirs connecting with the
gullet by a single capillary tube[605]; in _Pentatoma prasina_ there
appear to be _four_[606]; in _Nepa cinerea_, even _six_--the exterior
double pair in this insect, under a powerful lens, is found to consist
of spherical vesicles, resembling somewhat a bunch of currants[607];
and in _Syrphus arcuatus_ they are covered with _four_ rows of
similar ones[608]. In the flea they consist of two pair of spherical
reservoirs, each of which is connected with a short tube, which uniting
with that of the other forms a common capillary one connecting with the
mouth or gullet[609]; these organs sometimes terminate below in slender
vessels;--thus, in _Nepa_, the inner pair terminates in a single vessel
of this description[610], and in _Tabanus_ and _Hemerobius_ apparently
in many[611]. It admits of a doubt however, as was lately observed,
whether in the _Hemiptera_, which have usually more than a _pair_ of
these organs, some are not rather _food-reservoirs_ as in the _Diptera_.
The saliva-secretors open either into the _instruments_ of _suction_
themselves (_Tabanus_, _Musca_); or into the entrance of the _gullet_
(_Pentatoma_, &c.); or, lastly, into that of the _stomach_ (_Syrphus_,
_Bombylius_). Those which lie at the entrance of the _stomach_ consist
only of a blind uniform _tube_[612]; but there is commonly to be
distinguished in those that open into the _mouth_, a _reservoir_,
varying in shape in different species, and terminating in a capillary
tube, or tubes, at one or both extremities[613]. In Bugs, _two_ pair
of these vessels are often present, one of which opens into the
stomach (_Reduvius_), or gullet (_Pentatoma_), but the other into
the instruments of suction[614]. In the _Diptera_ they open into
the stomach when the insect feeds only upon the nectar of flowers
(_Syrphus_), and into the proboscis when it feeds upon both animal
and vegetable juices (_Tabanus_, _Musca_). The function of the fluid
secreted by these organs is to moisten or dilute the food before it is
received by the instruments of suction and passed to the stomach[615].
When a common house-fly applies its proboscis to a piece of sugar, it
is easy to see that it moistens and dissolves it by some fluid.
iii. _Varnish-secretor_ (_Colleterium_). In butterflies, moths, and
several other insects, one or more vessels called blind vessels open
into the oviduct, concerning the use of which, physiologists are not
agreed. In the cabbage butterfly there is a pair of ovate ones, or
rather a bilobed one, each lobe of which externally terminates in
long perplexed convolutions, not easily traced, filled with a yellow
fluid, which Reaumur and Herold think is used for varnishing or
gumming the eggs, so that they may adhere to the leaves on which they
are deposited: it may probably serve likewise for other uses[616].
Another vessel is also to be found in the above butterfly, which
enters the oviduct above this, filled with a thick white fluid, the
function of which is, probably, to lubricate the passage[617]. A
similar organ is found in _Phryganea grandis_[618].
iv. _Jelly-secretor_ (_Corysterium_). This is a remarkable organ,
related to the preceding, which secretes the jelly of _Trichoptera_,
some _Diptera_, &c.; this organ in the former, at least in _Phryganea
grandis_, is of an irregular shape, with four horns or processes[619].
_Poison-secretor_ (_Ioterium_). This organ, which is most conspicuous
in the _Hymenoptera_ Order, has not received much notice, except in
the case of the Hive-bee and the _Scolia_: in the former, it is an
elliptical membranous vesicle or reservoir, furnished at its lower
extremity with a tube which renders to the sting, and at the other by a
blind, long, filiform, secretory, vessel, which according to Swammerdam
divides into _two_ terminal blind branches[620], though Reaumur could
detect but _one_[621]; in this vessel the poison is secreted and
stored up. In _Scolia_ there are two secretory vessels, which enter
the reservoir in the middle on each side[622]. In the _Scorpion_,
we learn from Marcel de Serres that the poison-secretor is clothed
externally with a horny thickish membrane, containing two yellowish
glands, composed of an infinity of spherical glandules, terminating
in a canal, enlarged towards its base so as to form a reservoir, and
leading to the extremity of the sting[623]. Connected by a slender
tube with each mandible in _spiders_ is a vessel with spiral folds,
which seems properly to belong to _this_ head--though Treviranus
calls it a _saliva_-vessel[624]--since in the _Mygale avicularia_ and
other spiders, the effect of the bite is said to be so venomous as to
occasion considerable inflammation, and sometimes death[625].
