advantage than from those which devour the destructive Aphides,
whose ravages, as we have seen above, are more detrimental to us
in this island than those of any other insect. A great variety of
species, of different orders and genera, are employed to keep them
within due limits. There is a beautiful genus of four-winged flies,
whose wings resemble the finest lace, and whose eyes are often as
brilliant as burnished metals (_Hemerobius_), the larvæ of which,
Reaumur, from their being insatiable devourers of them, has named
the lions of the Aphides. The singular pedunculated eggs from which
these larvæ proceed I shall describe when we come to treat upon the
eggs of insects; the larvæ themselves are furnished with a pair of
long crooked mandibles resembling horns, which terminate in a sharp
point, and like those of the ant-lion are perforated, serving the
insect instead of a mouth; for through this orifice the nutriment
passes down into the stomach. When amongst the Aphides, like wolves
in a sheep-fold, they make dreadful havoc: half a minute suffices
them to suck the largest; and the individuals of one species clothe
themselves, like Hercules, with the spoils of their hapless victims.
Next in importance to these come the aphidivorous flies (many species
of _Syrphidæ_), whose grubs are armed with a singular mandible,
furnished like a trident with three points, with which they transfix
their prey. They may often be seen laid at their ease under a leaf or
upon a twig, environed by such hosts of Aphides, that they can devour
hundreds without changing their station; and their silly helpless
prey, who are provided with no means of defence, so far from thinking
of escaping, frequently walk over the back of their enemy, and put
themselves in his way. When disposed to feed, he fixes himself by his
tail, and, being blind, gropes about on every side, as the Cyclops
did for Ulysses and his companions, till he touches one, which he
immediately transfixes with his trident, elevates into the air, that
he may not be disturbed by its struggles, and soon devours. The havoc
which these grubs make amongst the Aphides is astonishing. It was
but last week that I observed the top of every young shoot of the
currant-trees in my garden curled up by myriads of these insects. On
examining them this day, not an individual remained; but beneath each
leaf are three or four full-fed larvæ of aphidivorous flies, surrounded
with heaps of the skins of the slain, the trophies of their successful
warfare; and the young shoots, whose progress had been entirely checked
by the abstraction of sap, are again expanding vigorously.
But even these serviceable insects must yield the palm to the lady-bird
or lady-cow (_Coccinella_), the favourite of our childhood, which, as
well as most of its congeners, in the larva state feeds entirely on
Aphides[472]; and the havoc made amongst them may be conceived from
the myriads upon myriads of these little interesting animals, which
are often to be seen in years when the plant-louse abounds. In 1807
the shore at Brighton and all the watering-places on the south coast
was literally covered with them, to the great surprise and even alarm
of the inhabitants, who were ignorant that their little visitors were
emigrants from the neighbouring hop-grounds, where in their larva
state each had slain his thousands and tens of thousands of the Aphis,
which under the name of the _Fly_ so frequently blasts the hopes of
the hop-grower. It is fortunate that in most countries the children
have taken these friendly Coccinellæ under their protection. In France
they regard them as sacred to the Virgin, and call them _Vaches à
Dieu_, _Bêtes de la Vierge_, &c.; and with us, commiseration for the
hard fate of a mother, whose "house is on fire and children at home,"
ensures them kind treatment and liberty. Even the hop-growers are
becoming sensible of their services, and, as I am informed, hire boys
to prevent birds from destroying them.--If we could but discover a
mode of increasing these insects at will, we might not only, as Dr.
Darwin has suggested, clear our hot-houses of Aphides by their means,
but render our crops of hops much more certain than they now are. Even
without this knowledge, nothing is more easy, as I have experienced,
than to clear a plant or small tree by placing upon it several larvæ
of Coccinellæ or of aphidivorous flies collected from less valuable
vegetables.
Lastly, to close this list of imparasitic insectivorous larvæ, I
may mention those of Geoffroy's genus _Volucella_ so remarkable for
their radiated anus, which live in the nests of humble-bees, braving
the fury of their stings and devouring their young; and the ant-lion
(_Myrmeleon_) and Reaumur's improperly named worm-lion (_Leptis_),
whose singular stratagems will be detailed in a subsequent letter,
both of which destroy great numbers of insects that are so
unfortunate as to fall into their toils.
The _parasitic_ larvæ, an extremely numerous tribe, must next be
considered. These, with the exception of a very few individuals,
belong to the order _Hymenoptera_, and were included by Linné under
his vast genus _Ichneumon_, so named from the analogy between their
services and those of the Egyptian Ichneumons (_Viverra Ichneumon_),
the former as destroyers of insects, being equally important with the
latter as devourers of serpents, the eggs of crocodiles, &c.
