bill of fare than the larger animals, each individual species is
commonly limited to a more restricted diet. Many both of animal and
vegetable feeders are absolutely confined to one kind of food, and
cannot exist upon any other. The larva of _Gasterophilus Equi_ can
subsist no where but in the stomach of the horse or ass, which animals
therefore this insect might boast with some show of reason to have been
created for its use rather than for ours, being to us useful only,
but to it indispensable. The larvæ of _Syrphus Pyrastri_ according to
De Geer eat no other Aphis but that of the rose[708]. Most Ichneumons
and _Sphecina_ prey each upon a single species of insect only, which
therefore they would seem to have been formed for the express purpose
of keeping within due limits. Reaumur mentions having once found in a
parcel of decaying wood the nests of six different kinds of the latter
tribe, each of which was filled with flies of a distinct species[709].
_Cerceris auritus_ and _Philanthus lætus_ in the larva state feed
solely on the weevil tribe of _Coleoptera_, the latter being restricted
even to the short-rostrum'd family, as _Otiorhynchus raucus_,
&c.[710], while _Bembex rostrata_, another hymenopterous insect,
selects flies, as _Musca Cæsar_, &c.[711]
A very large proportion of species, however, are able to subsist
on several kinds of food. Amongst the carnivorous tribes, it is
indifferent to most of those which prey upon putrid substances from
what source they have been derived: and the predaceous insects, such
as the _Libellulina_, _Telephorus_, _Empis_, the _Araneidæ_, &c.
will attack most smaller insects inferior to them in strength, not
excepting in many instances their own species. The wax-moth larva
(_Galleria Cereana_) will for want of wax eat paper, wafers, wool,
&c.[712]: another Tinea described by Reaumur, and before adverted
to, attacks chocolate[713], which cannot have been its natural
food, even selecting that most highly perfumed; and the Tineæ which
devour dressed wool, but happily for the farmer and wool-stapler
refuse it when unwashed, must have existed when no manufactured
wool was accessible.--The vegetable feeders are under greater
restrictions, yet probably the majority can subsist on different
kinds of food. This is certainly true of most lepidopterous larvæ,
several of which as well as many _Coleoptera_ (_Haltica oleracea_,
&c.) are polyphagous, eating almost every plant. It is worthy of
remark, however, that when some of these have fed for a time on one
plant they will die rather than eat another, which would have been
perfectly acceptable to them if accustomed to it from the first[714].
Here too it must be borne in mind, that by far the greater part of
insects feed upon different substances in their different states of
existence, eating one kind of food in the larva and another in the
imago state. This is the case with the whole Order _Lepidoptera_,
which in the former eat plants chiefly, in the latter nothing but
honey or the sweet juices of fruit, which they have often been
observed to imbibe; and the same rule obtains also in regard to most
dipterous and hymenopterous insects. Those which eat one kind of food
in both states, are chiefly of the remaining orders.
I have said that insects, like other animals, draw their subsistence
from the vegetable or animal kingdoms. But I ought not to omit
noticing that some authors have conceived that several species feed
upon mineral substances[715]. Not to dwell upon Barchewitz's idle
tale of East Indian ants which eat iron[716], or on the stone-eating
caterpillars recorded in the Memoirs of the French Academy[717],
which are now known to erode the walls on which they are found,
solely for the purpose of forming their cocoons; Reaumur and
Swammerdam have both stated the food of the larvæ of _Ephemeræ_ to
be earth, that being the only substance ever found in their stomachs
and intestines, which are filled with it. This supposition, which
if correct renders invalid the definition by which Mirbel (and my
friend Dr. Alderson of Hull long before him) proposed to distinguish
the animal and vegetable kingdoms, is certainly not inadmissible;
for, though we might not be inclined to give much weight to
Father Paulian's history of a flint-eater who digested flints and
stone[718], the testimony of Humboldt seems to prove that the human
race is capable of drawing nutriment from earth, which, if the
odious Ottomaques can digest and assimilate, may doubtless afford
support to the larvæ of Ephemeræ. Yet after all it is perhaps more
probable that these insects feed on the decaying vegetable matter
intermixed with the earth in which they reside, from which after
being swallowed it is extracted by the action of the stomach: like
the sand that, from being found in a similar situation, Borelli
erroneously supposed to be the food of many _Testacea_, though in
fact a mere extraneous substance.
