mouth a fluid which seems in some degree analogous to the _saliva_ of
larger animals. Thus many, as _Lepidoptera_, _Hemiptera_, _Diptera_,
&c., can dilute their food, and render it fitter for deglutition. I
have seen a common fly when not employed in eating, emit a globule
of fluid as big as a grain of mustard-seed from its proboscis, and
retract it again. On a former occasion I observed to you that many
predaceous, carnivorous, and some herbivorous beetles, when alarmed
emit a drop of coloured acrid fluid from the mouth[641]. That this
is not secreted in any of the ordinary salival vessels is evident
from Ramdohr's dissections of those beetles[642], who, had there
been such an organ, would doubtless have discovered it: but as the
stomach of all of them is distinguished by those minute _cœca_ or
blind vessels, which he denominates shags (_zotten_)[643], perhaps
these may be the secretors of this fluid, probably analogous to the
gastric juice[644]; in which case its _primary_ office would be the
_digestion_ of the food. We are not however warranted in considering
_every_ fluid effused from the mouth as saliva. The glutinous
material with which wasps cement the woody fibres for their paper
edifices[645]; that with which some sand-wasps moisten the sand
which they scrape away, of which they form the singular tubes that
lead to their nests[646]; and that with which the aphidivorous larvæ
fix themselves previously to their becoming pupæ[647],--may be a
secretion distinct from saliva; possibly intermediate between it and
gum or the matter of silk, and secreted by peculiar organs. In the
wasp, however, Ramdohr discovered nothing of the kind[648]; and in
_Syrphus_, as before observed, the saliva-secretors are very peculiar
in their structure, as if appropriated to the secretion of a peculiar
fluid[649]. Something similar has been observed by Reaumur with
regard to the larva of _Crioceris merdigera_, which forms its cocoon
with a kind of froth produced from the mouth[650].
iii. _Varnish_ or _Gum_. The eggs of various insects, when they leave
the oviduct, are covered with a kind of varnish or gum by which they
adhere to the substances that the young larvæ are to feed upon, or
are placed in a proper position for their hatching in an appropriate
station. Several instances of this have been already mentioned[651];
I shall therefore not enlarge further upon the subject. With
regard to the secretion itself, little has been recorded except
its _colour_, which has been before noticed. Some _Lepidoptera_
also as we learn from Reaumur and Bonnet[652], use a varnish in the
construction of their cocoons.
iv. _Jelly_ or _Gluten_. This secretion is particularly conspicuous
in the _Trichoptera_ and some _Diptera_, serving as a bed or _nidus_
for those eggs that are committed to the water,--upon which I have
nothing to add to what has been before said[653]. Under this head
also may be noticed the fluid, secreted in peculiar vesicles, that
lubricates the oviduct and the passages of the sexual organs[654].
v. _Oils._ Oily substances are sometimes produced by insects. The
common oil-beetle (_Meloe Proscarabæus_) when touched sends forth
a drop of this kind of fluid, of an orange colour, from each joint
of its legs[655]: something similar I have observed in _Coccinella
bipunctata_: Ray mentions a locust taken in Spain which emits a
yellow oleaginous fluid from between the claws of its fore legs[656];
but the precise nature of these substances has not been ascertained,
nor whether they are secreted by peculiar organs.
vi. _Milk._ A milky fluid is produced by the larva of _Chrysomela
Populi_. Willughby observed a similar effusion from pores in the
upper surface of the body of _Acilius sulcatus_; and other insects
emit it from other parts of their body[657].
vii. _Honey._ It is certain that honey is not an _animal_ secretion;
yet the saccharine matter collected from the nectaries of flowers,
from which it is derived, seems to undergo some _alteration_ in the
stomach; for the consistence of honey is greater than that of any
vegetable nectar, and its taste does not vary greatly, while that of
the nectar in different plants is probably not the same. Reaumur also
has observed, that each honey-cell in a bee-hive is always covered
by a cream-like layer of a thicker consistence than the rest, which
apparently serves to prevent the more liquid honey, which from time to
time is introduced under it, from running out[658]. Now if honey were
the unaltered nectar of plants, it is difficult to conceive how this
cream could be collected in proper proportions. The last-mentioned
naturalist likewise ascertained, that if bees, in a season in which
the fields afford a scarcity of food, be supplied with _sugar_, they
will from this substance fill their cells with _honey_ which differs
in no respect from the common sort, except that its flavour is a
little heightened[659]:--a similar argument may be deduced from the
circumstance of the bees imbibing the juices of _fruits_ of various
kinds as they are well known to do[660]. It seems therefore evident
that the honey collected by bees undergoes some modification in their
honey-stomach before it is regurgitated into the cells, and therefore
may be regarded in some degree as a peculiar secretion.
