spiders, are the _Hydrophili_, a kind of water-beetles. Some of
these, as _H. lividus_, carry them about with them, like _Lycosa
saccata_, attached to the under side of their body, as M. Miger
observed[112]; and others when they are finished desert them. That of
the great water-beetle (_Hydrophilus piceus_) was long ago described
and figured by Lyonnet[113]; and a more detailed account of it has
since been given by M. Miger[114]. In form it somewhat resembles a
turnip when reversed, since it consists of a pouch of the shape of
an oblate spheroid, the great diameter of which is three quarters
of an inch; and the small, half an inch, from which rises a curved
horn, about an inch long and terminating in a point[115]. The animal
is furnished with a pair of anal spinners, which move from right to
left, and up and down, with much quickness and agility: from these
spinners a white and glutinous fluid appears to issue, that forms
the pouch, which it takes the animal about three hours to construct.
The exterior tissue is produced by a kind of liquid and glutinous
paste, which by desiccation becomes a flexible covering impermeable
to water; the second, which envelops the eggs, is a kind of light
down of great whiteness, that keeps them from injuring each other.
The tissue of the horn is of a silky nature, porous and shining, and
greatly resembling the cocoons of _Lepidoptera_. This part, contrary
to what Lyonnet supposes, appears calculated to admit the air, the
water soon penetrating it when submerged. At its base is the opening
prepared for the egress of the larvæ, when hatched, which is closed
by some threads, that, by means of the air confined in the cocoon or
pouch, hinder the water from getting in[116]. This nidus does not
float at liberty in the water till after the eggs are hatched, the
parent animal always attaching it to some plant. By means of this
anomalous process for a beetle, which this insect is instructed by
Providence thus to perfect, the precious contents of its little ark
are secured from the action of the element which is to be the theatre
of their first state of existence, from the voracity of fishes, or
the more rapacious larvæ of its own tribe, until the included eggs
are hatched, and emerge from their curious cradle.
I shall next amuse you with a few instances, in which the Allwise
Creator instructs the parent insect, instead of defending her eggs
with a covering furnished by her internal organs, to provide it from
without, either from her own body or from some other substance.
Most commonly, indeed, the female leaves her cluster of eggs without
any other covering than the varnish with which in this case they are
usually besmeared. Either they are deposited in summer and will soon be
hatched, or they are of a substance calculated to encounter and resist
the severities of the season. But many species, whose eggs are more
tender or have to resist the cold and wet of winter, defend them in the
most ingenious manner with a clothing of different kinds of substance.
_Cassida viridis_, a tortoise beetle, Rösel tells us, covers her
group of eggs with a partially transparent membrane. _Arctia Salicis_
F., a moth, common on willows, wholly conceals hers with a white
frothy substance, which when dry is partly friable and partly
cottony, and being insoluble in water effectually protects them from
the weather[117]. The female of _Lophyrus Pini_ (a saw-fly), having
by means of her double saw made a suitable longitudinal incision
in the leaf of a fir, and placed in it her eggs in a single row
end to end, stops it up with a green frothy fluid mixed with the
small pieces of leaf detached by her saws, which when dry becomes
friable: a necessary precaution, since these eggs are extremely
brittle[118]. _Arctia chrysorhœa_, _Hypogymna dispar_, and several
other moths, surround theirs with an equally impervious and more
singular clothing--_hair_ stripped from their own bodies. With this
material, which they pluck by means of their pincer-like ovipositor,
they first form a soft couch on the surface of some leaf: they then
place upon it successively layers of eggs, and surround them with a
similar downy coating, and when the whole number is deposited cover
the surface with a roof of hairs, which cannot be too much admired;
for those used for the interior of the nest are placed without order,
but those employed externally are arranged with as much art and skill
as the tiles of a roof, and as effectually keep out the water, one
layer resting partly on the other, and all having the same direction,
so that the whole resembles a well-brushed piece of shaggy cloth or
fur. When the mother has finished this labour, which often occupies
her for twenty-four hours, and sometimes even twice that period, her
body, which before was extremely hairy, is almost wholly naked--she
has stripped herself to supply clothing to her offspring, and
having performed this last duty she expires. The female moths which
thus protect their eggs are often furnished with an extraordinary
quantity of hair about the anus for this express purpose; and Reaumur
conjectures, that the singular anal patch of scales resembling those
of the wings, but considerably larger, which is found in the female
of _Lasiocampa Pityocampa_, is destined for the same purpose[119].