The habits of the whole of this tribe[473], which properly includes
a great number of distinct genera, are similar. They all oviposit in
living insects, chiefly while in the larva state, sometimes while
pupæ (_Misocampus Puparum_); and even while in the egg state (_Ich.
Ovulorum_, L[474].); but not, as far as is known, in perfect insects.
The eggs thus deposited soon hatch into grubs, which immediately
attack their victim, and in the end ensure its destruction. The
number of eggs committed to each individual varies according to its
size, and that of the grubs which are to spring from them; being in
most cases one only, but in others amounting to some hundreds.
From the observations hitherto made by entomologists, the great body
of the Ichneumon tribe is principally employed in keeping within
their proper limits the infinite host of _lepidopterous_ larvæ,
destroying, however, many insects of other orders; and perhaps if
the larvæ of these last fell equally under our observation with
those of the former, we might discover that few exist uninfested
by their appropriate parasite. Such is the activity and address
of the Ichneumonidans, and their _minute_ allies (_Pupivora_,
Latr.), that scarcely any concealment, except perhaps the waters,
can secure their prey from them; and neither bulk, courage, nor
ferocity avail to terrify them from effecting their purpose. They
attack the ruthless spider in his toils; they discover the retreat
of the little bee, that for safety bores deep into timber; and
though its enemy Ichneumon cannot enter its cell, by means of her
long ovipositor[475] she reaches the helpless grub, which its parent
vainly thought secured from every foe, and deposits in it an egg,
which produces a larva that destroys it[476]. In vain does the
destructive _Cecidomyia_ of the wheat conceal its larvæ within the
glumes that so closely cover the grain; three species of these
minute benefactors of our race, sent in mercy by Heaven, know how to
introduce their eggs into them, thus preventing the mischief they
would otherwise occasion, and saving mankind from the horrors of
famine[477]. In vain also the _Cynips_ by its magic touch produces
the curious excrescences on various trees and plants, called galls,
for the nutriment and defence of its progeny: the parasite species
attached to it discovers its secret chamber, pierces its wall however
thick, and commits the destroying egg to its offspring. Even the
clover-weevil is not secure within the legumen of that plant; nor
the wire-worm in the earth, from their ichneumonidan foes. I have
received from the late Mr. Markwick that of the former, and Mr.
Paul has shown me the destroyer of the latter, which belongs to
Latreille's genus _Proctotrupes_. Others are not more secured by
the repulsive nature of the substance they inhabit; for two species
at least of Ichneumon[478] know how to oviposit it in stercorarious
larvæ without soiling their wings or bodies.
The ichneumonidan parasites are either external or internal. Thus the
species above alluded to, which attacks spiders, does not live within
their bodies, but remains on the outside[479]; and the larva of _Ophion
luteum_, which adheres by one end to the shell of the bulbiferous egg
that produced it, does not enter the caterpillar of _Euprepia villica_,
the moth upon which it feeds[480]. But the great majority of these
animals oviposit within the body of the insect to which they are
assigned, from whence, after having consumed the interior and become
pupæ, they emerge in their perfect state. An idea of the services
rendered to us by those Ichneumons which prey upon noxious larvæ may
be formed from the fact, that out of thirty individuals of the common
cabbage caterpillar (the larvæ of _Pontia Brassicæ_) which Reaumur put
into a glass to feed, twenty-five were fatally pierced by an Ichneumon
(_Microgaster globatus_[481]). And if we compare the myriads of
caterpillars that often attack our cabbages and broccoli with the small
number of butterflies of this species which usually appear, we may
conjecture that they are commonly destroyed in some such proportion--a
circumstance that will lead us thankfully to acknowledge the goodness
of Providence, which by providing such a check has prevented the utter
destruction of the Brassica genus, including some of our most esteemed
and useful vegetables.
The parasites are not wholly confined to the order _Hymenoptera_:
some insects of other orders, though comparatively very few, destroy
our little enemies in the same way. _Tachina Larvarum_, and another
like it described by De Geer, lay their eggs in caterpillars and
other larvæ[482]; and Reaumur describes several other flies of
similar habits[483]. The order also of _Strepsiptera_, lately
established[484], appears to be altogether parasitic; but with this
difference from the _Pupivora_, that these extraordinary animals are
found only upon _Hymenoptera_ in their perfect state, and do not
appear to destroy the insects upon which they prey, but probably
prevent their breeding. The species at present known are formed
into two genera, _Xenos_ and _Stylops_, which are confined to
_Melitta_[485] and _Vespa_[486].
The next description of insect destroyers are those which devour them
in their _first_ and _last_ states.--No beetles are more common after
the summer is confirmed, than the species of the genus _Telephorus_.