The majority of insects, either imbibing their food in a liquid
state, or feeding on succulent substances, require no aqueous fluid
for diluting it. Water, however, is essential to bees, ants, and
some other tribes, which drink it with avidity; as well as in warm
climates to many _Lepidoptera_, which are there chiefly taken in
court yards, near the margins of drains, &c. Even some larvæ which
feed upon juicy leaves have been observed to swallow drops of dew;
and one of them (_Odenestis potatoria_), which (according to Goedart)
after drinking lifts up its head like a hen, has received its name
from this circumstance. That it is not the mere want of succulency in
the food which induces the necessity of drink, is plain from those
larvæ which live entirely on substances so dry that it is almost
unaccountable whence the juices of their body are derived. The grub
of an Anobium will feed for months upon a chair that has been baking
before the fire for half a century, and from which even the chemist's
retort could scarcely extract a drop of moisture; and will yet have
its body as well filled with fluids as that of a leaf-fed caterpillar.
By far the greater part of insects always feed themselves. The
young however of those which live in societies, as the hive- and
humble-bees, wasps, ants, &c. are fed by the older inhabitants of the
community, which also frequently feed each other. Many of these last
insects are distinguished from the majority of their race, which live
from day to day and take no thought for the morrow, by the circumstance
of storing up food. Of those which feed themselves, the larger
proportion have imposed upon them the task of providing for their
own wants; but the tribe of Spheges, wild bees, and some others, are
furnished in the larva state by the parent insect with a supply of food
sufficient for their consumption until they have attained maturity.
As to their _time_ of feeding, insects may be divided into three
great classes: the day-feeders, the night-feeders, and those which
feed indifferently at all times. You have been apt to think, I dare
say, that when the sun's warmer beams have waked the insect youth, and
"Ten thousand forms, ten thousand different tribes,
People the blaze,"
you see before you the whole insect world. You are not aware that a
host as numerous shun the glare of day, and, like the votaries of
fashion, rise not from their couch until their more vulgar brethren
have retired to rest. While the painted butterfly, the "fervent
bees," and the quivering nations of flies, which sport
"Thick in yon stream of light, a thousand ways,
Upward and downward thwarting and convolved,"
love to bask in the sun's brightest rays, and search for their food
amidst his noon-tide fervor, an immense multitude stir not before the
sober time of twilight, and eat only when night has overshadowed the
earth. Then only, the vast tribe of moths quit their hiding-places;
"the shard-born[719] beetle with his drowsy hum," accompanied by
numerous others of his order, sallies forth; the airy gnat-flies
institute their dances; and the solitary spider stretches his net.
All these retire into concealment at the approach of light.--Some
few larvæ (_Agrotis exclamationis_, &c.) have similar habits, and
those of one singular genus before adverted to (_Nycterobius_) are
remarkable for providing in the night a store of food which they
consume in the day; but to the generality of these the period of
feeding is indifferent, and most of them seem to eat with little
intermission night and day.
Insects like other animals take in their food by the mouth (in
_Chermes_ and _Coccus_, indeed, the rostrum seems to be, but really
is not, inserted in the breast, between the fore-legs), but there is
one exception to this rule. The singular _Uropoda vegetans_, which
is such a plague to some beetles, derives its nutriment from them by
means of a filiform pedicle or umbilical cord attached to its anus;
and what increases the singularity, sometimes several of these mites
form a kind of chain, of which the first only is fixed by its pedicle
to the beetle, each of the remainder being similarly connected with
the one that precedes it; so that the nutriment drawn from the beetle
passes to the last through the bodies and umbilical cords of the
individuals which are intermediate[721]. Some have regarded these
bodies as true eggs; and their analogy with the pedunculated eggs of
_Trombidium aquaticum_, which also seem to derive nourishment from
the water-boatmen, &c. to which they are fixed, and still more the
circumstance of their ultimately losing their pedicle and detaching
themselves from the infested beetles, give plausibility to the
idea. Yet these animals are certainly furnished with feet, and have
according to De Geer[722] a part resembling a mouth--characters which
cannot be attributed to any egg.
