those resulting, namely, from some accidental _external_ injury or
_internal_ derangement, and those produced by _parasitic_ assailants.
I. Under the _first_ head we may begin with _wounds_, _fractures_,
_mutilations_, and other _extraneous_ causes of disease. To
these--insects are peculiarly subject; and though they are not, like
the _Crustacea_ and _Arachnida_[921] and some other invertebrate
animals, endowed with the power of _reproducing_ a mutilated limb,
yet their wounds appear to heal very rapidly, and at the time they
are inflicted to produce little pain[922]. But if those important
members, their _antennæ_, are mutilated, insects seem to suffer a
kind of derangement; the great organ of their communication with
each other, and in various respects with the external world, being
removed, all their instincts at once fail them. I formerly related
how the amputation of these affects the _queen-bee_[923]. A similar
result, as Huber tells us[924], follows, when the same experiment is
repeated on the _workers_ or _drones_: they immediately become unable
to take any further part in the labours of the hive; they can no longer
guide themselves except in the light; if they petition one of their
fellow-citizens for honey, they are unable to direct their tongue to
its mouth to receive it; they remain near the entrance of the hive, and
when the light is intercepted they rush out of it to return no more.
Insects occasionally are subject to _tumours_ or a preternatural
enlargement of their parts and organs. The antennæ of bees sometimes
swell at their extremity so as to resemble the bud of a flower ready
to open, becoming at the same time very yellow, as does the fore part
of the head[925]. I once saw a specimen of a _Hydrobius_--agreeing
with _H. fuscipes_ in every other respect even to the most minute
punctum--which had a large tumour on each side of the _prothorax_,
evidently accidental, occasioned probably by the stoppage of the
pores by which the superfluous moisture and air escape when it
undergoes its last change. The converse of this I have observed to
take place sometimes in the same part of _Geotrupes foveatus_, the
ordinary lateral _foveæ_ becoming very considerably enlarged;--this
was the case with the specimen from which Mr. Marsham made his
description of that insect. The species is, however, very distinct in
other respects, and may always be known by its small size. It happens
now and then also, that these tumours represent _blisters_. I saw
one once on one elytrum of a beetle and not on the other. Those of
_Serropalpus_ (as Mr. MacLeay, on the authority of M. Clairville,
informs me) are particularly subject to this disease. But, of all
the organs, the wings are most exposed to derangements of this kind.
De Geer, in a specimen of _Pieris Cratægi_ just excluded from the
chrysalis, observed that one of these was distended by a considerable
quantity of extravasated green fluid--two or three large drops
following an incision. This disease appeared to arise from the lower
membrane not adhering to the upper; so that the nervures--which are
rather longitudinal channels, being open below, than tubes--were
not closed to confine the fluid to its proper course. The malady,
which might be called a dropsy of the wing, carried off the insect
the day after its exclusion[926]. Reaumur observed that the wings of
some flies were affected by an _air_-dropsy, as he calls it, which
appeared to arise from the air escaping from its natural channels,
and thus separating, the two membranes that form the wing, and
filling the cavity produced by their separation[927].
Sometimes also _monstrosities_ are to be met with in these animals,
or variations from a symmetrical structure in organs that are pairs.
I have a beetle in which the terminal joint of one of the maxillary
palpi is short, ovate, and acute; and that of the other, long,
semiovate, and rather obtuse. A specimen of _Blaps mortisaga_ in
my cabinet, taken by Mr. Denny, besides the terminal mucro of the
_elytra_, has a long diverging lateral one. Goeze had the larva
of a _Semblis_ brought to him in which one of the two fore-legs,
though perfect in all its parts, was only half the length of the
other[928]; which he regarded as a reproduction, but it seems rather
a malformation. Müller mentions a most extraordinary fact of one of
the _Noctuidæ_, which when disclosed from the pupa retained the head
of the larva[929]. One of the most remarkable instances of this kind
that have fallen under my own observation, may be seen in a specimen
of _Chrysomela hæmoptera_ in the cabinet of our friend Curtis; in
which one of the thighs produces a double tibia, but only one of
these is furnished with a tarsus.