v. _Scent-secretors_ (_Osmateria_). Amongst other means with which
insects are gifted for the annoyance of their foes and pursuers, are
the powerful _scents_ which many of them emit when alarmed and in
danger. Concerning the _internal_ organs by which these effluvia are
_secreted_ we possess but little information, but more notice has
been taken of the _external_ ones by which they are _emitted_. We may
conclude in general, that the secretory organs are membranous sacs
or vesicles, perhaps terminating in longer or shorter blind filiform
vessels, sometimes secreting a fetid fluid, and at others a fetid
gaseous effluvium. The _Iulidæ_, at least _Iulus_ and _Porcellio_[626],
cover themselves when alarmed, with a _fluid_ of this kind, or emit
one, for this faculty is not peculiar to the species noticed by Savi. I
observed early in the year, when I handled _Iulus terrestris_, that it
was covered with a slimy secretion, of a powerful scent, which stained
my fingers of an orange colour. The spiraculiform pores that mark the
sides of the animal are the outlets by which this fluid is emitted,
and not spiracles as has been supposed: each of these orifices, as
we learn from Savi, terminates internally in a black vesicle, which
is the reservoir of the fluid[627]. The most remarkable insect for
its powers of annoyance in this way, is one on that account called
the _bombardier_ (_Brachinus crepitans_), which can fire numerous
volleys of stinking vapour at its assailants before its ammunition is
exhausted[628]. M. Dufour has given a very particular account of the
organ that secretes this vapour;--it consists of a double apparatus,
one on each side, in the cavity of the abdomen, both formed of two
distinct vessels. The _first_, which is the innermost, presents
itself under two different aspects, according as it is contracted or
dilated: in the former case it is a whitish, irregularly rounded, soft
body, apparently glandular, placed under the last abdominal segments;
communicating at one end with the reservoir, and terminating constantly
at the other in a very long and slender filament: in the second case,
or when it is dilated, it resembles an oblong, membranous, diaphanous
sac, filled with air, then occupying the whole length of the abdomen,
and appearing free except where it communicates with the reservoir. The
_second_ vessel or reservoir is a small, spherical, brown or reddish
body, constant in its form, internally hollow, placed under the last
dorsal segment, precisely above the _rectum_, and opening by a small
pore into the _anus_[629]: so that the tail of this little beetle may
be regarded as a battery mounted with two pieces of cannon, which our
alert bombardier fires alternately without intermission till all his
ammunition is expended. The ground-beetles (_Eutrech_in_a_) in general
have a pair of these anal scent-secretors, which discharge an acrid
and caustic fluid, and sometimes a volatile one[630]. The external
organ of the scent-secretors in _Gyrinus_ consists of two minute hairy
cylindrical retractile tubes, of a red colour[631]. Numerous insects
of other tribes and genera emit _scents_ from their anus, and from
various other parts of the body, of which having before given you a
very full account[632], I shall proceed to the consideration of the
secretions themselves: but first I must observe, that in many cases,
as in some of the cottony and powdery _Aphides_, _Chermes_, &c., the
substance secreted appears to be a transpiration through the pores of
the body, a kind of excretion from the superabundance of its fluid
contents[633]. In many, however, this secretion transpires through
appropriate orifices: thus in _Chermes Abietis_, which produces those
curious galls resembling the cone of a fir[634], the flocoons of
seeming cotton that cover it proceed from little oval concavities on
its back, four of which are arranged in a transverse line on each
dorsal segment of the abdomen: these concavities have minute tubercles
probably terminating in a pore[635]. In _Aphis Fagi_ the cottony
flocoons are almost an inch long[636].
* * * * *
The _secretions_ of insects may be considered under the following
heads--_Silk_; _Saliva_; _Varnish_ or _Gum_; _Jelly_; _Oils_;
_Milk_; _Honey_; _Wax_; _Poisons_ and _Acids_; _Odorous fluids_ and
_Vapours_; and _Luminous matter_.
i. _Silk._ This valuable product of insects, while in the
silk-secretor, assumes in the _Lepidoptera_ the appearance of a viscid
gum, but the moment it is exposed to the air it hardens into a silken
thread. It is remarkable for the following qualities:--it dries the
instant it comes in contact with the air; it is then insoluble not
only in water but in the most active solvents, and even _heat_ has no
effect upon it to melt or soften it: indeed, without these qualities
it would be of no use to us[637]. As soon as it leaves the spinneret
it becomes the thread we call silk, which being drawn through _two_
orifices is necessarily _double_ through its whole length. This
thread varies considerably in colour and texture, as has been before
stated[638], and sometimes resembles cotton or wool rather than silk.
In spiders it is of a much softer and more tender texture than that of
other spinning insects; and Mr. Murray seems to have proved that it is
imbued, in the case of the gossamer, with negative electricity: in the
_sericterium_ the fluid that produces it is sometimes white or grey,
and at others yellow[639]. A remarkable gnat (_Ceroplatus tipuloides_),
living on an agaric, carpets its station of repose and its paths with
something between silk and varnish, which it spins, not in a _thread_,
but in a _broad_ riband[640].