in _Philos. Trans._ 1781, and Percival's _Ceylon_, 307--.
[453] _Oriental Memoirs_, i. 362.
[454] _Morning Herald_, Dec. 31st, 1814.
[455] The ship here alluded to was the Albion, which was in such a
condition from the attack of insects, supposed to be white ants, that,
had not the ship been firmly lashed together, it was thought she would
have foundered on her voyage home.--The late Mr. Kittoe informed me
that the _Droguers_ or _Draguers_, a kind of lighter employed in the
West Indies in collecting the sugar, sometimes so swarm with ants, of
the common kind, that they have no other way of getting rid of these
troublesome insects than by sinking the vessel in shallow water.
[456] Luke x. 19.
LETTER IX.
_BENEFITS DERIVED FROM INSECTS._
INDIRECT BENEFITS.
My last letters contained, I must own, a most melancholy though not
an overcharged picture of the injuries and devastation which man, in
various ways, experiences through the instrumentality of the insect
world. In this and the following I hope to place before you a more
agreeable scene, since in them I shall endeavour to point out in
what respects these minute animals are made to benefit us, and what
advantages we reap from their extensive agency.
God, in all the evil which he permits to take place, whether spiritual,
moral, or natural, has the ultimate good of his creatures in view. The
evil that we suffer is often a countercheck which restrains us from
greater evil, or a spur to stimulate us to good: we should therefore
consider every thing, not according to the present sensations of pain,
or the present loss or injury that it occasions, but according to its
more general, remote, and permanent effects and bearings;--whether
by it we are not impelled to the practice of many virtues which
otherwise might lie dormant in us--whether our moral habits are not
improved--whether we are not rendered by it more prudent, cautious,
and wary, more watchful to prevent evil, more ingenious and skilful to
remedy it--and whether our higher faculties are not brought more into
play, and our mental powers more invigorated, by the meditation and
experiments necessary to secure ourselves. Viewed in these lights, what
was at first regarded as wholly made up of evil, may be discovered to
contain a considerable proportion of good.
This reasoning is here particularly applicable: and if the ultimate
benefit to man seems in any case problematical, it is merely because
to discover it requires more extended and remote views than we are
enabled by our limited faculties to take, and a knowledge of distant or
concealed results which we are incompetent to calculate or discover.
The common good of this terraqueous globe requires that all things
endowed with vegetable or animal life should bear certain proportions
to each other; and if any individual species exceeds that proportion,
from beneficial it becomes noxious, and interferes with the general
welfare. It was requisite therefore for the benefit of the whole system
that certain means should be provided, by which this hurtful luxuriance
might be checked, and all things taught to keep within their proper
limits: hence it became necessary that some should prey upon others,
and a part be sacrificed for the good of the whole.
Of the counterchecks thus provided, none act a more important part
than insects, particularly in the vegetable kingdom, every plant
having its insect enemies. Man, when he takes any plant from its
natural state and makes it an object of cultivation, must expect
that these agents will follow it into the artificial state in which
he has placed it, and still prey upon it; and it is his business
to exert his faculties in inventing means to guard against their
attacks. It is a wise provision that there should exist a race of
beings empowered to remove all her superfluous productions from the
face of nature; and in effecting this, whatever individual injury may
arise, insects must be deemed general benefactors. Even the locusts
which lay waste whole countries clear the way for the renovation
of their vegetable productions, which were in danger of being
destroyed by the exuberance of some individual species, and thus are
fulfilling the great law of the Creator, that of all which he has
made nothing should be lost. A region, Sparrman tells us, which had
been choked up by shrubs, perennial plants, and hard half-withered
and unpalatable grasses, after being made bare by these scourges,
soon appears in a far more beautiful dress, clothed with new herbs,
superb lilies, and fresh annual grasses, and young and juicy shoots
of the perennial kinds, affording delicious herbage for the wild
cattle and game[457]. And though the interest of individual man is
often sacrificed to the general good, in many cases the insect pests
which he most execrates will be found to be positively beneficial to
him, unless when suffered to increase beyond their due bounds. Thus
the insects that attack the roots of the grasses, and, as has been
before observed, so materially injure our herbage, the wire-worm, the
larvæ of _Melolontha vulgaris_, _Tipula oleracea_, &c., in ordinary
seasons only devour so much as is necessary to make room for fresh
shoots, and the production of new herbage; in this manner maintaining
a constant succession of young plants, and causing an annual though
partial renovation of our meadows and pastures. In the rich fields
near Rye in Sussex I particularly observed this effect; and I have
since at home remarked, that at certain times of the year dead plants
may be every where observed, pulled up by the cattle as they feed,
whose place is supplied by new offsets. So that, when in moderate
numbers, these insects do no more harm to the grass than would the
sharp-toothed harrows which it has been sometimes advised to apply
to hide-bound pastures, and the beneficial operation of which in
loosening the sub-soil these insect-borers closely imitate.
