that Ichneumons were appropriated to the _eggs_ of other insects.
He observed one proceed from those of the emperor-moth (_Saturnia
Spini_): finding two holes in each egg, one larger than the other,
he conjectured that one was made when it entered, and the other when
it emerged. In this case the egg of the Ichneumon must be fixed on
the outside of the egg it was to feed upon; though some appear
to pierce it with their ovipositor, and consequently introduce
their egg within: for he says afterwards; "I have seen with my own
eyes a certain kind of wild flies deposit their eggs _upon_ other
eggs, and _bore_ and pierce others with an aculeus--by which they
have introduced the egg[956]." Count Zinanni, a correspondent of
Reaumur's, saw an Ichneumon pierce the eggs with her ovipositor
repeatedly; which in about fifteen days were filled with the pupa,
and in six more produced the imago[957]. _I. Ovulorum_ L. is the
only _known_ species of egg-devourers; but most likely there are
many, varying in size, according to the size of the egg they
inhabit. Probably _I. Atomus_ L., and _I. Punctum_ Shaw, are of this
description[958]. It is wonderful what a number these little flies
destroy:--out of a mass of more than _sixty_ eggs which was brought
to De Geer, not _one_ had escaped the Ichneumon[959]. But the most
extraordinary thing is, that even these little creatures we are told
are destroyed by another still more minute[960].
Though the animals we are speaking of usually destroy only a _single_
egg, yet some appear not so to confine themselves. Geoffrey informs
us that the larva of one of the Ichneumons whose females are without
wings (_Cryptus_) devours the eggs of the nests of spiders, and from
its size--it is nearly a quarter of an inch long--it must require
several of them to bring it to maturity[961]. One of those also
which destroys the gnat infesting the wheat (_I. inserens_) appears
to devour them in their _egg_ state, and could not be brought to
perfection by the food that a single one would furnish[962].
The Ichneumons that are parasitic upon _larvæ_ are the most numerous
of all. Some of them are deposited by the parent fly on the _outside_
of their prey, and others introduced into its _interior_. _Ophion
luteum_ is one of the former tribe; it plants its eggs in the skin
of the caterpillar of the puss-moth (_Cerura Vinula_). Each egg
is furnished with a footstalk terminating in a bulb[963], which
is so deeply and firmly fixed that it is impossible to extract it
without detaching a portion of the animal with it, and even when
the caterpillar changes its skin it is not displaced. After it is
hatched, the grub, while feeding, keeps its posterior extremity in
the egg-shell, to which it adheres so pertinaciously, that it is
scarcely possible to disengage it without crushing it. It fixes
itself by its mandibles to the skin of the caterpillar, and keeps
constantly sucking the contents of its body till it dies: sometimes
nine or ten of these larvæ inhabit a single caterpillar[964].
Reaumur has given an account of other external Ichneumons. Upon one
caterpillar that he examined, they were so numerous as to render the
poor animal quite a spectacle, and they underwent their metamorphosis
attached to it[965]. One species of this description avenges the
cause of insects upon their most pitiless foes, the all-devouring
spider--for in the midst of her toils and lines of circumvallation it
makes her its prey. De Geer, meeting one day with a young spider of a
common kind, observed with surprise, engaged in sucking it, a small
white grub, which was firmly attached to the abdomen near the trunk.
Putting it by in a glass, after some days he examined it again;
when he observed that it had spun the outline of a vertical web, had
stretched threads from the top to the bottom of the glass and from
one side to the other, and had also spun the radii that meet in the
centre, and this was all;--but what was remarkable, the larva that
had fed upon it was suspended in the centre of this web, where it
was engaged in spinning its own cocoon, while the spider, exhausted
by this last effort, had fallen dead to the bottom of the glass. It
cannot be asserted positively that this suspension of the larva of
the Ichneumon in the centre of the web _always_ takes place; but if
it does, as seems most probable, it shows that this little parasite
is endowed with an instinct which causes it so to act upon the spider
as may induce it to spin a web so nicely timed as to be sufficiently
complete at the period of its death and of the change of the
Ichneumon, for the latter to cast it down and assume its station[966].
