Aquatic insects—Lyonnet’s water-beetle—A floating cradle—Larva
and pupa—An ingenious contrivance—Nothing useless—The
imaginary philosopher—How the cradle is made—The mysterious
“mast”—Later observation—The giant water-bug—An oppressed
husband.
SPIDERS having brought us to the water, it may be as well, or even
better, in view of the title of this work, to say something about
water-insects. Of these, so long as the water be fresh, and not salt,
there are many, and the largest, perhaps, if he exceeds some of the
dragon-fly larvæ and the Giant Water-Bug of America, must be the Great
Water-Beetle—_Hydrophilus piceus_—which is larger even than the much
commoner one—_Dytiscus_—which everybody knows, and which is _the_
water-beetle to most people.
It is the fate of some animals to become associated for all time in our
minds with the name of some particular man, as, for instance, the bee is
with that of Huber, and the ephemera with that of Swammerdam. Again, the
fame of Lyonnet, though he was skilled in eight languages, and became
cypher secretary and confidential translator to the United Provinces of
Holland, is principally bound up with a certain caterpillar, viz. that
of the goat moth, of which creature, though only an amateur in such
matters, he made dissections and executed plates, which have never yet
been surpassed, and are supposed to be entirely unsurpassable. In a
lesser degree his memory is associated with this particular
water-beetle—the great one, into the heart of whose mystery he was the
first to pierce: “In the beginning of July,” he tells us, “I had noticed
in the ditches a kind of cocoon which I did not recognise. It was
whitish, of the size of the end of the finger, nearly spherical, but
rather oval and flattened. The surface, which looked like tow, was not
quite smooth. One of the two ends was flatter than the other, and
furnished with a raised rim. From the space within this rim projected a
sort of little tapering mast about as long as the cocoon.”[143]
These cocoons, when opened, were found to contain about a hundred eggs.
Lyonnet kept them in water till the eggs hatched. “The larvæ,” he says,
when this had taken place, “remained one day enclosed in the cocoon
before escaping. Then they made an oval aperture in the lower part of
the flattened end of the cocoon, and escaped through this into the
water.”[143] Here they fed upon snails, their manner of eating which is
thus described: “The larva seizes the snail with its mandibles, then
bends its body backwards and rests the snail upon the broad back, which
serves as a table (as with the larva of the grain-eating ant of Texas).
In this position, holding the snail in its legs, the larva breaks the
shell, and devours it.”[143]
When full-fed the larvæ left the water, and one of them was placed by
Lyonnet in a box full of moistened earth. This it entered, and, some
days afterwards, changed into a large white pupa or chrysalis, about
which there was one curious feature, viz. that “on each side of the
head”—or, as an entomologist would say nowadays, “on the fore part of
the prothorax—were three brown, strong hooks. Two others of the same
kind were found at the hinder end of the body.”[143] These hooks were
solid, so that they could contain no part of the perfect insect, and
Lyonnet points to them as good examples of _apparently_ useless
structures. Their office, indeed, he himself knows, but he does not
reveal it till the usual philosopher has been imagined who denies that
they can have any. Then, of course, comes the anticipated discomfiture
of this unwary person—so frequent in the eighteenth century—who,
unwarned by experience, has walked quietly into the trap. “In the damp
earth which the pupa requires the above-described hooks fulfil a
purpose, unexpected by us, but, at the same time, of great importance.
The skin of the pupa is very delicate. Lying on damp earth, it could
hardly escape injury, and the weight of the body might easily give it a
distorted shape. But (_Monsieur le Philosophe_) the pupa protects itself
from these dangers by assuming an unusual attitude. It extends itself
back downwards in a horizontal position, and supports the weight of its
body by the three sets of hooks, as upon a tripod. In this attitude,
though surrounded on all sides by moist earth, it keeps its body from
actual contact with any object until it has assumed its final shape.