Nor would it be difficult to show that the ordinary good effects
of some of those insects, which torment ourselves and our cattle,
preponderate over their evil ones. Mr. Clark is inclined to think
that the gentle irritation of _Gasterophilus Equi_ is advantageous
to the stomach of the horse rather than the contrary. On the same
principle it is not improbable that the Tabani often act as useful
phlebotomists to our full-fed animals; and that the constant motion
in which they are kept in summer by the attacks of the Stomoxys
and other flies, may prevent diseases that would be brought on by
indolence and repletion. And in the case of man himself, if I do not
go so far as with Linné to give the louse the credit of preserving
full-fed boys from coughs, epilepsy, &c., we may safely regard as
no small good, the stimulus which these, and others of the insect
assailants of the persons of the dirty and the vicious, afford to
personal cleanliness and purity.
I might enlarge greatly upon the foregoing view of the subject: but
this is unnecessary, as numerous facts will occur in subsequent
letters which you will readily perceive have an intimate bearing
upon it; and I shall therefore proceed to point out the more evident
benefits which we derive from insects, arranging them under the two
great heads of _direct_ benefits, and those which are _indirect_;
beginning with the latter.
The insects which are _indirectly_ beneficial to us, may be
considered under three points of view: First, as removing various
nuisances and deformities from the face of nature: Secondly, as
destroying other insects, that but for their agency would multiply so
as greatly to injure and annoy us: and Thirdly, as supplying food to
useful animals, particularly to fish and birds.
To advert in the _first_ place to the former. All substances must be
regarded as nuisances and deformities, when considered with relation
to the whole, which are deprived of the principle of animation.
In this relation stand a dead carcase, a dead tree, or a mass of
excrement, which are clearly incumbrances that it is desirable to
have removed; and the office of effecting this removal is chiefly
assigned to insects, which have been justly called the great
scavengers of nature. Let us consider their little but effective
operations in each of their vocations.
How disgusting to the eye, how offensive to the smell, would be the
whole face of nature, were the vast quantity of _excrement_ daily
falling to the earth from the various animals which inhabit it,
suffered to remain until gradually dissolved by the rain or decomposed
by the elements! That it does not thus offend us, we are indebted to
an inconceivable host of insects which attack it the moment it falls;
some immediately beginning to devour it, others depositing in it eggs
from which are soon hatched larvæ that concur in the same office with
tenfold voracity: and thus every particle of dung, at least of the most
offensive kinds, speedily swarms with inhabitants which consume all
the liquid and noisome particles, leaving nothing but the undigested
remains, that soon dry and are scattered by the winds, while the grass
upon which it rested, no longer smothered by an impenetrable mass,
springs up with increased vigour.
