ARTHROPODS AND THEIR CLASSES.--THE MARGINED
WATER BEETLE; THE GREAT WATER BEETLE;
THE COCKTAIL BEETLE
Having got together our apparatus, which, as we have seen, need be
neither costly nor complicated, the next step will be to acquire some
knowledge of the group from which the examples here treated of will be
taken--the Ar´thropods, or animals with hollow-jointed limbs. These
are the ‘Insects’ of the Linnaean classification, and, for the matter
of that, of popular phraseology; for though few people would now
venture to call a Lobster an ‘insect,’ we still style some of its near
relatives Water ‘Fleas,’ as Swammerdam did two hundred years ago.
The Arthropods form a phylum, or main division of the Animal Kingdom.
Above this phylum comes that of the Molluscs, or soft-bodied animals,
such as the Oyster, the Snail, and the Cuttlefish. Still higher are the
Lancelet, the Sea-squirts, and some few others, that bridge the chasm
between the phyla without, and that phylum with, a backbone. And to
this last Man himself belongs.
Two reasons contributed to the selection of the Arthropods as a subject
for work with the pocket lens: (1) the great interest which surrounds
many of the group; and (2) the ease with which specimens may be
procured and kept under observation.
Every one has pretty clear notions as to the general ‘make’ of a
Vertebrate or backboned animal. An Invertebrate animal has, of course,
no backbone or the semblance of one; the nervecord, where present, lies
on the under surface, and forms a ring round the gullet, and the heart
lies on the upper surface or back. We may verify this by pulling to
pieces a dead insect.
But a phylum, or main division, is much too large to be considered as a
whole. It must, therefore, be broken up into smaller groups, which are
called Classes, generally reckoned as five in number. These, again, may
be grouped into two divisions, according as their members breathe by
means of air-tubes (_tracheae_) or by gills. Our scheme then will
stand thus:--
{ { Peripatus.
{ Breathing by { Centipedes and Millipedes.
{ air-tubes { Insects.
ARTHROPODS { { Spiders and their kin.
{
{ Breathing by { Lobsters, Crabs, Sand-hoppers,
{ gills { and Woodlice.
This scheme looks well on paper; and on the whole is workable. But
among our examples chosen from the Class of Insects, we shall find some
that breathe by gills in their larval stage, and by air-tubes when
adult. And among the Crabs are some, the gills of which have ceased
to perform their normal function, so that these animals cannot live
in water for a single day. And then there are the Sand-hoppers and
Woodlice.
The body of an Arthropod may be represented by a series of similar
rings, thus:
[Illustration]
This similarity is clearly apparent in the Centipede, but is concealed
in the Beetle, the Shrimp, and the Spider. It seems, at first sight,
to be altogether lost in the Crab, and does really vanish in the adult
stage of some parasitic Crustaceans.
It may be plausibly objected that our ideal Arthropod resembles nothing
so much as a worm. In many respects this is true. A primitive Arthropod
was worm-like, as is a Centipede. And Arthropods and Worms were
formerly classed together in one group, as Annulo´sa or ringed animals.
The chief external difference lies in the nature of the appendages
borne by the various rings or segments.
We may represent those of the Worms thus [image], for they are
bristles, or groups, or modifications of bristles. Those of the
Arthropods may be represented thus [image], for the appendages are
really jointed, though, of course, in a fashion different from those of
a backboned animal.
The jointed appendages of Arthropods may be modified to fulfil very
different functions. They may serve as legs for walking, hands for
climbing or seizing prey, jaws for masticating food, feelers or organs
of touch and sense, and, strange as it may seem, in one group, as eyes.
It is well to get some notion of how these joints are formed. To take
the body first: the skin connecting the segments is much thinner than
that of the segments themselves, which is thickened by the deposition
of chitine, and, in some cases, also of carbonate and phosphate of
lime. A portion of the body, then, may be represented thus, [image]
where the heavy lines denote the segments, and the thin ones the spaces
between the segments. It will be seen that this arrangement allows
of considerable play, and also of a telescopic movement by which the
segments can be brought close together.
It is easy to construct a kind of model that shall exemplify these
movements. Make a tube of calico, some six inches long, and having
stuffed it with cotton-wool, paste on it strips of brown paper one inch
in width, leaving an interval between each, as in the last diagram.
Then we shall be able to understand how Arthropods can bend the body or
move it from side to side. And the limb joints are made on a similar
plan.
[Illustration: FIG. 14.--Cape Peripatus (natural size).]
The most archaic Arthropod--Perip´atus--must be mentioned. It is not
found in Britain, nor even in Europe; so that, unless we travel, we
shall only know it from books, or from museum specimens. But it is an
extremely interesting creature, for it is of worm-like aspect, and
breathes by air-tubes, opening all over the body, which has no external
segments. The limbs are imperfectly jointed, and each of them bears
two claws. Most naturalists make this genus a Class by itself, while
some put it with the Centipedes. There are about a dozen species, four
of which are African, two Australian, and the rest are found in South
America and the West Indies. Besides these there are some doubtful
species.
In habit they resemble the Centipedes, and they ensnare the insects on
which they feed by ejecting sticky slime from the small processes near
the mouth. The left process is shown in the illustration, just below
the antenna of that side.