Preysler informs us that the grub of _T. fuscus_ destroys a great
many other larvæ[487], and I have observed the imago devour these and
also _Diptera_.--Linné has with justice denominated the _Cicindelæ_
the tigers of insects. Though decorated with brilliant colours, they
prey upon the whole insect race; their formidable jaws which cross
each other are armed with fearful fangs, showing to what use they
are applicable; and the extreme velocity with which they can either
run or fly, renders hopeless any attempt to elude their pursuit.
Their larvæ are also equally tremendous with the imago, having
eight eyes, four on each side, seated on a lateral elevation of the
head, two above and two very minute below, which look like those
of spiders, and besides their threatening jaws armed with a strong
internal tooth, being furnished with a pair of spines resembling
somewhat the sting of a scorpion, which stand erect upon the back
of the abdomen, and give them a most ferocious aspect[488]. This
last apparatus, according to Clairville, serves the purpose of an
anchor for retaining them at any height in their deep cells[489].
Most of the aquatic beetles, at least the Gyrini and Dytisci, prey
upon other insects both in their first and final state. The larvæ
of the latter have long been observed and described under the name
of _Squillæ_, and are remarkable for having their mandibles adapted
for suction like those of Hemerobius and Myrmeleon; but they are not
like them deprived of a mouth, being able to devour by mastication
as well as by suction.--Another tribe of this order which abounds
in species, those predaceous beetles which form Linné's great genus
_Carabus_ (_Eutrechina_[490]), is universally insectivorous. One
of the most destructive is the grub of a very beautiful species,
an English specimen of which would be a great acquisition to your
cabinet, it being one of our rarest insects[491], I mean _Calosoma
Sycophanta_. This animal takes up its station in the nests of
_Lasiocampa processionea_ and other moths, and sometimes fills
itself so full with these caterpillars, which we cannot handle or
even approach without injury, as to be rendered incapable of motion
and appear ready to burst. Another beautiful insect of this tribe,
_Carabus auratus_, known in France by the name of _Vinaigrier_, is
supposed to destroy more cockchafers than all their other enemies,
attacking and killing the females at the moment of oviposition, and
thus preventing the birth of thousands of young grubs[492]. Lastly
come the _Brachyptera_, many of which prey upon insects as well as on
putrescent substances. Mr. Lehmann tells us that some of them are
very useful in destroying a weevil, _Apion flavifemoratum_[493], the
great enemy of our crops of clover seed.
Amongst the devourers of insects in their _perfect_ state only, must
be ranked a few of the social tribes, ants, wasps, and hornets. The
first-mentioned indefatigable and industrious creatures kill and carry
off great numbers of insects of every description to their nests, and
prodigious are their efforts in this work. I have seen an ant dragging
a wild bee many times bigger than itself; and there was brought to me
this very morning while writing this letter, an _Elater_ quite alive
and active, which three or four ants in spite of its struggles were
carrying off. An observing friend of mine[494], who was some time in
Antigua, informed me that in that island, a kind of ant which construct
their nests in the roofs of houses, when they meet with any animal
larger than they can carry off alive, such as a cock-roach, &c., will
hold it by the legs so that it cannot move, till some of them get
upon it and dispatch it, and then with incredible labour carry it up
to their nest. Madam Merian, in her account of the periodical ants
mentioned to you before[495], and which is confirmed by Azara[496],
notices their clearing the houses of cock-roaches and similar animals;
and _Myrmica omnivora_ is very useful in Ceylon in destroying the
former insect, the larger ant, and the white ant[497].
You are not perhaps accustomed to regard wasps and hornets as of
any use to us; but they certainly destroy an infinite number of
flies and other annoying insects. The year 1811 was remarkable for
the small number of wasps, though many females appeared in the
spring, scarcely any neuters being to be seen in the autumn[498]; and
probably in consequence of this circumstance, flies in many places
were so extremely numerous as to be quite a nuisance. Reaumur has
observed that in France the butchers are very glad to have wasps
attend their stalls, for the sake of their services in driving away
the flesh-fly; and if we may believe the author of Hector St. John's
_American Letters_, the farmers in some parts of the United States
are so well aware of their utility in this respect, as to suspend in
their sitting-rooms a hornet's nest, the occupants of which prey upon
the flies without molesting the family.
There are other devourers of insects in their perfect state, the
manners and food of whose larvæ we are unacquainted with. St.