Numerous are the tribes of insects to which this office is assigned,
though chiefly if not entirely selected from the two orders
_Coleoptera_ and _Diptera_. A large proportion of the genera formed,
by different authors, from Scarabæus of Linné, viz. _Scarabæus_,
_Copris_, _Ateuchus_, _Sisyphus_, _Onitis_, _Onthophagus_,
_Aphodius_, and _Psammodius_; also _Hister_, _Sphæridium_; and
amongst the _Brachyptera_, the majority of the _Staphylinidæ_,
many _Aleocharæ_, especially of Gravenhorst's third family, many
_Oxyteli_ and some _Omalia_, _Tachini_ and _Tachypori_, of that
author, including in the whole many hundred species of beetles--unite
their labours to effect this useful purpose: and what is remarkable,
though they all work their way in these filthy masses, and at
first can have no paths, yet their bodies are never soiled by the
ordure they inhabit. Many of these insects content themselves
with burrowing in the dung alone; but _Ateuchus pilularius_[458],
a species called in America the _Tumble-dung_, whose singular
manœuvres I shall subsequently have to advert to, _Copris lunaris_,
_Geotrupes stercorarius_ and many other lamellicorn beetles, make
large cylindrical holes, often of great depth, under the heap, and
there deposit their eggs surrounded by a mass of dung in which they
have previously enveloped them; thus not only dispersing the dung,
but actually burying it at the roots of the adjoining plants, and
by these means contributing considerably to the fertility of our
pastures, supplying the constant waste by an annual conveyance of
fresh dung laid at the very root; by these canals, also, affording a
convenient passage for a portion of it when dissolved to be carried
thither by the rain.
The coleopterous insects found in dung inhabit it in their perfect
as well as imperfect states: but this is not the case with those of
the order _Diptera_, whose larvæ alone find their nutriment in it;
the imago, which would be suffocated did it attempt to burrow into a
material so soft, only laying its eggs in the mass. These also are
more select in their choice than the _Coleoptera_--not indeed as to
delicacy,--but they do not indiscriminately oviposit in all kinds,
some preferring horse-dung, others swine's-dung, others cow-dung,
which seems the most favourite pabulum of all the dung-loving
insects, and others that of birds. The most disgusting of all is the
rat-tailed larva that inhabits our privies, which changes to a fly
(_Eristalis tenax_) somewhat resembling a bee.
Still more would our olfactory nerves be offended, and our health
liable to fatal injuries, if the wisdom and goodness of Providence
had not provided for the removal of another nuisance from our
globe--the _dead carcases_ of animals. When these begin to grow
putrid, every one knows what dreadful miasmata exhale from them,
and taint the air we breathe. But no sooner does life depart from
the body of any creature, at least of any which from its size is
likely to become a nuisance, than myriads of different sorts of
insects attack it, and in various ways. First come the Histers and
pierce the skin. Next follow the flesh-flies, some, that no time may
be lost, (as _Sarcophaga carnaria_, &c.) depositing upon it their
young already hatched[459]; others (_Musca Cæsar_, &c.) covering it
with millions of eggs, whence in a day or two proceed innumerable
devourers. An idea of the dispatch made by these gourmands may be
gained from the combined consideration of their numbers, voracity and
rapid development. One female of _S. carnaria_ will give birth to
20,000 young; and the larvæ of many flesh-flies, as Redi ascertained,
will in twenty-four hours devour so much food, and grow so quickly,
as to increase their weight two hundred fold! In five days after
being hatched they arrive at their full growth and size, which is a
remarkable instance of the care of Providence in fitting them for
the part they are destined to act: for if a longer time was required
for their growth, their food would not be a fit aliment for them, or
they would be too long in removing the nuisance it is given in charge
to them to dissipate. Thus we see there was some ground for Linné's
assertion under _M. vomitoria_, that three of these flies will devour
a dead horse as quickly as would a lion.