Professor Sedgwick, who described these animals in the _Quarterly
Journal of Microscopical Science_ (1888), and, more popularly,
in the _Cambridge Natural History_, says, that ‘the exquisite
sensitiveness and changing form of the antennae, the well-rounded plump
body, the eyes set like small diamonds on the side of the head, the
delicate feet, and, above all, the rich colouring and velvety texture
of the skin, all combine to give these animals an aspect of quite
exceptional beauty.’
Unfortunately, an illustration in black-and-white can only render
form. We must take the beauty of the colouring for granted. One thing,
however, cannot escape the most cursory examination of the picture--the
resemblance of the creature, in some respects, to a worm, and, in
others, to a caterpillar, which, as everybody knows, is the larval
stage of a butterfly. If this resemblance sets us thinking how it came
about, and what it means, Peripatus will, for the present, have done
its work for us.
* * * * *
With these general notions of Arthropods, we may pass on to put our
pocket lens to some practical use. Our first subject shall be the
Margined Water Beetle (_Dytis´cus margina´lis_), which can be
taken in almost any open pond in the country. Water covered with
duckweed should be avoided in hunting for these beetles, which prefer
ponds with a clear surface, so that they may easily come to the top to
breathe.
Every one has a good general notion of the principal Insect-groups,
technically called Orders--Beetles, Cockroaches and Grasshoppers,
Butterflies, Bees and Wasps, and Flies. Insects may be defined
as animals with hollow-jointed limbs, and divided into three
regions--head, thorax, and abdomen. The head bears a pair of antennae;
the thorax carries three pairs of legs, and (generally) two pairs of
wings; the abdomen is without appendages. Insects when adult breathe
by tubes that open to admit air. In Chapter VI we shall see that many
larvae obtain an air supply in different ways.
[Illustration: FIG. 15.--Margined Water Beetle (male).]
Beetles may be taken as very good types of true Insects. They
constitute the Order Coleop´tera, or Insects with sheathed wings, only
the hinder pair being used for flight (Fig. 18), and at other times
they are folded under the wing-cases, or el´ytra, as in Fig. 15.
We may advantageously compare our Beetle with Peripatus, and note the
points of agreement and of difference.
Now, if our captive Beetles are to yield us the greatest possible
amount of profit, we shall keep them under observation for some time,
so as to watch their habits.
In keeping these Beetles we shall not require a large aquarium. A small
gathering of aquatic weed will be necessary to keep the water in good
condition and the aquarium ready for its tenants.
My interest in these Beetles was quickened by a letter in the
_Field_ (Oct. 28, 1893), in which a correspondent at Weybridge
asked ‘for information as to what animal or bird bisects so neatly the
shells of the Water Snail (_Planorbis_).’ I thought then, and
know now, that the shells were ‘bisected,’ if that is the proper word,
by Water Beetles. From that time I have had, and still have, several
living in small aquaria, but for a long time was unable to get direct
evidence on the subject.
[Illustration: FIG. 16.--Shells of Molluscs broken up by
Dytiscus.
(From a photograph by Cherry Kearton.)]
Many experiments were tried, and at last these proved successful.
Several specimens of Dytiscus[7] were obtained, and put into a small
aquarium in which was no other food for them than some snails and other
molluscs. The Beetles were carefully watched, and were several times
seen trying the snails. In crawling along the inner surface of the
glass, Planorbis and Limnaea both protrude the foot to a considerable
extent, and pieces were ripped out by the strong mandibles of the
Beetles before the shells were actually broken up.
All the shells represented in Fig. 16 were taken from this aquarium,
so that there is good evidence as to what creatures broke them up and
devoured their inmates. In these, as in the specimen kindly sent me by
Mr. Tegetmeier, the Natural History Editor of the _Field_, the
bisection is not complete, though in all cases it is carried far enough
to allow of the extraction of the mollusc. The large Limnaea shell in
the centre has been attacked, but it seems to have been left when the
beetles discovered it was empty. (The empty shell was noted before the
Beetles were put into the tank.) Another Limnaea shell is figured, from
which the snail has been picked out, and that of a fresh-water mollusc.
After these observations had been recorded in the _Field_[8], I
found that I had been anticipated by about forty years. I picked up,
at a bookstall, a copy of G. B. Sowerby’s _Popular History of the
Aquarium_, and there I found that the author had distinctly seen
Dytiscus at this kind of work. He says[9]: ‘I have only once witnessed
him in the act of seizing an unfortunate Planorbis or Flat-coiled Water
Snail. At first, the Dytiscus seemed to be roaming about in quest
of something, first under, then over, the leaves of a water-lily.
At last, in a rather dark corner, he seemed to perceive suddenly a
Planorbis which was browsing upon the stem of a plant just under the
shade of a broad leaf. He darted at this, seized it, and then, putting
his tail out of water, for the purpose of taking in a fresh supply of
air, moved slowly down, bearing the snail with him. He held it by his
fore-feet, turning round the coil until the aperture of the shell was
opposite his mandibles, then he began nibbling away at the animal. In
vain did the poor mollusc try to withdraw within its shelly fortress,
for the beetle picked off the edges of the shell bit by bit, so as to
expose the body as fast as it was withdrawn. All the way down to the
bottom of the tank was this process continued, air-bubbles rising to
the top, and bits of broken shell falling, till the beetle with his
burden reached a stone near the bottom, where I left him still busy at
his work.’