In the variety of their instruments of nutrition, which you must
bear in mind are often quite different in the larva and perfect
states, insects leave all other animals far behind. In common with
them, a vast number (the orders _Coleoptera_, _Hymenoptera_, and
_Orthoptera_, and the larvæ of _Lepidoptera_, some _Diptera_, &c.)
are furnished with jaws, but of very different constructions, and
all admirably adapted for their intended services; some sharp, and
armed with spines and branches for tearing flesh; others hooked for
seizing, and at the same time hollow for suction; some calculated
like shears for gnawing leaves; others more resembling grindstones,
of a strength and solidity sufficient to reduce the hardest wood
to powder: and this singularity attends the major part of these
insects, that they possess in fact two pairs of jaws, an upper and
an under pair, both placed horizontally, not vertically, the former
apparently in most cases for the seizure and mastication of their
prey; the latter, when hooked, for retaining and tearing, while the
upper comminute it previously to its being swallowed[723].
To the remainder of the class of insects, a mighty host, jaws would
have been useless. Their refined liquid food requires instruments
of a different construction, and with these they are profusely
furnished. The innumerable tribes of moths and butterflies eat
nothing but the honey secreted in the nectaries of flowers, which are
frequently situated at the bottom of a tube of great length. They
are accordingly provided with an organ exquisitely fitted for its
office--a slender tubular tongue, more or less long, sometimes not
shorter than three inches, but spirally convoluted when at rest, like
the main spring of a watch, into a convenient compass. This tongue,
which they have the power of instantly unrolling, they dart into the
bottom of a flower, and, as through a siphon, draw up a supply of the
delicious nectar on which they feed. A letter would scarcely suffice
for describing fully the admirable structure of this organ. I must
content myself therefore with here briefly observing that it is of
a cartilaginous substance, and apparently composed of a series of
innumerable rings, which, to be capable of such rapid convolution,
must be moved by an equal number of distinct muscles; and that,
though seemingly simple, it is in fact composed of three distinct
tubes, the two lateral ones cylindrical and entire, intended, as
Reaumur thinks, for the reception of air; and the intermediate one,
through which alone the honey is conveyed, nearly square, and formed
of two separate grooves projecting from the lateral tubes; which
grooves, by means of a most curious apparatus of hooks like those
in the laminæ of a feather, inosculate into each other, and can be
either united into an air-tight canal, or be instantly separated, at
the pleasure of the insect[724].
Another numerous race, the whole of the order _Hemiptera_, abstract
the juices of plants or of animals by means of an instrument of a
construction altogether different--a hollow grooved beak, often
jointed, and containing three bristle-formed lancets, which, at
the same time that they pierce the food, apply to each other so
accurately as to form one air-tight tube, through which the little
animals suck up[725] their repast; thus, forming a pump, which, more
effective than ours, digs the well from which it draws the fluid[726].
A third description of insects, those of the order _Diptera_,
comprising the whole tribe of flies, have a sucker formed on the same
general plan as that last described, but of a much more complicated
and varied structure. It is in like manner composed of a grooved case
and several included lancets; but the case, although horny, rigid and
beak-like in some, is in others fleshy, flexible, and more resembling
the proboscis of an elephant, and terminates in two turgid liplets: and
the accompanying lancets are themselves included in an upper hollow
case, in connexion with which they probably compose an air-tight tube
for suction. The number and form of these instruments is extremely
various. In some genera (_Musca_) there is but one, which resembles a
sharp lancet. Others (_Empis_, _Asilus_,) have three, the two lateral
ones needle-shaped, that in the middle like a scymetar; together
forming so keen an apparatus, that De Geer has seen an Asilus pierce
with it the elytra of a lady-bird; and I have myself caught them with
not only an _Elater_ and weevil, but even a _Hister_ in their mouths.
In many horse-flies we find four; two precisely resembling lancets,
and two, even to the very handles, buck-hafted carving-knives[727].