But the great bulk of the parasitic Hymenopterous devourers of larvæ
have their assigned station _within_ the body. As Entomologists in
breeding insects have paid their principal attention to _Lepidoptera_,
it necessarily follows that their Ichneumon infesters must be most
generally known; but doubtless the larvæ of the other Orders are not
wholly liberated from this scourge: they also require to be kept within
due limits, and have their appropriate parasites. Some, however, in
most of them have been detected: of which I shall now proceed to state
to you the most interesting examples, beginning with the _Coleoptera_.
_Alysia Manducator_[967], remarkable for having mandibulæ that do
not close, and toothed at the end, usually attends masses of dung,
both of man and cattle, probably for the purpose of depositing
its eggs in some of the _Coleopterous_ larvæ that inhabit it. Mr.
Stephens, one of the most accurate observers as well as one of the
best Entomologists of the present day, informs me that he once
captured three specimens of _Timarcha tenebricosa_, from each of
which forty or fifty minute Ichneumons emerged. An insect also of
this Order, that is a great benefactor to mankind, as a destroyer
of the plant-lice,--I mean the lady-bird (_Coccinella_), in its
larva state is itself subject to the attack, as we learn from De
Geer, of one of these small parasites[968]. He detected them also
in that of two species of weevils: and in the pupa of some large
grub of a beetle inhabiting the wood of the elm, perhaps that of the
stag-beetle, he found the pupa of one of those Ichneumons that have
an exserted ovipositor[969]. Doubtless, did we know their history, we
should find that numberless species have their internal assailants
belonging to this tribe.
_Orthopterous_ larvæ seem not to have been yet announced as affording
a pabulum to these animals: but the late Dr. Arnold, whose tact
for observation with regard to the manners and economy of insects
has rendered his loss irreparable, discovered that the remarkable
parasitic genus _Evania_ was appropriated to the all-devouring
_Blatta_. Whether it attacked it in its egg or larva state I have
not been informed. This little benefactor is here extremely rare,
at least in the _country_; perhaps in _towns_, where the cock-roach
abounds, it may be more common.
The observations of naturalists have chiefly been confined to the
_Hemipterous_ genus _Aphis_; but these early attracted their notice.
Leeuwenhoek has given a particular and entertaining account of the
proceedings of _I. Aphidum_. As soon as the little flies approached
their prey, they bent their abdomen, which is rather long, between
their legs, so that the anus projected beyond the head; then with
their ovipositor they pierced the body of the Aphis, at the same time
carefully avoiding all contact with it in every other part: whenever
they succeeded in their attempt, a tremulous motion of the abdomen
succeeded. Only a single egg is committed to one Aphis: when hatched,
the latter becomes very smooth and appears swelled; it is, however,
full of life, and moves when touched. Those that are thus pricked
separate themselves from their sound companions, and take their
station on the _underside_ of a leaf. After some days the inclosed
grub pierces the belly of the Aphis, and attaches the margin of the
orifice to the leaf by silken threads; upon this it dies, becomes
white, and resembles a brilliant bead or pearl[970]. De Geer observed
also an Ichneumon on the Coccus of the elm, _I. Coccorum_[971].
Amongst the _Neuropterous_ tribes likewise, probably the _Ichneumonidæ_
commit their usual ravages; but their exploits, as far as I recollect,
have met with no historian. I have a small species related to
_Chelonus_, which a memorandum made when I took it tells me was
obtained from _Æshna viatica_; yet I do not remember ever tracing that
species to its final change, so that I must have taken this Ichneumon
from the _perfect insect_. It suffices, however, to prove that this
tribe is also exposed to the attack of these parasites. Where larvæ
and pupæ are _aquatic_, it seems probable, if any attack is made upon
them, that it must take place after they have quitted the water.
In the _Hymenoptera_ Order itself, almost every genus has been
ascertained to have its Ichneumon parasites. Not even the fortified
habitations of the gall-flies (_Cynips_) can escape them, almost
every species becoming their prey; a circumstance which puzzled
not a little some of the older naturalists, when they at one time
saw a fly not remarkable for its colours or brilliancy emerge from
the curious moss-like _Bedeguar_ of the wild rose, and at another
were struck by the appearance of one of those splendid minims of
nature which almost dazzle the sight of the beholder[972]. Immunity,
however, from this pest seems to have been granted to the _gregarious
Hymenoptera_; at least none has yet been discovered to attack the
ant, the wasp, the humble-bee, or the hive-bee; in which last, had
there been one appropriated to it, it could never have escaped the
notice of the Reaumurs and the Hubers. The _solitary_ bees, however,
as we have seen above[973], do not escape; and _Epipona spinipes_,
a _solitary_ wasp which feeds its own young with a number of green
caterpillars[974], is itself, when a larva, though concealed in a
deep burrow, the prey of the grub of an Ichneumon, which by means
of a long ovipositor introduces its egg into its body[975]. Even
these parasites, whose universal office it is in their first state
to prey upon insects, are themselves subject to the same malady.