The diseases of insects which arise from some _internal_ cause are
not very numerous. The first that I shall mention is a kind of
_vertigo_. "Ants have also their maladies," says M. P. Huber: "I
have noticed one extremely singular; the individuals attacked by it
lose their power of guiding themselves in a straight line, they can
walk only by turning round in a circle of small diameter and always
in the same direction. A virgin female shut up in one of my glasses
was seized on a sudden with this distemper; she described a circle
of an inch in diameter, and made about a thousand turns in an hour,
or not quite seventeen in a minute. She continued constantly turning
round for seven days, and when I visited her in the night I found
her still in motion. I gave her honey--and I think that she ate some
of it." He observed that some workers were attacked by a similar
disease: one of these, however, had the power of walking from time
to time in a straight line; when placed upon its head it continued
its gyrations[930]. Similar motions of a little moth, mentioned on
a former occasion[931], may perhaps have been produced by the same
cause. Bees are also subject to vertigo, which has been attributed
to their eating poisonous honey[932]--but may not this disease in all
these cases arise from some derangement of the nervous system? One
of the ants which was so affected had lost one of its antennæ; but
as this was not the case with the others, no great stress is to be
laid upon the circumstance. Huber does not inform us whether those
attacked by this disease recovered or not.
I have observed more than once, that the _flesh-fly_ and some others
of the same tribe are subject in particular seasons to a kind of
_convulsions_. When thus attacked, they kick and struggle, and seem
unable to fly. Sometimes they lie upon their backs without motion,
but if a finger be placed near them their convulsive motions are
renewed. When thrown into the air, instead of flying, they fall to
the ground. Had this distemper occurred earlier or later in the year
I should have attributed it to the benumbing effects of cold; but as
my observations were made one year (1816) in _May_, and in another
(1811) in the latter end of _June_, this could scarcely be the case.
In the year last mentioned I observed that many flies died under its
influence. In wet seasons this tribe is subject to another disease,
which proves fatal to many of them, and indeed to other _Diptera_.
A white crust appears to be formed upon the abdomen both above and
below, of a granular appearance, much resembling fine moist sugar.
On the back of that part this crust does not cover the margins of
the segments, which gives it the appearance of white bands; so
that deceived by it, I have often at first flattered myself that I
had met with some new species. The under-side of the abdomen is
wholly covered by it, divided in the middle into two longitudinal
masses, the anal segment being bare. De Geer has noticed this or
a similar disease, which, when flies are attacked by it, causes
the abdomen to swell so as even to burst, and the segments become
dislocated. Upon opening the abdomen it is found filled with a white
unctuous substance, which often accumulates (as above described)
on its external surface[933]. Dr. Host says that in this disease
when the animal is dead, the wings, which were before incumbent,
become extended, and its almost invisible pubescence grows into long
hairs[934]. De Geer seems to think that these flies are thus affected
in consequence of having eaten some poisonous food[935]; but I rather
suspect, as I have observed it become prevalent chiefly in wet
seasons, that it arises from a superabundance of the nutritive fluid,
or of the fat, so that it seems to be a kind of _plethora_. I once
observed a fly fixed to a pane of glass, round which was a semicircle
of what appeared to be merely vapour, whose radius was nearly
three-fourths of an inch. Taking it for an aqueous fluid that had
transpired from the dead animal, I paid no further attention to it at
that time: but observing from day to day that the moisture did not
evaporate, after two or three months had elapsed, I had the curiosity
to examine it more closely, and, upon scraping some of it off with a
penknife, I found it was a white substance of a fatty nature. In this
case, then, the fat must have exploded on all sides with considerable
violence from half the body or the abdomen. Probably this was a more
intense degree of plethora. When I examined this appearance the fly
had fallen off, and I could not find it.
Mr. Sheppard once brought me a panicle of grass, the glumes of which
were rough with hairs, or small bristles, to which several specimens
of a fly related to _Xylota pipiens_ adhered by their proboscis.
At first I thought that having been entrapped by the bristles, and
unable to extricate themselves, they had perished from want of food;
but since when touched they readily dropped from the glumes, some
other cause, perhaps disease, probably occasioned this singular
suspension of themselves.