This puts the matter beyond doubt, if any before existed. I at once
wrote to Mr. Tegetmeier to let him know that my experiments had,
unknown to me, been anticipated, long ago, by Mr. Sowerby. Had
he rescued his Planorbis shell, it would have compared very well
with those forwarded to the _Field_ office in 1893. They had
been exhibited at the Malacological Society, and no one was able
to solve the mystery of their mutilation. This shows, to quote the
_Field_[10] on the subject, ‘how easily statements that have been
recorded may subsequently be overlooked and entirely forgotten.’
To return to our Beetle. The male is a handsome creature, from an inch
to an inch and a quarter long, clad in olive-green, bordered with
yellow, and exceedingly active. His mate is smaller, more soberly clad
in brown, without the yellow markings, and the wing-cases are more or
less furrowed.
The first thing to notice is the shape of the body, oval and smooth,
offering no resistance to the water. The hind pair of legs are
flattened and fringed with hairs, so as to make capital paddles. In
swimming the right and left legs are moved together.
Now, though this Beetle lives in the water, it is made, so far as
concerns its breathing apparatus, after the fashion of a Land Beetle,
and consequently is compelled to come to the surface pretty frequently
for a supply of air, which it obtains in this wise. Directly it ceases
paddling it floats to the top of the water; and as the head is heavier
than the tail the latter projects a little above the surface. Then the
wing-cases are raised, and air flows in under them to the breathing
holes on each side. The operation is not a long one, and as soon as it
is over the Beetle is ready for another ramble round his dwelling-house.
But if we do not supply our captive with food that he may take for
himself, it is only right that we should feed him, which may be done at
intervals--say, every other day. ‘Little, and often,’ is an excellent
motto to guide us in our feeding; and though its adoption may entail
some trouble, it will be more than compensated by the success that
will attend our endeavours to keep the inmates of our aquarium in good
condition. And the operation of feeding our Beetle will show us that he
has some capital sense-organs, which are of as much, if not of more,
use to him than his eyes.
He is a flesh-eater. Let us take a small piece of meat or fish in a
pair of forceps, or stuck on a pointed stick, and hold it at a little
distance from his great eyes. The chances are that he will not see it.
Even if we put it in front of him, he is quite likely to disregard it,
for he has nothing corresponding to a nose, with which he may smell.
From his head there spring a pair of long feelers--the antennae--and by
means of these we will let him know that his dinner is ready. That is
effected by drawing the food along the side of one of the antennae. The
creature undergoes a sudden change. Till the antenna was touched with
the food he was resting on his swimming legs. But in a moment down goes
his tail and up goes his head, he stretches out his raptorial legs, and
clutches wildly at the forceps or stick, as the case may be, holding so
tight that he may be dragged round and round the glass vessel. Let go
he will not, of his own accord; and it would be a difficult matter to
shake him off. Similar experiments may be tried with other Beetles, and
the result will be to impress on the mind the fact that the feelers are
capital sense-organs.
If we are to turn our Beetle to the best account, we shall need to
handle him. It may be inconvenient to wait till he dies, so we will
kill him quickly and painlessly by plunging him into boiling water, and
he may be preserved by putting him into a tube containing about equal
parts of water and spirit, or a five per cent. solution of formalin.
Dissections should properly be made under water. The Beetle should be
fastened, back upwards, to a piece of cork weighted with lead, and
placed in a deep saucer, or dissecting dish, and covered with water.
But a good deal of rough dissection, as is ours, may be done in air,
and the Beetle may be fastened to any convenient piece of board, or
even held in the palm of the left hand. Very little practice is needed
to run over the external parts of a large Beetle in this manner.
[Illustration:
FIG. 17.--Outline of Dytiscus (male). _a_,
antenna; _b_, maxillary palp; _c_, eye; _d_,
fore-leg; _e_, thorax; _f_, middle leg; _g_,
elytron; _h_, suture; _i_, hind leg; _j_, claw;
_k_, tarsus or foot; _l_, tibia or shank; _m_,
femur or thigh; _n_, first three joints of foot, widened
into a plate with suckers beneath.]
First, let us look over our Beetle, and get some general notions of
its make. As it lies, back upwards, it is clear that it consists of
three parts or regions------------[image], the first of which is the
head, the second the thorax, and the third the abdomen. Not only in our
Beetle, but in Insects generally, these parts correspond to the words
that denote them, in that the thorax is longer than the head, and the
abdomen longer than the thorax, as shown by the three dashes, a few
lines above.
These divisions are well shown in Fig. 17, where other parts are also
marked. It will pay to go over our own specimen with this figure before
us, and so make acquaintance with the several parts, to some of which
we shall return in greater detail.
[Illustration: FIG. 18.--Male Dytiscus in flight.]
At this point, if we have not done so before, it will be convenient
to fasten our Beetle, in the position figured, by a stout pin driven
between the thorax and the abdomen, just above the suture (_h_).
We want to raise one of the wing-cases.
If a needle be taken in each hand, between the thumb and first two
fingers, and that in the left hand be used to steady the creature, the
wing-case on the right may be raised with the needle in the right hand,
and then cut off. The small filmy membrane, of somewhat triangular
shape, which comes off with the wing-case, is the winglet. There is
one on each side; and their vibration causes the humming noise made by
these insects in flight. When the water dries up in one pond, or food
becomes scarce, they will leave and fly off to another.