As soon as the various tribes of _Muscidæ_ have opened the way, and
devoured the softer parts, a whole host of beetles, _Necrophori_,
_Silphæ_, _Dermestes_, _Cholevæ_, and _Staphylinidæ_, actively second
their labours. Wasps and hornets also come in for their portion of
the spoil; and even ants, which prowl every where, rival their giant
competitors in the quantity consumed by them; so that in no very
long time, especially in warm climates, the muscular covering is
removed from the skeleton, which is then cleansed from all remains
of it by the little _Corynetes cæruleus_ and _ruficollis_, (which
last is so interesting, as having been the means of saving the life
of Latreille[460],) and several _Nitidulæ_[461]. Even the horns of
animals have an appropriate genus (_Trox_) which inhabits them, and
feeds upon their contents. And not only are large animals thus disposed
of, even the smallest are not suffered long to annoy us. The burying
beetle (_Necrophorus Vespillo_) inters the bodies of small animals,
such as mice, several assisting each other in the work; and those to
which they commit their eggs afford an ample supply of food to their
larvæ[462]. Ants also in some degree emulate these burying insects, at
least they will carry off the carcases of insects into their nests;
and I once saw some of the horse-ants dragging away a half-dead snake
of about the size of a goose-quill[463]. Some insects will even attack
living animals and make them their prey, thus contributing to keep them
within due limits. The common earth-worm is attacked and devoured by a
centipede (_Geophilus electricus_). Mr. Sheppard saw one attack a worm
ten times its own size, round which it twisted itself like a serpent,
and which it finally mastered and devoured.
But insects are not only useful in removing and dissipating dead
animal matter; they are also intrusted with a similar office with
respect to the _vegetable_ kingdom. The interior of rotten trees
is inhabited by the larvæ of a particular kind of crane-fly with
pectinated antennæ (_Ctenophora_[464]), and other insects, which there
find an appropriate nutriment; and a similar diet is furnished to
the grubs of the rose-beetle (_Cetonia aurata_) by the dead leaves
and stalks usually to be found in an ant's nest. _Staphylinidæ_,
_Sphæridia_, and other _Coleoptera_, are always found under heaps of
putrescent vegetables; and an infinite number are to be met with in
decomposing fungi, which seem to be a kind of substance intermediate
between animal and vegetable. The Boleti in particular have a genus of
coleopterous insects appropriated to them[465], and the Lycoperdons
another.--Stagnant waters, which would otherwise exhale putrid
miasmata and be often the cause of fatal disorders, are purified by
the innumerable larvæ of gnats, Ephemeræ, and other insects which
live in them and abstract from them all the unwholesome part of their
contents. This, Linné says, will easily appear if any one will make the
experiment by filling two vessels with putrid water, leaving the larvæ
in one and taking them out of the other. For then he will soon find
the water that is full of larvæ pure and without any stench, while
that which is deprived of them will continue stinking[466].
Benefits equally great are rendered by the wood-destroying insects.
We indeed, in this country, who find use for ten times more timber
than we produce, could dispense with their services; but to estimate
them at their proper value, as affecting the great system of nature,
we should transport ourselves to tropical climes, or to those under
the temperate zones, where millions of acres are covered by one
interminable forest. How is it that these untrodden regions, where
thousands of their giant inhabitants fall victims to the slow ravages
of time, or the more sudden operations of lightning and hurricanes,
should yet exhibit none of those scenes of ruin and desolation that
might have been expected, but are always found with the verdant
characters of youth and beauty? It is to the insect world that this
great charge of keeping the habitations of the Dryads in perpetual
freshness has been committed. A century almost would elapse before
the removal from the face of nature of the mighty ruins of one
of the hard-wooded tropical trees, by the mere influence of the
elements. But how speedy its decomposition when their operations are
assisted by insects! As soon as a tree is fallen, one tribe attack
its bark[467], which is often the most indestructible part of it;
and thousands of orifices into the solid trunk are bored by others.
The rain thus insinuates itself into every part, and the action of
heat promotes the decomposition. Various fungi now take possession
and assist in the process, which is followed up by the incessant
attacks of other insects, that feed only upon wood in an incipient
state of decay. And thus in a few months a mighty mass, which seemed
inferior in hardness only to iron, is mouldered into dust, and its
place occupied by younger trees full of life and vigour. The insects
to which this duty is intrusted have been already mentioned in a
former letter (p. 235--); but none of them do their business so
expeditiously or effectually as the Termites, which ply themselves in
such numbers and so unremittingly, that Mr. Smeathman assures us they
will in a few weeks destroy and carry away the trunks of large trees,
without leaving a particle behind; and in places where, two or three
years before, there has been a populous town, if the inhabitants, as
is frequently the case, have chosen to abandon it, there shall be a
very thick wood, and not the vestige of a post to be seen.