The maladies to which _bees_ and _silkworms_ are subject are more
interesting to us than those of _flies_, on account of their utility
as _cultivated_ insects. One of the worst distempers which attacks
the first of these animals is a kind of _looseness_ or _dysentery_:
this happens early in the year, when they are fed with too much honey
without any portion of bee-bread[936], and sometimes destroys whole
hives. Their excrements, instead of a yellowish red, then become
black, and the odour they emit is insupportable; the bees no longer
observe their usual neatness, inducing them to leave the hive when
they void their excrements, but they defile it, their cells, and each
other. Several remedies have been prescribed for this disease. To
prevent it, a syrup made by an equal mixture of good wine and honey
is recommended; and as a cure, to place in the hive combs containing
cells filled with bee-bread[937]. But one of the worst maladies to
which these useful animals are subject, is that called by Schirach
_Faux Couvain_. It originates with the larvæ; and is caused either
by their being fed with unwholesome food, or when the queen, as
sometimes happens, lays her eggs so that the head of the grub is not
in a proper position for emerging from the cell when the period for
its disclosure is arrived:--the consequence is, that in both cases it
dies and becomes putrid, which sometimes produces a real pestilence
in a hive. The remedy for this evil is to cut away the infected
combs, and to make the bees undergo a fast of two days[938]. The hive
should be cleaned and fumigated, by burning under it aromatic plants.
The cultivators of the _silkworm_ in France have given names to
several diseases to which that animal is subject. One is called _La
Rouge_, and is supposed to be occasioned either by too great heat,
or by too sudden a transition from cold to heat. It takes place
when the caterpillar is first hatched; which lives perhaps, but in
a very sickly state, till it should spin its cocoon and assume the
pupa, when it expires. Another degree of the same disease is called
_Les Harpions_ or _Passis_. A second distemper of this animal is
_Des Vaches_, _Le Gras_ or _La Saune_: this is a mortal disease,
supposed to be of a putrid nature, and produced by mephitic air; it
shows itself after the second moult, but rarely after the subsequent
ones. When a caterpillar is first attacked, changing the air may
prove a remedy; but when the disease has made progress, it is best
to burn or bury them, since if the poultry pick them up they might
be poisoned by them. A third disease of silkworms is called _Les
Morts Blancs_, or _Tripes_, which is also occasioned by impure air,
when the leaves the animal feeds upon are heaped so as to produce
fermentation. The caterpillars attacked by it die suddenly, and
preserve after their death the semblance of life and health. Too
great heat, whether artificial or natural, occasions _La Touffe_, a
fourth, which, when the heat continues long, destroys all those that
are arrived at their last stage of existence in their larva state.
Black points scattered over different parts of the body, or livid
and blackish spots in the vicinity of the spiracles, followed by a
yellowish or reddish tint, are symptoms of a fifth malady, called _La
Muscardine_. After this the animal soon dies, and becomes mouldy, but
does not stink. This disease is not contagious, and is thought to
be caused by a moist heat, attended by pernicious exhalations. _La
Luzette_, _Luisette_, or _Clairène_, is another malady, which shows
itself most commonly after the _fourth_ moult. It seems to arise from
some original defect in the _egg_. The caterpillars attacked by it
may be known by their clear red and afterwards dirty white colour;
their body becomes transparent, and the matter of silk exudes in
drops from their spinnerets; consequently, though as voracious as
the rest, they are never able to construct a cocoon, and should be
destroyed. _Les Dragées_ is the name given to cocoons which include
a larva that never becomes a pupa. The cause of this disorder has
not been ascertained, and whole broods are sometimes subject to it,
which, as in the last, seems to imply some defect in the _eggs_.
But as the caterpillar spins its cocoon, and the silk is as good as
usual, it is a malady of no great importance. Lastly, sometimes the
mulberry leaves have a gummy rather acrid secretion, which purges the
silkworms; their excrement is no longer solid; they become weak and
languid; and if the secretion is abundant, their transpiration is
impeded, and at the time of moulting they are become so feeble as to
be unable to cast their skin[939].