The wing lies folded upon the abdomen. A good deal of very interesting
matter has been written on the way in which Insects fold their wings,
but we can see for ourselves how this Beetle folds them. All we have to
do is to take the wing, and draw it gently away from us, and so unfold
it. We may use finger and thumb, or a small pair of forceps. When let
go, it will spring back to its old position. Reference to the expanded
wing in Fig. 18, and to the diagrams Figs. 19 and 20, will show how the
wing is folded.
[Illustration: FIG. 19.--To show fold of (right) wing of
Dytiscus.]
[Illustration: FIG. 20.--To show fold of (right) wing of
Dytiscus.]
The cross-mark in the diagram represents a joint in the chitinous rod
that forms the wings. This lies just above the cell (which is left
white in Fig. 18). The shorter part of the rod is bent down, forming an
acute angle (Fig. 20); of course, carrying with it the membranous part
of the wing.
This may seem a little difficult. But if it be tried on a specimen, no
real difficulty will be experienced. When the wing has been unfolded,
it will, if let go, spring back to its old position, the shorter part
lying underneath, and the chitinous rod fitting into a groove formed by
the projecting sides of the segments of the abdomen.
To this point the sum of our knowledge about Dytiscus amounts
to this: It is aquatic in habits; its body is divided into three
regions; and it has a pair of membranous wings, covered by chitinous
wing-cases, or sheaths, technically called _el´ytra_ (each being
an _el´ytron_). Wing-cases of this kind are the distinguishing
mark of the Beetles, or _Coleop´tera_, though they are not always
so well developed as in the specimen with which we are dealing. This we
can discover for ourselves by examining all the Land Beetles met with
in a country ramble or in a stroll round the garden.
Now let us unpin our Beetle, turn it on its back, and examine it from
the under side. Head, thorax, and abdomen may be made out more clearly
than before, and we can see that the last two regions are divided into
segments.
Let us deal with the head first. This may be easily separated from the
thorax with a dissecting needle, or with a pocket-knife--an exceedingly
handy tool. The huge goggle-eyes cannot escape observation; and, even
without a magnifier, they may be seen to be compound--that is, made up
of a number of facets, which show like a fine network.
Just in front of the eyes are the antennae, which serve as organs of
touch and perhaps also of other senses.
Kirby has recorded facts which seem to show that the antennae (in
some cases) are also organs of hearing. Other authorities, after many
observations, have come to the same conclusion. The matter, however,
is beset with difficulty. It is certain that some Insects have their
ears in their legs; and for the present, at any rate, we may be
satisfied to know that the antennae are sense-organs, certainly of
touch, probably of smell, and, in some cases, of hearing. An excellent
authority on the subject is Sir John Lubbock’s book, _The Senses of
Animals_[11], which contains references to very many original papers.
[Illustration: FIG. 21.
FIG. 21.--Upper surface of head of Dytiscus. _a_,
labrum, or upper lip; _b_, clypeus or shield; _c_,
mandible dissected out, and (_d_) reversed; _e_, eye;
_f_, antennae.]
[Illustration: FIG. 21 A.
FIG. 21 A.--Under surface. _a_, mentum or chin; _b_, ligula
or tongue; _c_, labial palp; these three together forming
the labium, or lower lip; _e_, eye; _f_, antennae. Above the
maxillae, or lower jaws (_d_ _d_), are shown dissected out:
_d^1_, inner or palpiform lobe; _d^2_, maxillary palp; _d^3_,
lacinia or blade; _d^4_, the palpifer or piece that bears the
palp (_d^2_); _d^5_, stipes or stalk; _d^6_, the cardo or hinge.]
Now we may pass to the mouth parts. It will be good practice to dissect
these out, either in air or in water. We may hold a Beetle between the
finger and thumb of the left hand, and separate all the parts with a
needle held in the right. It is a good plan to gum these parts on a
card, for comparison with the figures in our favourite book--whatever
that may be--on Natural History, and also with the mouth parts of
insects of other Orders. For however much these may differ in form,
and in the uses to which they are put, they are really modifications
of the same parts.
In Fig. 21 we have the upper side and in Fig. 21A the under
side of the head represented, so that we may easily get acquainted
with the different parts, and the names given to them. The cut should
be gone over several times, and the parts in the picture compared with
those in the specimen under consideration. It is good practice to
endeavour to draw what is seen from the specimen itself, and then to
compare the result with the work of the trained artist. And the mouth
parts of Dytiscus may be compared with the mouth parts of the Cockroach
(Fig. 33).
[Illustration: FIG. 22.--Disposition of mouth parts.]
Returning to practical work, the first thing is to separate
the _labrum_, or upper lip, from the head. Then the large
_mandibles_ should be dissected out, and cleaned (by soaking in
caustic potash) from the muscles which will come away with them. Behind
these are a smaller pair of jaws, the _maxillae_, furnished with
a pair of palps, called maxillary palps from their position. These are
to be dissected out; and then the lower lip, or _labium_, may be
separated by passing a sharpened needle along the line where it joins
the chin. The palps on the lower lip are called labial palps.
When these parts are cleaned and dried, they should be gummed on card,
as shown in Fig. 22, where the long lines represent the upper and lower
lips respectively, and the shorter ones the mandible and maxilla of
each side.
So much for the head. Now we discover that what appeared to be the
thorax, when we were looking at the upper surface of the Beetle, and
what is called the thorax in descriptions of Beetles, is really but
a portion of that region, which is seen to be divided into segments.