Ichneumonidan devourers are kept in check by other Ichneumonidan
devourers. These in some cases are so numerous as to destroy the
tithe of the kinds they attack[976]. Thus an ever-watchful PROVIDENCE
prevents these parasites from becoming so numerous as to annihilate
in any place the species necessary for the maintenance of the general
economy and proportion of animal and vegetable productions. Amongst
the assailants of the _Hymenoptera_, none seem to have a more
laborious task assigned them than those that pierce the various galls
in which the larvæ of the _Cynips_ tribe are inclosed. To look at an
oak-apple, we should think it a work of difficulty, requiring much
sagacity and address, for one of our little flies to discover the
several chambers lurking in its womb, and to direct their ovipositor
to each of them. Its CREATOR, however, has enabled it instinctively
to discover this, and furnished it with an appropriate elongated
instrument, which will open a way to the deep and hidden cells in
which the grubs reside, penetrate their bodies, and to each commit
an egg. When it prepares to perforate the gall, the Ichneumon begins
by depressing this organ, that it may extricate it from its sheath;
it next elevates its body as high as possible, and bending the
instrument till it becomes perpendicular to the body and to the gall,
so as to touch the latter with its point, it then gradually plunges
it in, till it is quite buried[977]. A very remarkable Hymenopterous
parasite (_Leucospis_), which when unemployed turns its ovipositor
over the back of its abdomen, so that its end points to its head, is
said to deposit its eggs in the nest of the mason-bee, most probably
in the larva: but the curious observations that are stated to have
been made by M. Amédée Lepelletier upon its history have not yet
been given to the public[978].
_Dipterous_ insects, likewise, do not escape from these pests of
their Class: but few observations, however, have been recorded as
to the species assailed by them. We learn from De Geer, that a
gnat (_Cecidomyia Juniperi_), which forms galls upon the juniper
is devoured by an external Ichneumon[979]; that which injures the
wheat in the ear, whose ravages I formerly mentioned to you[980],
affords food to three of these parasites,--one I lately mentioned
as probably devouring its eggs; another pierces the glumes of the
floret, where its destined prey is concealed; and the third enters
it. I once placed a number of the larvæ of the gnat upon a sheet of
paper, at no great distance from each other, and then set down one
of these last Ichneumons in the midst of them. She began immediately
to pace about, vibrating her antennæ very briskly: a larva was soon
discovered, upon which she fixed herself, the motion of her antennæ
increasing intensely; then bending her abdomen obliquely under her
breast, she inserted her ovipositor, and while the egg was depositing
these organs became perfectly motionless. The larva when pricked gave
a violent wriggle. This operation was repeated with all that had not
already received an egg, for only one is committed to each larva. I
have often seen it mount one that was already pricked, but it soon
discovered its mistake, and quitted it untouched[981]. The Hessian
Fly also (_Cecidomyia Destructor_) related to the preceding, whose
alarming ravages I formerly described to you[982], has a peculiar
parasite attached to it, which keeps it in check. The only other
Dipterous insects that I have seen mentioned as affording pabulum
to an Ichneumon, are--one of the aphidivorous flies mentioned by
De Geer, who does not note the species, to the larva of which the
Ichneumon commits only a single egg, producing a grub that intirely
devours its interior[983];--and two described by Scopoli, one, the
larva of a _fly_ frequenting hemp; and the other, which feeds on a
_Boletus_, that of a _gnat_[984].