In the case of many caterpillars of _Lepidoptera_ that died, Bonnet
found by dissection that the disease was remotely occasioned by a
_diarrhea_, which taking place immediately before they became pupæ,
prevented the inner membrane of their intestines from being rejected,
as it would have been if no extraordinary cause had prevented it,
attached to the hard excrement. He found this membrane converted into
a jelly occupying great part of the stomach, which he conjectured was
the proximate cause of their death[940].
To conclude this head--_spiders_ are reputed to be subject to the
_stone_: I do not say _Calculus in Vesica_; but we are informed by
Lesser that Dr. John Franck having shut up fourteen spiders in a
glass with some valerian root, one of them voided an ash-coloured
calculus with small black dots[941].
II. I now come to that class of diseases which appears to prevail
almost universally amongst insects--I mean those resulting from the
attack of _parasitic_ enemies. Thus millions and millions annually
perish before they have arrived at their perfect state. Diseases of
this kind proceed either from _vegetable_ or _animal_ parasites. I
shall begin with the first, which will not occupy us long.
i. As insects pass often no small portion of their life in a state
of torpidity, in which they remain chiefly without motion, it
will not seem wonderful, should any partial moisture accidentally
accumulate upon them, that it affords a seed plot for certain minute
fungi to come up and grow in. Persoon observes with regard to his
genus _Isaria_, that one species grows upon the _larvæ_ of insects
(_I. truncata_), and another upon _pupæ_ (_I. crassa_[942]):--as
he does not say upon _dead_ larvæ and pupæ, as upon a former
occasion[943], perhaps in these cases these plants may constitute an
insect disease; but I lay no stress upon it, and only mention the
circumstance here as connected with the history of these animals. Mr.
Dickson has described a _Sphæria_ under the name of _entomorhiza_
that grows upon _dead_ larvæ; it has a slender long stipes and
spherical granulated head: on the pupa of a species of _Cicada_ in my
cabinet, another kind of _Sphæria_, with a twisted thickish stipes
and oblong head, springs up in the space between the eyes. I observed
something similar but longer, in the grub of some large beetle in
M. Du Fresne's museum at Paris; and I have a memorandum of having
noticed something of the kind on the rostrum of a _Calandra_. Bees
and humble-bees have been sometimes thought to have some species
of _mucor_ or other _Fungilli_ occasionally growing upon them; but
Mr. Brown is of opinion that _stamina_ which they have filched
from flowers have been mistaken for these _Fungilli_, since he has
detected those of _Orchideæ_ in some of this tribe, and upon a beetle
shown to him by Mr. MacLeay, one which he knew to be the stamen of
an _Aristolochia_. I once observed a bunch of what I mistook for a
singular _mucor_ that adorned the vertex of a humble-bee, between the
antennæ, which doubtless were of the same description; and I even
saw one upon its wing. Upon a former occasion I mentioned a parallel
circumstance with respect to a species of _Xylocopa_[944].
ii. The _animal_ parasites that infest insects are either themselves
_insects_; or _worms_.
1. Their _insect_ infesters, as far as we know at present, are
confined to the Orders _Strepsiptera_, _Hymenoptera_, _Diptera_,
and _Aptera_: they attack them sometimes in their _egg_ state, most
frequently when they are larvæ, occasionally when pupæ, and very
rarely in their perfect state. Upon many of these I have formerly
enlarged[945], and I shall now add such further circumstances as
I then omitted. The _Strepsiptera_ Order, as at present known,
consists only of two genera, _Stylops_ and _Xenos_; the first being
appropriated to the imago of _Andrena_, a kind of _bee_, and the
latter to that of the _wasps_. Their eggs appear to be deposited in
the abdomen of these insects in which they feed, till having attained
their full growth they perforate the membrane that connects its
segments; and at the proper time their pupa-case bursts, they emerge,
and take their flight. Sometimes four or five infest a single bee.
Whether the latter dies upon their quitting it I have not been able
to ascertain, but from their flying, when the little parasite is very
near leaving them, with their usual activity, it should seem that
this disease is not mortal; but it probably prevents their breeding:
I do not recollect observing the exuviæ of one in a _male_ bee[946].