The covering on the upper surface protects only the first segment,
the middle and hinder ones being covered by the wing-cases and the
_scutellum_ (a triangular piece jutting backward from the second
segment, and meeting the suture). This is not represented in Fig. 17;
but we may put in with our pen a tiny triangle, with its base towards
the head, and its apex towards the tail--this will meet the case.
The first segment bears no appendage above, but to the under side is
attached the first pair of legs. The middle segment also carries a pair
of legs, and on its upper surface are the wing-cases, to the under side
of which, and to the body, the winglets are joined. The last segment
bears the wings above, and the last pair of legs below, these being
placed very far back, so as to give them greater power in propelling
the animal through the water.
It will be convenient to examine the legs next. First, however, it
will be well to look at a normal leg of an Insect (the Cockroach), and
learn the names of the different parts. First comes the _coxa_ (_a_) or
haunch, next the _trochanter_ (_b_), then the _femur_ (_c_) or thigh,
the _tibia_ (_d_) or shank, and the _tarsus_ (_e_) or foot, ending in
a pair of claws. There are three pairs of legs in perfect Insects, and
usually the same number in larval forms, though in some of these legs
are entirely wanting.
[Illustration: FIG. 23.--Leg of Cockroach.]
In the males of the Margined Water Beetle and many of its near
relations the first pair of legs deserve special attention. The first
three joints of the tarsus have coalesced to form a disk or cup, which
in our specimen bears two smaller ones on its inner surface. A power
of 20 will show the disk nearly as well as it appears in Fig. 24.
The purpose of this disk, or clasper, which is absent in females, is
obvious. It was formerly supposed to act as a sucker, but Professor
Lowne and Professor Miall[12] have shown that it does not act by
atmospheric pressure, but by a viscid secretion discharged from the
cup-like hairs with which the inner surface is set.
[Illustration: FIG. 24.--Tarsus of Dytiscus (magnified).]
The middle pair of legs in the male also bear cup-like hairs on the
corresponding joints of the tarsus, and in very much greater number.
Professor Miall quotes Simmermacher to the effect that while the large
disk on the fore-leg has 170 sucking-hairs, the enlarged joints of
the tarsus of the middle leg bear no less than 1590. These hairs are
plainly discernible with the half-inch Steinheil, and I have made them
out with the inch, and think that I could show them to anybody else
with that power. I have not looked for these sucking-hairs on the
middle leg of other Beetles of the same family which have disks on
their fore-legs, but they do exist in some other genera.
If we watch a male Dytiscus in life, in a small aquarium, we shall soon
be convinced that Lowne and Miall are correct in their statement that
the cup-hairs discharge an adhesive substance. We shall see this all
the more plainly if there is much floating vegetation. For, in swimming
about, the Beetle will often come in contact with some of this, and it
will adhere to the cup-hairs. His struggles to free himself from the
encumbrance will show that the attachment is not altogether under his
control. The offending weed is rubbed against the spines of one of the
other legs till it is removed.
[Illustration: FIG. 25.--Female Dytiscus swimming.]
The spines with which the legs are set are worthy of a good deal of
attention, and, like the adhesive cup-like hairs, though in different
fashion, they doubtless assist the animal in holding its prey. The
first and middle legs end in strong claws; those of the last pair are
not so well developed.
The last pair of legs are the swimming organs. The tibia and tarsus
are fringed with long stiff hair behind, so as to hold the water
when the Beetle swims. A peculiar arrangement of the first joint of
the tarsus allows the edge to be presented to the water when the limb
is carried forward for the return stroke, thus offering the least
possible resistance. This Dr. Sharp has compared to the action of a
rower in feathering his oar. There is, however, this difference, which
it is well to note. The oar is feathered after the stroke; the Beetle
feathers its legs before the stroke. It is the first motion when it
begins to swim, and the action is not peculiar to the male.
We now come to the third region, the abdomen. Like the thorax it is
visibly divided into segments, though the division between them is
not so great. Much difference of opinion exists as to the number of
segments in the abdomen of a typical insect. Some authorities maintain
there are eleven, while others put the number as low as five. This,
however, is theoretical rather than practical. It is enough for us
to know that the number apparently varies greatly, owing to the
coalescence of two or more of the segments.
[Illustration:
FIG. 26.--Upper surface of abdomen of typical Beetle.]
The head in Insects, we have seen, carries the eyes, antennae, and
feeding organs. The thorax bears the legs and wings. The abdomen bears
no appendages, except in some cases, on the last segment; these are
called _cerci_. It may be, however, that the stings of bees and
the ovipositors of saw-flies and other insects are modified appendages.
On examining the abdomen of Dytiscus we shall probably be struck with
the difference in appearance between the upper and the under surfaces.
The latter is hard, smooth, and shiny; the former, when the wings are
removed, is seen to be covered with felt-like hair.
Our interest is with the upper surface. Along the abdomen on each
side lie spiracles, stigmata, or openings to the breathing tubes. The
first and last are larger than the rest, and their general form can be
readily made out with an inch magnifier, and with the half-inch we may
get some idea of the detail shown in Fig. 27.
[Illustration: FIG. 27.--Spiracle of Dytiscus
(magnified).]