Reaumur had once brought to him a nidus of eggs clothed still more
curiously: they surrounded a twig in a spiral direction, like those
of _Lasiocampa Neustria_, but were much more numerous, and were
thickly covered with fine down, not pressed close, but standing off
horizontally, which assumed much the same appearance as a fox's tail
would if twisted spirally round a branch[120].
A procedure nearly similar was observed by De Geer in some species of
Aphides (_A. Alni_ and _A. Pruni_), which covered their eggs with a
white cottony down detached from their belly by means of their hind
legs[121]. In this case, however, the eggs were separately coated
with the down, but there was no general covering to the group.
Several insects make the leaves and other parts of plants serve as
coverings for their eggs. _Tenthredo Rosæ_ L., a saw-fly, and other
species of the same genus, with their saws make an incision in the
green twigs of shrubs and trees, and fill it with a line of eggs
placed end to end, taking care that, as the eggs grow after they are
laid, they are placed at such distances as to leave room for their
expansion[122]. _Rhynchites Bacchus_, a brilliant weevil, well known
to the vine-dressers for the injury it does[123], rolls with much art
the leaves of the vine, so as to form a cavity, in which it places
its eggs; other species practise similar manœuvres; and some probably
place their young progeny in the interior of twigs, making an opening
for that purpose with their rostrum--at least, I once saw _Rhynchites
Alliariæ_ L. with its rostrum plunged up to the antennæ in the twig of
a crab-tree. Others of this tribe, as we know, place their eggs in the
interior of fruits and grain, as the nut, acorn, and common weevils.
It is probable that most of the above coverings serve another
purpose besides the protection of the eggs from wet and cold--that
of sheltering them from the action of too great light, which, as Dr.
Michellotti by numerous experiments has ascertained, is fatal to
the included germe[124]. On this account it is perhaps that so many
insects fasten their eggs to the under side of leaves. Those exposed
in full day have usually an opaque and horny texture.
Some insects are spared all trouble in providing a covering for their
eggs, their own bodies furnishing one in every respect adapted to
this purpose. Not to mention the _Onisci_, or wood-lice, since they
rather belong to the _Crustacea_, which have a four-valved cell under
the breast, in which they carry their eggs, as the kangaroo does
its young in its abdominal pouch, the whole body of the female of
those strange animals the _Cocci_ becomes a covering for her eggs,
which it incloses on every side. To make this intelligible to you,
further explanation is necessary. You must have noticed those singular
immovable tortoise-shaped insects, which are such pests to myrtles and
other greenhouse plants. These are the young of a species of _Coccus_
(_C. Hesperidum_ L.), and _their_ history is that of the whole race.