The great body of insect parasites, however, belong to the
_Hymenoptera_ Order, and chiefly to the Linnean genus _Ichneumon_.
The insects of this order have been denominated _Principes_, because
of the wonderful instincts of ants, wasps, bees, and other gregarious
tribes that belong to it; and they merit a name of honour not less
for the benefits that they confer upon mankind, by keeping within
their proper limits the various insect-destroyers of the produce of
the globe. It deserves notice that when these latter increase to a
degree to occasion alarm, their parasites are observed to increase
in a much greater, so as to prevent the great majority of them from
breeding[947]. Though these benefactors of the human race constitute
numerous genera, at present not well ascertained, I shall speak of
most of them under the common name of _Ichneumon_.
The appearance of these little four-winged flies puzzled much the
earlier naturalists:--that a caterpillar usually turning to a
_moth_ or _butterfly_ should give birth to myriads of _flies_, was
one of those deep mysteries of nature which they knew not how to
fathom[948]: even the penetrating genius of our great Ray, though he
ultimately ascertained the real fact[949], was at one time here quite
at fault; for he seems at first to have thought, when from any defect
or weakness nature could not bring a caterpillar to a butterfly, in
order that her aim might not be entirely defeated, that she stopped
short, and formed them into more imperfect animals[950].
Before I detail more particularly the proceedings of Ichneumons, I
shall make a few general remarks upon them. The structure of the
instrument by which they are enabled to deposit their eggs in their
appropriate station has been before sufficiently described[951]; it
is long or short according to the situation and circumstances of the
larva which receives them: if this lives in the open air, and the
access to it is easy, it is usually _short_ and retracted within the
body; but if it lies concealed in deep holes or cavities, or shuns
all approach, it is often very long. Thus in _Pimpla Manifestator_,
which commits its eggs to the grub of a wild bee inhabiting the bottom
of deep holes bored in posts and rails, the ovipositor is nearly an
inch and half in length, and in some extra-European species three
inches. How the egg is propelled so as to pass in safety from the
oviduct, along this extended and very slender instrument to the grub
for which it is destined, has not been certainly ascertained: but from
an observation of Reaumur's[952] it should seem that it is aided in
its passage by some fluid ejected at the same time with it, or is so
lubricated as to slide easily without being displaced. The flies we are
speaking of, by some authors are called _Muscæ vibrantes_, because when
searching for the destined nidus of their eggs their antennæ vibrate
incessantly, and it is by the use of these wonderful organs that they
discover it wherever it lurks. Bergman observed that _Fœnus Jaculator_
searches for the latent grub of certain bees and other _Hymenoptera_
with its antennæ[953]: and from Mr. Marsham we learn that _Pimpla
Manifestator_, before it inserts its ovipositor in the nest of the grub
of _Chelostoma maxillosa_, explores it first with one antenna and then
with the other, plunging them all the while intensely quivering up
to the very root[954]. With respect to their _size_, Ichneumons vary
greatly; some being so extremely minute as to be invisible to the naked
eye, unless moving upon glass; while others, as to their _length_,
emulate the giants amongst insects. The former, unless appropriated
to the eggs themselves, usually commit many eggs to a single larva,
while the latter are directed by their instinct to introduce into
them only one. Some of the former description are endowed with the
faculty of leaping[955]. The food of Ichneumons, and indeed of other
internal parasites, is chiefly the _epiploon_ or fat of the larva, but
they never touch any vital organ; so that it continues to feed, and
probably more voraciously, grow, cast its skin, and often it changes to
a chrysalis, although at the same time inhabited by an army of these
little devourers.
Ichneumons, as far as has been at present ascertained, are parasitic
upon other insects chiefly in their _three_ first states, a solitary
instance only having been observed of their inhabiting an _imago_;
but from their first exclusion as eggs from the ovary till their
assumption of that state they give them no rest. I shall therefore
first treat of those that infest the _eggs_; next those appropriated
to _larvæ_; and lastly those that devour _pupæ_.