Huber says that he has ascertained by a great number of observations
that electricity is singularly favourable to the secretion of the
substance of which honey is formed by flowers; the bees never collect
it in greater abundance, nor is the formation of wax ever more
active, than when the wind is in the south, the air humid and warm,
and a storm gathering[661].
viii. _Wax_ generally transpires through the pores of the skin of
those insects that produce it, either partially or generally, and it
is secreted from honey or other saccharine substances taken into the
stomach. In the hive-bee, as has been before stated, it is produced
_partially_[662], but in many other insects it is a _general_
transudation of the body. This is particularly the case with a large
number of the _Homopterous Hemiptera_; and those flocoons that look
like cotton, and cover the body of several _Chermes_ and _Aphides_,
if closely examined will be found of the nature of _wax_: this I have
particularly noticed with respect to _Chermes Fagi_, in which the
cotton-like flocoons are often so long as to cause the insect to look
like a feather, and a leaf covered by them exhibits a very singular
appearance, as if clothed with the fine down of a swan[663].
Probably the white powder or threads that appear to transpire through
the skin of many other insects is of a waxy nature. In the larva of a
beetle described by Reaumur, the flocoons are so arranged as to give
the animal some resemblance to a hedgehog, and when rubbed off they
are reproduced in twelve hours[664]. Gyllenhal, speaking of _Peltis
limbata_, observes, that when alive it is covered with a white powder
resembling mould, which if rubbed off returns again as long as the
animal lives[665].
It will not be improper to include under this head what further
account I have to give of _Lac_, which though regarded as a _resin_,
since _Cocci_ sometimes certainly produce _wax_[666], probably has
some analogy with the latter substance. When the females of this
_Coccus_ (_C. Lacca_) have fixed themselves to a part of the branch
of the trees on which they feed (_Ficus religiosa_ and _indica_,
_Butea frondosa_, and _Rhamnus Jujuba_[667]), a pellucid and
glutinous substance begins to exude from the margins of the body, and
in the end covers the whole insect with a cell of this substance,
which when hardened by exposure to the air becomes lac. So numerous
are these insects, and so closely crowded together, that they often
entirely cover a branch; and the groups take different shapes, as
squares, hexagons, &c., according to the space left round the insect
which first began to form its cell. Under these cells the females
deposit their eggs, which after a certain period are hatched, and
the young ones eat their way out. Though indisputably an _animal_
secretion, many of the properties of lac are not very different from
those of the juices of the trees on which the animal feeds, and which
therefore would seem to undergo but little alteration.
Wax seems also to form a constituent part of some insects which are
not found to secrete it. The yellow substance deposited in vessels
containing _spiders_ in alcohol is said to be a true _wax_, and may
be obtained from these animals by gently heating them[668].
ix. _Poisons_ and _Acids_. The _bite_ as well as the _sting_ of many
insects is followed by inflamed tumours, so that the _sialisteria_ of
some _bugs_, _Diptera_, _Aptera_ and _spiders_, may be regarded as
producing a poisonous fluid; but we know nothing of the real nature
of it, nor of that of other venomous insects, except the _ant_--whose
celebrated _acid_ may be considered under the present head,--the
_bee_, the _wasp_, and the _scorpion_.
Contrary to the once received doctrine that no _acid_ was to be found
in any animal, except as the effect of disease in the alimentary canal,
many insects secrete peculiar and powerful ones. I have on a former
occasion related an instance in which an acid of this description,
secreted in its _sialisteria_, is employed by a moth to soften its
cocoon[669]; and Lister mentions a species of _Iulus_ which produced
one resembling that of ants[670]; but this last is the most powerful
of all. The fact that blue flowers when thrown into an ant-hill become
tinged with red has been long known; but Mr. Fisher of Sheffield, about
1670, seems to have been the first who ascertained that this effect is
caused by an _acid_ with which ants abound, and which may be obtained
from them by distillation or infusion in water[671]. Margraff and
other chemists confirmed this discovery[672]; and concluding that this
acid was of a peculiar kind, they gave it the name of the _Formic
acid_. This name, however, is now exploded; the subsequent experiments
of Deyeux, Fourcroy and Vauquelin having ascertained that the acid
of ants is not of a distinct kind, but a mixture of the _Acetic_ and
_Malic_[673]. These acids are in such considerable quantities, and so
concentrated in these animals, that, when a number of _Formica rufa_
are bruised in a mortar, the vapour is so sharp that it is scarcely
possible to endure it at a short distance. It also transpires from
them, for they leave traces of it on the bodies which they traverse:
and hence, according to the experiments of Mr. Coleridge, the vulgar
notion that ants cannot pass over a line of chalk is correct; the
effervescence produced by the contact of the acid and alkaline
being so considerable, as in some degree to burn their legs[674].
The circumstance of much of the food of ants being of a saccharine
nature may account for this copious secretion of acid, the use of
which is probably to defend themselves and their habitations from the
attack and intrusion of their enemies: if a frog be put into a nest
of _Formica rufa_ that has been deranged, it will be suffocated in
five minutes[675]. That which they _ejaculate_ from their _anus_ when
attacked, as formerly stated[676], must be secreted in an _ioterium_;
but their very _blood_ seems of an acid nature. It is very probable,
as Dr. Thomson has observed[677], that acids may be obtained from many
other insects, and that they are various modifications of the acetic.