Dytiscus breathes in this way. Floating up to the top of the water,
the end of the abdomen projects above the surface. If one watches the
Beetle the wing-cases will be seen to rise a little. The air retained
by the felted hairs is given off, and a further supply taken in. Then
the wing-cases are lowered again; the Beetle gives two or three strokes
with its swimming legs, and descends below the surface to ramble round
the tank in search of food.
[Illustration: FIG. 28.--Tracheal tubes (magnified).]
This air-supply between the wing-cases and the abdomen is taken in at
the spiracles and distributed through the tracheal tubes throughout
the body. These tubes branch and subdivide till they end in small
twig-like vessels comparable to the capillaries of the human body.
They consist of two layers--the inner strengthened by what probably is
a spiral fibre, though Packard believes that, in some cases at least,
it consists of similar rings. But we must not pursue this subject. It
would lead us beyond our appointed limits.
* * * * *
Another Beetle fairly common in stagnant waters round London and in
the southern counties is that to which the name Great Water Beetle
(_Hydroph´ilus pic´eus_) of right belongs. This name is sometimes
wrongly applied to Dytiscus, with which its rightful owner has
little in common, except its aquatic habitat. Its scientific name is
_Hydrophilus piceus_; but we shall speak of it as Hydrophilus.
It is not a very easy matter to take this Beetle with a net, by
sweeping in the ordinary way, for it likes to get into the middle of
a mass of vegetation, where it is sure of a good food supply, and is
probably safe from the attacks of Dytiscus, who not unfrequently makes
a meal of his larger relation. A good plan is to pass the net under a
mass of weed and then shake it to and fro in the water. By this means
any Beetles in the weed will be dislodged from their hiding-places, and
fall down into the bottle.
They have, in confinement, the same habit of making a snug place
for themselves; and more than once I have fancied that a Beetle of
this species had escaped from the aquarium, when all the time it was
hidden in a thick patch of water-moss. They are practically vegetable
feeders, though Dallas says that they are not such strict vegetarians
as to deny themselves a meal of animal food when they meet with a dead
mollusc or larva in the course of their wanderings. I have never known
them to indulge in animal food, dead or living, but I have known them
refuse it.
Hydrophilus is the largest British Water Beetle, and, with the sole
exception of the Stag-Beetle, the largest British member of the Order.
Its total length is very little less than two inches, and across the
middle of the back it measures about half as much. It is more slenderly
built than Dytiscus, and the contrast in the size and armature of the
legs is very striking (Fig. 29). There is also a great difference in
their method of progression through the water. Dytiscus moves both legs
simultaneously, while Hydrophilus walks rather than swims, moving one
leg after the other.
If we cannot collect this Beetle for ourselves--which we should
endeavour to do, if possible--it may be bought of almost any dealer
in what are called ‘aquarium requisites.’ But prices rule higher
for Hydrophilus than for Dytiscus. Bateman says that this species
is rarer than formerly, and that specimens cost from 1_s._ to
2_s._ 6_d._ a pair, ‘according to the dealer and the season.’
From this I gather that I must have gone to a shop where the prices
were reasonable, for I have never paid more than 6_d._ for a
Hydrophilus, and then have been allowed to pick out a male. At the same
shop I have paid 2_d._ for Dytiscus.
[Illustration: FIG. 29.--Great Water Beetle. _a_,
male; _b_, female; _c_, larva; _d_, pupa.]
In keeping this Beetle we shall need a larger vessel than was required
for Dytiscus. (In both cases the aquarium should be covered, for if
food be scarce, and sometimes for other reasons, both these Beetles
may take to flight.) The aquarium should be well supplied with growing
water-weed, but none that is choice or valuable should be put in, for
in moving about over the weed the animal will damage almost if not
quite as much as it eats. This difficulty can be easily got over by
supplying it with anacharis, water-crowfoot, milfoil, or any other
common plant that grows rapidly and is easily procurable.
The only specimen that I have taken myself was captured a few miles
north of London. It exhibited a strange instance of depraved appetite.
In the large tank into which it was put were growing vallisneria,
frog-bit, and water-crowfoot in plenty. These it was never seen to
touch. The tank, at one time, had been used for newts, and floating
on the surface was a piece of virgin cork. It had served the former
inmates as a kind of island continent, and had never been removed. To
the under side of this the Beetle would moor himself, head downwards,
and nibble away, as if cork were the natural diet of a British Water
Beetle.
In a few days the Beetle died. It was put into spirit, and soon after
became the subject of a post-mortem. But its strange diet was not the
cause of its death, which was sufficiently accounted for by injuries
inflicted before its capture, probably by a larval or an adult Dytiscus.
It would be mere waste of time to go over this Beetle and describe it
point by point, as was done with Dytiscus. If what was there written
was of any value, readers will be able to apply for themselves the
method laid down. There are, however, some points of difference to
which it will be well to invite attention.
It is a good plan to lay specimens of these Beetles side by side
for comparison. Hydrophilus is the larger of the two; and differs in
colour as well as in size. Its hue is black with an olive tinge; and
in certain lights a blue-black metallic gloss may be seen on the outer
margins of the wing-cases. These are marked with faint longitudinal
lines, and each bears three rows of dots running in the same direction.