Part of them never become much bigger than the size of which you
ordinarily see them, and when full-grown disclose minute two-winged
flies, which are the males. The size of the females, which glue
themselves to a twig or leaf as if lifeless, now augments prodigiously,
and the whole body, distended with the thousands of eggs which it
includes to the bigness of a large pea, without any vestige of head or
limb, resembles a vegetable excrescence or gall-apple rather than an
insect. If you remove one of them, you will perceive that the under
part of its abdomen is flat and closely applied to the surface of the
branch on which it rests, only a thin layer of a sort of cotton being
interposed between them. In laying her eggs the female _Coccus_ does
not, like most insects, protrude them beyond her body into day-light;
but as soon as the first egg has passed the orifice of her oviduct,
she pushes it between her belly and the cottony stratum just mentioned,
and the succeeding eggs are deposited in the same manner until the
whole are excluded. You will ask how there can be found space between
the insect's belly and the cotton, to which at first it was closely
applied, for so large a mass of eggs? To comprehend this, you must
consider that nearly the whole contents of its abdomen were eggs; that
in proportion as these are extruded a void space is left, which allows
the skin of the under side of the body to be pushed upwards, or towards
that of the back, affording room between it and the cottony web for
their convenient stowage. If you examine the insect after its eggs are
all laid, you will find that they have merely changed their situation;
instead of being on the upper side of the skin forming the belly, and
within the body, they now are placed between it (now become concave
and nearly touching the back) and the layer of cotton. As soon as the
female _Coccus_ has finished her singular operation she dies; but her
body, retaining its shape, remains glued upon the eggs, to which it
forms an arched covering, effectually protecting them, until they are
hatched, from every external injury. Some species lay so many eggs,
that the abdomen is not sufficiently large to cover the whole mass,
but merely one side of it, the remainder being enveloped in cottony
web[125].
* * * * *
I am next to consider the situation of those eggs that are excluded
by the mother in groups _without any_ other _covering_ than the
varnish with which they are usually besmeared in their passage from
the oviduct. The females only place them upon or near the food
appropriated to the young larvæ, to which they adhere by means of
the varnish just mentioned. These groups consist of a greater or
less number; and when the eggs are hatched by the heat of the sun,
the larvæ begin to disperse and attack with voracity the food that
surrounds them. It is thus that most butterflies and moths attach
their eggs to the stems, twigs, and leaves of plants; that the lady
birds (_Coccinellæ_), the aphidivorous flies (_Syrphi_ &c.), and
the lace-winged flies (_Hemerobii_), deposit them in the midst of
plant-lice (_Aphides_); that the eggs of some flesh-flies are gummed
upon flesh; those of crickets and grasshoppers buried in the earth;
those of gnats and other Tipulidans set afloat upon, or submerged in,
the water.
Frequently the whole number of eggs laid by one female is placed in
one large group, more commonly, however, in several smaller ones,
either at a distance from each other on the same plant, or on distinct
plants. The object in the latter case seems to be, in some instances,
to avoid crowding too many guests at one table, in others to protect
the unhatched eggs from the voracity of the larvæ first excluded, which
would often devour them if in their immediate neighbourhood.
In the disposition of the eggs which compose these groups much
diversity prevails. Sometimes they are placed without order in
a confused mass: more frequently, however, they are arranged
in different, and often in very beautiful modes. The common
cabbage-butterfly (_Pieris Brassicæ_) and many other insects place
theirs upon one end, side by side, so as, comparing small things with
great, to resemble a close column of soldiers, in consequence of which
those larvæ which, on hatching, proceed from the upper end, cannot
disturb the adjoining eggs. Many indeed have a conformation purposely
adapted to this position, as the hemisphærical eggs of the puss-moth
(_Cerura Vinula_), which have the base by which they are gummed
membranous and transparent, while the rest is corneous and opaque. The
same ready exit to the larva is provided for in the oblong eggs of the
emperor moth (_Saturnia Pavonia_), which are piled on their sides in
two or more lines like bottles of wine in a bin[126].
Where the larva does not emerge exactly from the end of the egg other
arrangements take place. The whirlwig-beetle (_Gyrinus natator_) and
the saw-fly of the gooseberry &c. (_Tenthredo flava_ L.) dispose
theirs end to end in several rows; the former upon the leaf of some
aquatic grass, the rows being parallel[127], the latter gummed to the
main nerves of gooseberry or currant leaves, the direction of which
they follow[128].