The _Lepidoptera_, however, is the Order over the larvæ of which the
Ichneumons reign with undisputed sway; attacking all indiscriminately,
from the minute one that forms its labyrinth within the thickness of
a leaf, to the giant caterpillar of the hawk-moth. The most useful
of all, however, the silkworm, appears at least with us, exempted
from this scourge. De Geer, out of fifteen larvæ that were mining
between the two cuticles of a rose-leaf, belonging to the first tribe
here alluded to, found that _fourteen_ were destroyed by one of
these parasites, only one coming forth to display itself in all its
brilliancy and miniature magnificence[985]. One of the most useful to
us is that which destroys the clothes-moth, which the same writer also
traced[986]. Another, equally serviceable, takes up its abode in the
caterpillar that ravages our cabbages and brocoli (_Pontia Brassicæ_)
which perish by hundreds from its attacks. As this falls frequently
under our notice, it will not be uninteresting to give a fuller account
of it. Reaumur has traced and related its whole history. One of these
little flies that he observed, was so intent upon the business in which
she was engaged, that she suffered him to watch her motions under a
lens, without being discomposed. She pursued nearly the same plan of
proceeding with that of the Ichneumon of the wheat-gnat just described;
except that she repeated her operations frequently on the same
caterpillar in different parts, alternately plunging in and extracting
her ovipositor. She seemed to prefer the spot where the segments of the
body are united, particularly where the eighth meets the ninth, and the
ninth the tenth. When the fly had completed its work and quitted the
caterpillar, Reaumur gave it food, and it did not seem less lively and
vigorous than others of its kind; in less than a fortnight it assumed
the pupa; and in four days the whole of its interior being devoured,
it died: but its parasites, perhaps not finding a sufficient supply of
nutriment in it, never came to perfection[987]. Sometimes, however,
these little grubs arrive at maturity before the caterpillar has become
a chrysalis, when they pierce the skin and begin to emerge. First
appears a little white tubercle, which gradually elevates itself in
a direction perpendicular to the body; while this is doing, a second
appears in another place; and so on, till fifteen or sixteen are seen
on each side, giving the caterpillar a very grotesque appearance. By
the alternate contraction and relaxation of their bodies the grubs
effect their complete liberation, which takes place with respect to
the whole in less than half an hour. When entirely disengaged, they
place themselves close to the sides of the caterpillar: even before
this they begin spinning, and draw unequal threads in different
directions, of which they form a cottony bed, which serves as the base
of the separate cocoon of each individual, which they next construct
of a beautiful silk thread of a lovely yellow, which, if it could be
unwound and in sufficient quantity, would yield a silk unrivalled in
lustre and fineness[988].
De Geer has recorded a very singular fact which deserves your notice.
An Ichneumon, appropriated to one of the _Tortrices_, had deposited
its eggs in two of their caterpillars; each produced a considerable
number; but those that emerged from one were all _females_, and
those from the other, _males_[989]. He observed a similar fact take
place with _Misocampus Puparum_[990]. One might conjecture from this
circumstance, that as in the queen-bee[991], so in these Ichneumons,
the eggs producing the two sexes were arranged separately in the
ovaries. Reaumur has related, that in one instance three or four
males were produced to one female; and in another four or five
females to one male[992].
But though the great majority of insects are subject to this
_Scolechiasis_[993] in their larva state, yet sometimes they are not
attacked by the _Ichneumon_ till they have become _pupæ_. Of this
kind is one just mentioned (_M. Puparum_), which commits its eggs to
the chrysalis of the butterfly of the nettle, _Vanessa Urticæ_: the
moment this caterpillar quits its skin to assume that state, while it
is yet soft they pierce it and confide to it their eggs[994]. De Geer
and others have supposed that this same Ichneumon attacks the _Cocci_
and _Coccinellæ_[995]; but this probably is an erroneous supposition.
_Cryptus Compunctor_ also attacks the pupæ of butterflies[996].
If we consider the great purpose of PROVIDENCE in giving being
to this tribe of destroyers--the keeping of insects within their
proper limits,--we may readily conceive that this purpose is more
effectually answered by destroying them in their _preparatory_
than in their _ultimate_ state, since at that time the laying
of their eggs and a future progeny could not so effectually be
prevented;--this will account for there being few or no Ichneumons
appropriated to them in their latter state.