From the circumstance that water is absorbed by _greasy_ moths,
that crystals of a salt are occasionally found adhering to them,
that they change blue litmus paper _red_,--it has been inferred that
their supposed _oiliness_ is in fact an _acid_ or acid salt, having
the property of attracting moisture from the air, the infected moths
being in fact not greasy, but _wet_; hence the application of chalk
and clay, usually recommended in this case, can have only a temporary
and superficial effect. The only effectual remedy, is steeping the
body in spirits of wine till all the acid is extracted[678]. This
acid is probably the same as Chaussier obtained from silkworms, since
called _Bombic Acid_[679].
The _poison_ of _bees_ and _wasps_, as to its chemical qualities, is
a transparent fluid, at first sweet to the taste, but immediately
afterwards hot and acrid like the milky juice of the _spurge_[680];
soluble in water, but not in alcohol; and separable from the former in
the state of white powder, when the latter is added giving a slight
_red_ tinge to paper stained with vegetable blue, and when dry and
chewed appearing tenacious, gummy and elastic. This last property,
as well as solubility in water and not in alcohol, is common also to
the poison of the _viper_, which however differs in being tasteless,
and not affecting vegetable blues. From hence Fontana concludes that
this fluid is united with an _acid_, but in a very small proportion,
and not with an _alkali_[681]. The venom of bees is extremely active;
a grain in weight, it is conjectured, would kill a pigeon in a
few seconds[682]. It is remarkable, however, that while in some
constitutions the sting of a single bee or wasp is sufficient sometimes
to induce alarming symptoms, in others numerous punctures will produce
little or no pain or inflammation. That this fluid, and not the
puncture of the sting, is the sole cause of the inflammation that
usually follows the wound inflicted by one of these animals, is proved
by the facts, that if it be introduced into one made by a needle, the
same effect ensues, and that when the whole contents of the poison-bag
have been exhausted by the insect's stinging three or four times in
succession, its weapon then becomes harmless[683].
The venom of _scorpions_, though much more potent, probably resembles
that of bees, &c., in many of its chemical qualities: it issues from
two pores in the sting before described[684], where, when the animal
is irritated, it accumulates under the form of two little drops of
a whitish colour: spread upon paper this fluid produces a spot like
what would be caused by oil or grease, and this part of the paper
becomes by desiccation firmer and transparent[685].
x. _Odorous fluids_ and _Vapours_[686]. The powerful scents which
different insects emit are extremely numerous, much more so indeed
than the generality of Entomologists have been aware, for there is
scarcely a scent odious or agreeable that may not be met with in the
insect world. This you will be convinced of, by following a practice
which I would recommend to you--that of smelling the insects you
take. Some of these scents are peculiar to particular parts or
organs, and some are exhaled generally by the whole body; some are
emitted by a fluid secretion, and others are gaseous effluvia. On a
former occasion I gave you a rather full account of these scents and
their organs[687]; I shall relate here only what I there omitted. To
begin with _sweet_ odours. Many beetles emit an agreeable scent. The
rose-scented Capricorn or musk-beetle (_Cerambyx moschatus_) has long
been noted for the delicious scent of roses which it exhales; this is
so powerful as to fill a whole apartment, and the insect retains it
long after its death. Captain Hancock also informed me that another
species of the same genus, _C. sericeus_, has in a high degree a
scent resembling that of the cedar[688] on which they feed. Though
most of the micropterous tribes (_Brachyptera_) have a _fetid_ smell,
yet there are some exceptions to this amongst them. One species
(_Philonthus suaveolens_ K. MS.) related to _P. micans_, which I once
took, smelt precisely like a fine high-scented ripe pear; another,
_Oxytelus morsitans_, like the water-lily; a third, _O. rugosus_,
like water-cresses; and lastly, a fourth (_P. fuscipes_) like
saffron[689]: _Trichius Eremita_, one of the Petalocerous beetles,
is stated to have the scent of Russia leather; _Geotrupes vernalis_,
in spite of its stercorarious food, of lavender-water[690]. Mr.
Sheppard has observed that _Dytiscus marginalis_ when recently taken
smells not unlike liquorice: Bonnet mentions a caterpillar that had
the scent of new hay. A little gall-fly (_Cynips Quercus Ramuli_)
has the remarkable odour of Fraxinella: the larva of another species
of this genus (_C. Rosæ_) has an odour which seemed to Reaumur
as attractive to cats as that of _Nepeta cataria_ or _Teucrium
Marum_[691]: some _Phalangia_ smell like walnut leaves[692]; and the
various species of the genus _Prosopis_ (_Melitta_ * b. K.) have a
very agreeable scent of _Dracocephalum moldavicum_[692].
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._