But the lackey-moths (_Lasiocampa Neustria_, _castrensis_, &c.)
adopt a different procedure. As their eggs, which are laid in the
autumn, are not to be hatched until the spring, the female does not,
like most other moths, place them upon a leaf, with which they might
be blown by the winter's storms far from their destined food, but
upon the twig of some tree, round which she ranges them in numerous
circles. If you examine your fruit-trees, you can scarcely fail
to find upon the young twigs collections of these eggs, which are
disposed with such admirable art, that you would take them rather
for pearls, set by the skilful hand of a jeweller, than for the eggs
of an insect. Each of these bracelets, as the French gardeners
aptly call them, is composed of from 200 to 300 pyramidal eggs
with flattened tops[129], having their axes perpendicular to the
circumference of the twig to which they are fastened, surrounding it
in a series of from fifteen to seventeen close spiral circles, and
having their interstices filled up with a tenacious brown gum, which,
while it secures them alike from the wintry blast and the attack
of voracious insects, serves as a foil to the white enamel of the
eggs that it encompasses. It is not easy to conceive how these moths
contrive to accomplish so accurately with their tail and hind feet an
arrangement which would require nicety from the hands of an artist;
nor could Reaumur, with all his efforts and by any contrivance,
satisfy himself upon this head. He bred numbers of the fly from the
egg, and supplied the females after impregnation with appropriate
twigs; but these, as though resolved that imprisonment should not
force from them the secret of their art, laid their eggs at random,
and made no attempt to place them symmetrically[130].
This illustrious Entomologist was more successful in discovering
the mode in which another insect, the common _gnat_, whose group of
eggs is, in some respects, as extraordinary as that last described,
performs its operations. The eggs of this insect, of a long
phial-like form, are glued together, side by side, to the number of
from 250 to 300, into an oblong mass, pointed and more elevated at
each end, so as considerably to resemble a little boat in shape.
You must not here suppose that I use the term _boat_ by way of
illustration merely; for it has all the essential properties of a
boat. In shape it pretty accurately resembles a London wherry, being
sharp and higher, to use a nautical phrase, fore and aft; convex
below and concave above; floating, moreover, constantly on the keel
or convex part. But this is not all. It is besides a _life-boat_,
more buoyant than even Mr. Greathead's: the most violent agitation
of the water cannot sink it; and what is more extraordinary, and a
property still a desideratum in our life-boats, though hollow it
never becomes filled with water, even though exposed to the torrents
that often accompany a thunder-storm. To put this to the test, I
yesterday (July 25, 1811) placed half a dozen of these boats upon the
surface of a tumbler half full of water; I then poured upon them a
stream of that element from the mouth of a quart bottle held a foot
above them. Yet after this treatment, which was so rough as actually
to project one out of the glass, I found them floating as before upon
their bottoms, and not a drop of water within their cavity.
This boat, which floats upon the surface of the water until the
larvæ are disclosed, is placed there by the female gnat. But how?
Her eggs, as in other insects, are extruded one by one. They are so
small at the base in proportion to their length that it would be
difficult to make them stand singly upright on a solid surface, much
more on the water. How then does the gnat contrive to support the
first egg perpendicularly until she has glued another to it--these
two until she has fixed a third, and so on until a sufficient number
is fastened together to form a base capable of sustaining them in
their perpendicular position? This is her process. She fixes her four
anterior legs upon a piece of leaf, or a blade of grass, and projects
her tail over the water. She then crosses her two hind legs, and in
the inner angle which they form, retains and supports the first laid
egg, as it proceeds from the anus. In like manner she also supports
the second, third, &c., all of which adhere to each other by means of
their glutinous coating, until she feels that a sufficient number are
united to give a stable base to her little bark; she then uncrosses
her legs, and merely employs them to retain the mass until it is of
the required size and shape, when she flies away, and leaves it to
its fate floating upon the water[131].