The next tribe of insect parasites are to be found in the _Diptera_
Order. The species that has been particularly noticed as such is
the _Tachina Larvarum_; its larva is polyphagous, laying its eggs
_upon_ the bodies of caterpillars of different kinds. Sometimes a
pair is placed on the first segment, sometimes on the head itself,
and sometimes near the anus. These eggs are very hard, convex, of an
oval figure, polished and shining like a mirror. They are fixed so
firmly that if you attempt to remove them with a penknife the skin
comes off with them. When hatched, they enter the body and feed on
the interior, and, undergoing their metamorphosis within it, do not
emerge till they enter their perfect state. The caterpillar thus
attacked lives long enough to spin its cocoon, when it dies[997].
Sometimes, however, these animals quit their prey sooner. Reaumur
saw a grub of one of the _Muscidæ_ come out of a caterpillar, and
then become a pupa, which was so large that he wondered how it could
have been contained in the animal it had quitted[998].
We have now done with those parasites that produce in insects the
disease I have called _Scolechiasis_[999]: the rest, which belong to
the _Aptera_ Order, will afford us examples both of _Phthiriasis_ and
_Acariasis_[1000].
I begin with the _first_. Mr. Sheppard once brought me a specimen
of a bird-louse (_Nirmus_) which he took upon a butterfly (_Vanessa
Io_): and should such a capture be more than once repeated, it would
afford a _certain_ instance of the _first_ of these diseases amongst
insects;--but most probably the specimen in question had dropped from
some bird upon the butterfly. The only remaining animal belonging
to the apterous hexapods that is parasitic on insects, is by many
supposed to be the larva of a giant-beetle (_Meloe Proscarabæus_). I
have before alluded to this animal[1001], and shall now resume the
subject. Gœdart, Frisch, and De Geer, observed that it deposited in
the earth one or two considerable masses, containing an infinite
number of very minute orange-coloured eggs adhering to each other,
which in about a month were hatched, and produced a number of small
hexapods distinguished by two pairs of anal setæ and a proleg, by
means of which they could move readily upon glass, as I have myself
seen: these little animals precisely corresponded with one found by
the latter author upon _Eristalis intricarius_; and when that fly
was placed amongst them, they immediately attached themselves to
it, so as to leave no doubt of their identity[1002]. A congenerous
species had been detected upon wild bees, and described by Linné
under the name of _Pediculus Apis_. De Geer is so thoroughly to be
depended upon for his veracity and accuracy of observation, that we
cannot suppose there is any incorrectness in his statement. If the
mass of eggs be, as he represents it, of the size of a hazel-nut, it
must have been the product of a very large insect: in confirmation
of this opinion it may be further observed, that the larva of the
kindred genus _Cantharis_ agrees with it in having anal setæ, though
it appears to differ in having only two conspicuous segments in the
trunk[1003]. Those which infest wild bees make their first appearance
upon _acrid_ plants, which the _Meloe_ likewise feeds upon; from
whence with wonderful agility they leap upon the _Andrenæ_, &c.
that visit these flowers. Strong, however, as all these facts
appear, still we cannot help exclaiming with the illustrious Swede
last named, Who could ever have imagined that the larva of this
great _beetle_ would be found upon the body of _flies_,--and we
may add, or _bees_? Who could ever imagine that it would feed like
a _bird-louse_ and resemble it so closely? that in the insertion
of its palpi it should exhibit a character _exclusively_ belonging
to that tribe[1004]? Another circumstance seems to indicate that
these hexapods at the time that they take their station in bees or
flies are perfect insects--they do not vary in size, at least not
materially. Where, we may also ask, if they are to become large
beetles, where do they take their principal growth? It cannot be
as parasites on the little bees or flies that they are usually
found upon; they must soon desert them, and like their kindred
blister-beetles, as is most probable, have recourse to vegetable
food. What an anomaly _in rerum natura_! It is much to be wished that
some skilful insect-anatomist would carefully dissect the _Meloe_;
or perhaps by digging round the roots of the ranunculuses and other
acrid plants the larva of that beetle might be discovered in a later
stage of growth, and so this mystery be cleared up. I should observe
here, that Scopoli has described three parasites as _Pediculi_; viz.
_P. rostratus_, _coccineus_, and _Cerambycinus_; the first of which
Fabricius has adopted under the name of _P. Gryllotalpæ_, but which
are all evidently hexapod _Acarina_[1005].