The greater length and more slender build of the legs of Hydrophilus
are at once apparent. There is also a marked difference in the tarsal
joints of the fore-legs of the male. The disks and cup-like hairs of
Dytiscus are absent in Hydrophilus, but in their stead the last joint
bears a sub-triangular plate, studded on the inner surface with spines,
which probably serve a similar purpose. A great deal of valuable
information about organs of this kind and their functions will be
found in chapter X of Darwin’s _Descent of Man_. Simmermacher’s
paper[13] should be consulted by all who have the opportunity. Our inch
magnifier will show us these spines quite clearly; and also a curious
little bunch of bristles, which Simmermacher says are probably organs
of touch.
It is a good plan to take Hydrophilus out of the water, and lay it upon
its back, so that the difference between it and Dytiscus may be clearly
seen. The Beetle should be handled carefully, for on the thorax is a
kind of keel, ending in a sharp spine, which extends over part of the
abdomen. This spine is free, and may easily wound the hands of those
who do not watch the motions of the creature pretty carefully. The fore
part of the abdomen and the thorax are covered with short close hairs,
and when the Beetle is in the water these parts entangle a layer of
air, which gives it the appearance of being covered with quicksilver.
The two Beetles differ also in their method of exchanging impure for
pure air. Dytiscus, as we have seen, takes in a fresh supply under
its wing-covers behind; Hydrophilus takes in a fresh supply in front,
employing for this purpose the antennae, which apparently do not
function as feelers, as is generally the case.
When Hydrophilus wants to take in a supply of pure air, it rises to
the top of the water, slowly and deliberately. Unlike Dytiscus, it
is never in a hurry. Then one of the antennae is pushed through the
surface film, thus communicating with the air, which descends to the
hair-covered thorax, whence it reaches the spiracles on the upper
surface of the abdomen. To allow of this the wing-cases are slightly
raised in front. The spiracles in Dytiscus are larger at the posterior
end of the abdomen: in Hydrophilus the largest spiracles are in front.
This is what might be expected, from the method adopted in each case
for procuring a fresh supply of air.
These Beetles have frequently bred in confinement; but no better
account than that of Lyonnet has ever been given of the operation of
the female in making her cocoon and depositing her eggs. As his account
is not generally available, a condensed translation of it is inserted
with his illustration.
[Illustration: FIG. 30.--Female Hydrophilus constructing
a cocoon. (After Lyonnet.)]
Lyonnet[14] wanted to find out how the female made the cocoons (Fig.
30), and this is how he set to work. He put some of these Beetles into
a large aquarium, with a good quantity of water and some duckweed.
On May 31 and the following day he noticed that one of the females
was swimming about in every direction, as if in search of something.
Thinking that this was because she had not the proper materials for her
work, he then put into the aquarium some thread-like alga of a kind
which he had seen attached to some cocoons, and on June 3 the Beetle
began to make a cocoon, but soon gave up the task, apparently because
she was troubled by other aquatic insects which had made a home in this
weed. These intruders were removed, and the Beetle set to work once
more. Lyonnet then noticed that, like a spider, she had her spinning
apparatus at the posterior end of the body. She extended the last
segments slightly, and opened the hindmost one, when he saw a nearly
circular opening, in which was a whitish disk (Fig. 30A,
_a_). On this disk were two little brown tubercles side by side,
nearly at right angles to the longitudinal axis of the body. From each
there projected a blackish-brown conical tube, about a line long, stiff
towards the base, but flexible and elastic towards the tip. These tubes
were the spinnerets, which acted together with a parallel movement, and
from each proceeded a separate thread.
And this is how she made her cocoon. She lay near the surface of the
water back downwards, the under part of the body and the second and
third pair of legs buried in the thread-like weed. The front legs were
free, and with these she shaped the weed over her abdomen. Then she
spun a covering of white silk against the under side of the weed. While
she was spinning, from time to time she used her front legs to press
and flatten the work against her body (Fig. 30B), giving it
the shape of a flattened arch, to which her body gave the requisite
curve. This, forming the top of the cocoon, was finished in about half
an hour. Then she turned (Fig. 30C), and spun the bottom of
the cocoon, moulding this, like the top, on the curve of her abdomen,
and uniting the top and bottom with silk which she spun. The work
occupied about an hour and a quarter.
The Beetle then remained nearly in that position for some two hours. At
first she was hidden in the cocoon quite up to the thorax. The body,
however, was withdrawn almost imperceptibly. During this time she was
busy laying her eggs in regular order, with the pointed ends upwards.
After this she came out of the cocoon, and closed the mouth (Fig.
30D), making the opening smaller by degrees. Then she made a
little mast (Fig. 30D, _b_), of the use of which Lyonnet
admits his ignorance, suggesting, however, that its construction may
serve to use up the silky matter remaining after the work is finished,
lest it should acquire harmful qualities in the body of the Beetle. The
true explanation seems to be that it serves to convey air to the eggs
inside the cocoon.
On July 17 Lyonnet was rewarded for his patient watching by seeing
a larva come out of the cocoon, and the next day some fifty more
appeared. What he saw and recorded it is in the power of others to see,
if they will imitate his patient observation.
The Cocktail Beetle, or Devil’s Coach Horse (_Ocypus olens_), is
an excellent specimen of a Land Beetle to examine, for it is of fairly
large size and extremely common. Moreover it does well in captivity, so
that there will be no difficulty in watching its habits in life, and
pickling it for closer examination when dead.