The blood-thirsty gnat has five, some acutely lanced at the extremity,
and others serrated on one side. The flea, the spider, the scorpion
have all instruments for taking their food of a construction altogether
different[728]. But it is impossible here to attempt even a sketch of
the variations in these organs which take place in the apterous genera,
and in many of the dipterous larvæ. Suffice it to say that they all
manifest the most consummate skill in their adaptation to the purposes
of the insects which are provided with them, and which can often employ
them not only as instruments for preparing food, but as weapons of
offence and defence, as tools in the building of their nests, and even
as feet.
Some insects in their perfect state, though furnished with organs of
feeding, make no use of them, and consume no food whatever. Of this
description are the moth which proceeds from the silk-worm, and
several others of the same order; the different species of gad-flies,
and the Ephemeræ, insects whose history is so well known as to afford
a moral or a simile to those most ignorant of natural history. All
these live so short a time in the perfect state as to need no food.
Indeed it may be laid down as a general rule, that almost all insects
in this state eat much less than in that of larvæ. The voracious
caterpillar when transformed into a butterfly needs only a small
quantity of honey; and the gluttonous maggot, when become a fly,
contents itself with a drop or two of any sweet liquid.
While in the state of larvæ the quantity of food consumed by insects
is vastly greater in proportion to their bulk than that required by
larger animals. Many caterpillars eat daily twice their weight of
leaves, which is as if an ox, weighing sixty stone, were to devour
every twenty-four hours three quarters of a ton of grass--a power
of stomach which our graziers may thank their stars that their oxen
are not endowed with. A probable proximate cause for this voracity
in the case of herbivorous larvæ has been assigned by John Hunter,
who attributes it to the circumstance of their stomach not having
the power of dissolving the vegetable matters received into it, but
merely of extracting from them a juice[729]. This is proved both by
their excrement, which consists of coiled-up and hardened particles
of leaf, that being put into water expand like tea; and by the
great proportion which the excrement bears to the quantity of food
consumed. From experiments, with a detail of which he has favoured
me, made by Colonel Machell on the caterpillars of _Euprepia Caja_,
he ascertained that, though a larva weighing thirty-six grains
voided every twelve hours from fifteen to eighteen grains weight of
excrement, it did not increase in weight in the same period more than
one or two grains. On the other hand, many carnivorous larvæ increase
in weight in full proportion to the food consumed, and that in an
astonishing degree. Redi found that the maggots of flesh-flies, of
which one day, twenty-five or thirty did not weigh above a grain,
the next weighed seven grains each; having thus in twenty-four hours
become about two hundred times heavier than before[730].
Some insects have the faculty of sustaining a long abstinence from all
kinds of food. This seems to depend upon the nature of their habits. If
the insect feeds on a substance of a deficiency of which there is not
much probability, as on vegetables, &c. it commonly requires a frequent
supply. If, on the contrary, it is an insect of prey, and exposed to
the danger of being long deprived of its food, it is often endowed with
a power of fasting, which would be incredible but for the numerous
facts by which it is authenticated. The ant-lion will exist without the
smallest supply of food, apparently uninjured, for six months; though,
when it can get it, it will devour daily an insect of its own size.
Vaillant, whose authority may be here taken, assures us that he kept
a spider without food under a sealed glass for ten months, at the end
of which time, though shrunk in size, it was as vigorous as ever[731].
And Mr. Baker, so well known for his microscopical discoveries, states
that he kept a darkling beetle (_Blaps mortisaga_) alive for three
years without food of any kind[732]. Some insects, not of a predaceous
description, are gifted with a similar power of abstinence. Leeuwenhoek
tells us that a mite, which he had gummed alive to the point of a
needle and placed before his microscope, lived in that situation eleven
weeks[733].