It may not be out of place to mention here a remarkable circumstance
which not seldom attends a kind of water-scorpion (_Naucoris_ F.)
occasionally to be met with in collections of Chinese insects. Its
back is often covered with a group of rather large eggs, closely
arranged; but whether these are its own eggs or those of some large
species of water-mite (_Hydrachna_ Maïll.) has not been clearly
ascertained. On the former supposition, the ovipositor must be
remarkably long and flexile to enable the animal to place the eggs
on its back. In confirmation of the latter it may be observed, that
the species of the genus _Hydrachna_ usually attach their eggs to the
body and legs of aquatic insects, as for instance _H. abstergens_ to
the water-scorpion (_Nepa cinerea_), &c.[132]
2. After having thus laid before you some of the procedures of those
insects that usually deposit their eggs in groups, either naked or
defended by coverings of various kinds, I next proceed to a rapid
survey of those of the species that commonly deposit them _singly_.
Some of these, as for instance the Admiral Butterfly (_Vanessa
Atalanta_), glue each egg carefully to its destined leaf by alighting
on it for a moment. Another butterfly (_Hipparchia Hyperanthus_)
whose caterpillar is polyphagous, drops hers at random on different
plants. In general it may be observed, that all those larvæ which
live in solitude, as in the interior of wood, leaves, fruits, grain,
animals, &c., proceed from eggs laid singly by the female, which
is usually provided with an appropriate instrument for depositing
them in their proper situation. Thus the nut-weevil (_Balaninus
Nucum_ Germ.) and also that of the acorn (_B. Glandium_) pierce a
nut or an acorn with their long beak, and then deposit in the hole
an egg, from which proceeds the maggot that destroys those fruits.
Leeuwenhoek asserts that the common weevil (_Calandra granaria_)
adopts the same process, boring a hole in every single grain of
corn before it commits an egg to it, and at the same time, by this
manœuvre, prepares a small quantity of flour to serve for the food
of the tender grub when it is first hatched[133]. It is probable
that the Rhyncophorous or weevil tribe in general chiefly use their
beaks for the purpose of depositing their eggs in different vegetable
substances, and perhaps principally in fruit or grain. The tribe of
gall-flies (_Cynips_) on the contrary, whose economy, detailed in a
former letter[134], interested you so much, bore an opening for the
egg with their spiral oviduct, which also conveys it.
Another large tribe of insects depositing their eggs singly, are those
which feed upon the bodies of other animals, into the flesh of which
they are either inserted, or placed so as speedily to find their way
into it. Some of these introduce them into _living_ animals, and then
leave them to their fate, as the _Ichneumons_ and gad-flies: others
deposit them along with the _dead_ body of an insect interred in a
hole, often prepared with great labour, as the different species of
sand-wasps (_Sphecidæ_), spider-wasps (_Pompilidæ_), &c.: the manners
of the latter of these tribes have been already adverted to[135], and
those of the _Ichneumonidæ_ will come more fully under consideration
when I treat of the _diseases_ of insects.
A similar labour in providing suitable habitations for their eggs
is undergone by various other insects whose larvæ live chiefly on
vegetable food, some inserting their egg within the substance the
larva devours, as those that prey on timber, twigs, roots, or the
like, and others on its surface. One would suppose at first, that the
exceedingly small egg which produces the subcutaneous larvæ would, by
the parent moth, be imbedded in the substance of the leaf which is
to exhibit hereafter their serpentine galleries: but this is not the
case, for she merely glues it on the outside; at least such was the
situation of the only egg of these very minute moths Reaumur had ever
an opportunity to observe[136].
Other insects, belonging to the tribe which lay their eggs singly, bury
them in the ground. Of this description are many of the lamellicorn
insects, the dung-chafers (_Scarabæidæ_ MacLeay) particularly, which,
inclosing their eggs in a pellet of dung, deposit them in deep
cylindrical cavities. Concerning the proceedings of some of these, as
well as of the whole race of bees, wasps, &c., which all lay single
eggs, I have before detailed to you many interesting particulars[137].
I must not conclude this subject without observing, that the female
_Pycnogonidæ_, an osculant tribe between Insects and _Crustacea_, carry
their eggs upon two pair of false legs[138].