Pierre speaks of a lady-bird, but it probably belonged to some
other genus, of a fine violet colour, with a head like a ruby,
which he saw carry off a butterfly[499]. Linné informs us that
_Clerus formicarius_ devours _Anobium pertinax_. A fly related
to the _Panorpa communis_ appears created to instill terror into
the pitiless hearts of the tyrants of our lakes and pools,--the
all-devouring _Libellulina_[500]. The _Asili_ also, which are always
upon the chase, seize insects with their anterior legs and suck them
with their haustellum. The cognate genus _Dioctria_, particularly
_D. œlandica_, prey upon _Hymenoptera_, by some unknown means
instantaneously killing the insect they seize. Many species also of
_Empis_, whose haustellum resembles the beak of a bird, carry off in
it _Tipulariæ_ and other small _Diptera_; and what is remarkable, you
can seldom take these insects in coitu, but the female has a gnat,
some fly, or sometimes beetle, in her mouth. Can this be to deposit
her eggs in, as soon as they are impregnated by the male? or is it
designed for the nuptial feast? Even _Scatophaga stercoraria_ and
_scybalaria_, and probably many others of the same tribe, feed upon
small flies, though their proboscis does not seem so well adapted for
animal as for vegetable food.
The most unrelenting devourers of insects appear to be those
belonging to my fourth division, which attack them under _every_
form. These begin the work of destruction when they are larvæ,
and continue it during the whole of their existence.--The earwig
that haunts every close place in our gardens, and defiles whatever
it enters, probably in some degree makes up for its ravages by
diminishing the number of other insects. The cowardly and cruel
_Mantis_, which runs away from an ant, will destroy in abundance
helpless flies, using its anterior tibiæ, which with the thigh form
a kind of forceps, to seize its prey. The water-scorpions (_Nepa_,
_Ranatra_, and _Naucoris_), whose fore legs are made like those of
the _Mantis_, the water-boatman (_Notonecta_), which always swims
upon its back, and _Sigara_, all live by rapine, and prey upon
aquatic insects. Some of this tribe are so savage that they seem to
love destruction for its own sake. One (_Nepa cinerea_) which was put
into a basin of water with several young tadpoles, killed them all
without attempting to eat one.
Those remarkable genera of the tribe of water-bugs (_Hydrocorisæ_
Latr.), which glide over the surface of every pool with such
rapidity, being gifted with the faculty of walking upon the water,
_Hydrometra_, _Velia_, and _Gerris_, subsist also upon aquatic
insects. A large number of the land-bugs (_Geocorisæ_ Latr.) plunge
their rostrum into the larvæ of _Lepidoptera_, and suck the contents
of their bodies; and _Reduvius personatus_, which ought on that
account to be encouraged, is particularly fond of the bed-bug.
But of all the insects that are locomotive and pursue their prey in
every state, none are greater enemies of their fellow tribes than
the _Libellulina_, and none are provided with more powerful and
singular instruments of assault. In the larva and pupa states, during
which they live in the water and prey upon aquatic insects, they are
furnished with two pair of strong jaws, covered by a kind of mask
armed with a pair of forceps or claws, which the animal has the power
of pushing from it to catch any thing at a distance[501]. When an
aquatic insect passes within its reach, it suddenly darts forth the
mask, opens the forceps, seizes the unfortunate victim, and brings it
within the action of its jaws.
When they assume the imago state, their habits do not, like those
of the white ants, become more mild and gentle, but on the contrary
are more sanguinary and rapacious than ever; so that the name given
to them in England, "Dragon-flies," seems much more applicable than
"Demoiselles," by which the French distinguish them. Their motions
it is true are light and airy; their dress is silky, brilliant and
variegated, and trimmed with the finest lace:--so far the resemblance
holds; but their purpose, except at the time of love, is always
destruction, in which surely they have no resemblance to the ladies.
I have been much amused by observing the proceedings of a species not
uncommon here, _Anax Imperator_ of Dr. Leach. It keeps wheeling round
and round, and backwards and forwards, over a considerable portion of
the pool it frequents. If one of the same species comes in its way, a
battle ensues; if other species of _Libellulina_ presume to approach,
it drives them away, and it is continually engaged in catching
case-worm flies and other insects (for the species of this tribe all
catch their prey when on the wing, and their large eyes seem given
them to enable them the more readily to do this,) that fly over the
water, pulling off their wings with great adroitness and devouring in
an instant the contents of the body. From the number of insects of
this tribe which are every where to be observed, we may conjecture
how useful they must be in preventing too great a multiplication of
the other species of the class to which they belong.
Lastly, under this head, not to dwell upon some other apterous
genera, devourers of insects, as the scorpion and centipede,
_Phalangium_ and _Galeodes_, must be enumerated the whole world of
Spiders, extremely numerous both in species and individuals, which
subsist entirely upon insects, spreading with infinite art and skill
their nets and webs to arrest the flight of the heedless and unwary
summer tribes that fill the air, which are hourly caught by thousands
in their toils; one of them (_Theridium_ 13-_guttatum_ Rossi), we are
told, even attacking the redoubted Scorpion[502].