During the day these animals usually lie concealed under stones or
pieces of earth, coming forth at dusk and during the night in search
of food. Occasionally, however, they may be met with in daylight,
leisurely stalking a smaller beetle or a fly; then with a dash seizing
the victim in their powerful mandibles, which are quite capable of
making an impression on the human skin, as those who handle these
Beetles unwarily will discover for themselves.
[Illustration: FIG. 31.--Cocktail Beetle. _a_,
larva; _b_, pupa.]
Nothing of an animal nature comes amiss to them, and if they cannot
capture living prey, they will make a hearty meal off carrion. This is
an advantage to us, for we may feed our captives with dead insects or
with small pieces of meat.
This Beetle is about an inch long, and of a deep dull black colour.
The head is joined to the thorax by a distinct neck, and the abdomen
is naked, owing to the fact that the wing-cases are very short. Its
wing-cases bear about the same proportion to those of the Margined
Water Beetle that a man’s frock-coat bears to a boy’s Eton jacket. And
this Beetle may be taken as a good type of a group--the Beetles with
short wing-cases (_Brachel´ytra_).
The attitude of this animal when irritated or alarmed is well depicted
in Fig. 31. It raises its head menacingly and opens its strong
mandibles to their full extent, at the same time turning up the end of
the abdomen, like a scorpion about to sting. From the last segment it
will often put forth a pair of white vesicles, from which is discharged
a volatile liquid of disagreeable odour, that probably acts as a
defence against insect-eating creatures.
The best way to capture one of these Beetles is to pick it up with what
Kirby calls the ‘natural forceps’--the finger and thumb. It may be
dropped into any convenient receptacle; the small metal boxes in which
vestas are sold will answer the purpose very well.
My specimen was given me by a friend, who kept it with another in a
round tin box. It lived with me for about three months in a four-ounce
bottle, that measured three inches in height, to the neck, and two
inches in diameter. The bottom was covered to the depth of about an
inch with garden soil, and the top tightly corked, to prevent the
prisoner’s escape. This precaution was necessary; for the inside of
the bottle, though cleaned from time to time, soon became covered with
a coating of earthy particles, which afforded the Beetle a pretty firm
foothold.
It was an extremely interesting pet, and its struggles to escape
by climbing up the sides of the bottle often afforded me much
entertainment. It seemed to have a glimmering notion that the only
way out was by the top, and knowing nothing of the cork it would rear
itself up against the side, and try to climb up by vigorous movements
of its fore-legs. It would also take advantage of any little lump of
earth projecting about the rest. It had not intelligence enough to make
anything like a mound for itself, though the inequalities were probably
the result of its burrowing under the surface. Its temper was none of
the best, for if it was disturbed with the forceps it would resent it
fiercely. The mandibles would be opened, the abdomen curled up, and out
would come the two vesicles as a means of defence. If the forceps were
put near the mandibles, they would be seized, and the Beetle would hold
on so tenaciously that it has often been lifted out of its bottle in
this fashion.
It was exceedingly voracious, and was generally fed on garden worms.
After a full meal its increase in size was very evident. This is not to
be taken to mean that insects grow after they have attained the perfect
or imago state, for this is not the case. But when they have had a long
fast, the segments approach each other, and are forced apart when the
creature is gorged with food. If a Beetle of this species were kept
fasting for some days, and then carefully measured, and measured again
after being plentifully supplied with worms or flies, there would be a
difference of some millimetres between the results.
Dallas has an interesting passage in his _Elements of Entomology_
respecting the boldness of the larval form, which is worth quoting.
‘I have seen one engaged in a struggle, which lasted about twenty
minutes, with a worm of some five inches in length, the larva being
scarcely more than an inch long. During this contest the little savage
crept under the worm, fixing his mandibles into the creature’s body in
various places, each bite apparently producing a considerable swelling.
Sometimes he would fasten upon the head of the worm, and retain his
hold with the pertinacity of a thoroughbred bulldog, although twisted
about in every direction by the struggles of his intended victim. At
last, however, he seemed to come to the conclusion that he had been
too ambitious in his desires, and went quietly off amongst the grass,
rather prematurely, as it seemed to me, for when the worm began slowly
to leave the field of battle, about an inch of his tail was attached to
the rest of his body solely by the intestine, a union which the jaws of
the larva would easily have dissolved.’
I have never seen a fight between a larva and a worm, for the few larva
I have kept have been fed on flies. But the adult Beetle which has once
fastened on a worm cannot be shaken off. It will grip its prey with the
first pair of legs, fixing the claws in the skin, and will finish a
worm three inches long at a meal.
A dead specimen should be looked over in the way recommended for
Dytiscus, raising the small wing-covers and unfolding the wings. The
spiracles are to be looked for at the sides of the abdomen, in the
groove formed by the meeting of the upper and under plates of each
segment. The short downy hair with which the body is covered should
be noticed, and the front legs are well worth examination. The tibia
or shank is armed with a strong spine, and between this part of the
leg and that which follows it is a notch, through which the Beetle
passes its antennae to clean them from dirt. The peculiar shape of
the joints of the tarsus or foot is very plainly discernible with the
appliances at our command, and by a careful management we may make out
the different kinds of hairs with which four out of the five of these
joints are furnished; some stout and spine-like, others finer, ending
in a pear-shaped bulb. These last probably serve the same purpose as
the sucking-disks of Dytiscus and the tarsal plates of Hydrophilus.