* * * * *
I observed in a former letter, that the devastations of insects are
not the same in every season, their power of mischief being evident
only at certain times, when Providence, by permitting an unusual
increase of their numbers, gives them a commission to lay waste any
particular country or district. The great agents in preventing this
increase, and keeping the noxious species within proper limits, are
other insects; and to these I shall now call your attention.
Numerous are the tribes upon which this important task devolves, and
incalculable are the benefits which they are the means of bestowing
upon us; for to them we are indebted, or rather to Providence who
created them for this purpose, that our crops and grain, our cattle,
our fruit- and forest-trees, our pulse and flowers, and even the
verdant covering of the earth, are not totally destroyed. Of these
insects, so friendly to man, some exercise their destructive agency
solely while in the larva state; others in the perfect state only;
others in both these states; and lastly, others again in all the
three states of larva, pupa, and imago. For order's sake, and to give
you a more distinct view of the subject, I shall say something on
each separately.
The first, those which are insectivorous only in their _larva_ state,
maybe further subdivided into _parasites_ and _imparasites_, meaning
by the former term those that feed upon a _living_ insect, and only
destroy it when they have attained their full growth; and by the
latter, those that prey upon insects already dead, or that kill them
in the act of devouring them.
The _imparasitic_ insect devourers chiefly belong to the
_Hymenoptera_ order; and though it is in the larva state that their
prowess is exhibited, the task of providing the prey is usually left
to the female, of which each species for the most part selects a
particular kind of insect. Thus many species of _Cerceris_ and the
splendid _Chrysidæ_ or golden wasps feed upon insects of their own
order. One of the latter (_Parnopes incarnata_) commits her eggs
to the progeny of _Bembex rostrata_: another (_Chrysis bidentata_)
attacks the young of _Epipone spinipes_.
_Bembex_ and _Mellinus_ confine themselves to _Diptera_, the former
preying upon _Eristalis tenax_, _Bombylii_, and the like[468];
the latter amongst others ridding us of the troublesome _Stomoxys
calcitrans_. One of these last I have observed stationed on dung
watching for flies, which when seized, she carried to her burrow.
_Epipone spinipes_, belonging to the family of Wasps, feeds upon
certain green apod larvæ, of which the female deposits ten or twelve
with each egg. _Ammophila vulgaris_ destroys caterpillars of a larger
size, and it is probable that most of the other Vespoid and Sphecoid
_Hymenoptera_, viz. _Trypoxylon_, _Philanthus_, _Larra_, _Crabro_,
&c. assist in this great work.
_Pompilus_, to which genus probably several species mentioned by
Reaumur as preying on these insects should be referred, has it in
charge to keep the number of spiders within due bounds: and some
Sand-wasps lend their aid. One of these last, mentioned by Catesby
(_Pronæus cæruleus_), has been known to seize a spider eight times
its own weight[469]. Another species of this genus, which is common
in the Isle of France, attacks an insect still more difficult, one
would think, to turn to its purpose, the all-devouring _Blatta_ or
cock-roach, and is therefore one of the great benefactors to mankind.
When this insect perceives a Blatta (called there Kakerlac and
Cancrelas) it stops immediately: both animals eye each other; but in
an instant the sand-wasp darts upon its prey, seizes it by the muzzle
with its strong jaws, and bending its abdomen underneath it, pierces
it with its fatal sting. Sure of its victim, it now walks or flies
away, leaving the poison to work its effect; but in a short time
returns, and, finding it deprived of power to make resistance, seizes
it again by the head, and drags it away, walking backwards to deposit
it in a hole or chink of a wall[470].
Grasshoppers are the prey of another sand-wasp, supposed to be the
_Sphex pensylvanica_ of Linné, a native of North America, each of
which in its larva state devours three of a large green species with
which its mother has provided it[471].