In some cases the very want of food, however paradoxical the
proposition, seems actually to be a mean of prolonging the life
of insects. At least one such instance has fallen under my own
observation. The aphidivorous flies, such as _Syrphus Pyrastri_, &c.
live in the larva state ten or twelve days, in the pupa state about a
fortnight, and as perfect insects sometimes possibly as long--the whole
term of their existence in summer not exceeding at the very utmost
six weeks. But one[734], which I put under a glass on the 2d of June,
1811, when about half grown, and, after supplying it with Aphides once
or twice, by accident forgot, I found to my great astonishment alive
three months after; and it actually lived until the June following
without a particle of food. It had therefore existed in the larva
state more than eight times as long as it would have lived in all its
states, if it had regularly undergone its metamorphoses--which is as
extraordinary a prolongation of life as if a man were to live 560
years. It is true that its existence was not worth having even to the
larva of a fly. For the last eight months it remained without motion,
attached by its posterior pair of tubercles to the paper on which it
was placed, manifesting no other symptoms of life than by moving the
fore part of the body when touched, and replacing itself on its belly
if turned upon its back. But this was quite enough to prove it still
alive.--I can attribute this singular result to no other circumstance
than its having been deprived of a sufficient quantity of food to bring
it into the pupa state, though provided with enough for the attainment
of nearly its full growth as larva. Possibly the same remote cause
might act in this case, as operates to prolong the term of existence of
annual plants that have been prevented from perfecting their seed; and
it would almost seem to favour the hypothesis of some physiologists,
who contend that every organised being has a certain portion of
irritability originally imparted to it, and that its life will be long
or short as this is slowly or rapidly excited--no great consolation
this for the advocates for fast-living, unless they are in good earnest
in their affected preference of a "short life and a merry one:" though
it must be admitted that they would have the best of the argument were
the alternative such a state of torpid insensibility as that with which
our larva purchased the prolongation of its existence.
* * * * *
After this general view of the food of insects, and of circumstances
connected with it, I proceed to give you an account of some
peculiarities in their modes of procuring it.
The vegetable feeders have for the most part but little difficulty
in supplying their wants. In the larva state they generally find
themselves placed by the parent insect upon the very plant or
substance which is to nourish them: and in their perfect state their
wings or feet afford a ready conveyance to the banquet to which by an
unerring sense they are directed. All nature lies before them, and
it is only when their numbers are extraordinarily increased, or in
consequence of some unusual destruction of their appropriate aliment,
that they perish for want. The description of their food renders
unnecessary those artifices to which many of the carnivorous insects
are obliged to have recourse: and none of them, if we except the
white-ants, whose cunning mode of insinuating themselves into houses
in tropical climates has been detailed in a former letter, can be
said to use stratagem in obtaining their food.
Of the carnivorous species, the greater proportion attack their prey
by open violence, such as the predaceous beetles, the Ichneumons,
burrowing wasps, and true wasps; the praying insects (_Mantis_);
the bugs (_Geocorisæ_ Latr.); dragon-flies (_Libellulina_), &c.;
which have been before adverted to. But a very considerable number,
chiefly, however, of one tribe, that of spiders, provide their
sustenance solely by artifice and stratagem, the singularity of
which, and the admirable adaptation of the instruments by which they
take their prey to the end in view, afford a most wonderful instance
of the power and wisdom of the Creator, and have attracted admiration
in all ages. A description of these, however, which will require a
detailed survey, I must refer to another letter.
I am, &c.
FOOTNOTES:
[703] _Ephem. German. An._ xii. _Obs._ 58. Rai. _Hist. Ins._ 261.
[704] _Linn. Trans._ xi. 11. _t._ 3. _f._ 5-7.
[705] De Geer, iv. 210.
[706] Brahm, _Insekten Kalender_, i. 190.
[707] Reaum. iv. 280.
[708] De Geer, vi. 112.
[709] Reaum. vi. 271.
[710] _Entomologische Bemerkungen_ (Braunschweig 1799), p. 6.
[711] Latreille, _Obs. sur les Hymenoptères_. _Ann. de Mus._ xiv. 412.
[712] Reaum. iii. 257.
[713] Ibid. iii. 277.
[714] Ibid. ii. 324.
[715] For an instance in which an insect, usually subsisting upon
animal food, derived nutriment from a mineral substance, see _Philos.
Magaz._ &c. for January 1823. 2--.
[716] Lesser, _L._ i. 259.
[717] x. 458.
[718] _Dictionnaire Physique._