Thus,” continues Lyonnet, turning full upon the stupefied philosopher,
“we see how necessary are those hooks, which at first sight appeared so
useless. To decide that this or that structure is superfluous because we
cannot guess its use is truly ridiculous in beings whose information is
so limited as ours.”[143] Applauding shouts (“_Mais certainement!_”
“_C’est vrai cela!_”) rend the air, and the imaginary philosopher goes
out in a state of painful confusion.
The above facts, first made known by Lyonnet, have been confirmed by
subsequent observers, such as Miger, and the pupa of another and much
smaller water-beetle is now known to support itself in the same manner,
or, rather, on the same principle, since the place of hooks at either
extremity is taken by spiny projections, with which the back is covered.
Lyonnet now turned his attention to the mature beetle, and especially to
the female, whom he was anxious to see make her cocoon. Having put a few
in a large wooden trough and supplied them with some floating weed, “I
had,” he says, “before long, the pleasure of seeing the female
_Hydrophilus_ betake herself to work under my eyes. I found, to my
surprise, that, like the spider, she had her spinneret at the hinder end
of the body. Two small brown prominences enclosed each a delicate
conical tube, from each of which a separate thread proceeded, and with
these the cocoon was woven in the following way. At first, lying
upside-down near the surface of the water, the beetle buried the hinder
part of her body, and the two hindermost pairs of legs, in the weed,
whilst with the first pair, which were free, she drew and pressed the
weed around the end of her body, moulding it to its shape. She then
began to weave what seemed the under half of her cocoon, but having
finished this part she turned over with it so that it became the upper
half, and then wove the real under one. The two curved surfaces were
then woven together, and in about an hour and a quarter the body of the
cocoon was finished. For about two hours after this the beetle remained
still, her back being uppermost. At first her body was buried in the
cocoon up to the thorax, but one could see that she was gradually
withdrawing it. During these two hours of apparent rest she laid her
eggs, not at hazard, but in regular order, side by side, the pointed
ends uppermost. This work accomplished, she closed the mouth of the
cocoon, and then began to spin the little mast, which gradually rose
above the surface of the water till it had attained the requisite
height, and the cocoon was then finished.”[143]
Lyonnet was unable to discover the use of the so-called mast, and it
remains a mystery to this day, so that the imaginary philosopher might
have a better chance here, were it not his _métier_ to be put to
confusion. It is hollow, and as the cocoon contains air, with which the
beetle supplies it—just as the water-spider does her diving-bell—Miger,
whose observations were made in 1807, some fifty years after those of
Lyonnet, supposed it might serve as the channel of entry. But, although
hollow, it has no orifice, but is closed at the end, and this does not
seem to accord with the above view. Mr. G. A. Laker, a modern observer,
does not think that the spike can serve as a balance to the cocoon,
since this is usually attached to some weed, or other supporting
substance. He, however, cut the spike off two of the cocoons, and the
eggs in both of these remained unhatched. Moreover, these cocoons
subsequently sank, whereas in their normal state they “are so
constructed that when floating loose the spike retains its proper
position, and even if the cocoon be held so that the spike is parallel
with the water and then suddenly released, it immediately rights
itself.”[144] The balance theory, therefore, certainly seems to have
something in its favour. Lyonnet’s own conjecture was that the mast, as
he calls it—a designation against which Miger protests—might merely
represent the waste silk which the beetle felt impelled to get rid of.
The time taken by the beetle in making the whole cocoon is about five
hours, whilst the mast, spike, or turned-up point, as Miger severely
calls it, takes it half an hour. It is curious that whereas Lyonnet’s
cocoons held “about a hundred” eggs, Mr. Laker gives the number as
“usually between fifty and sixty.”[144]
As space has its exigencies—and long may it continue to have—I will here
merely mention such names as _Gyrinus_, _Dytiscus_, _Hydrobius_,
_Donacia_, etc., “and let _them_ speak for me,” but having paid some
attention to the great water-beetle, silence in regard to the giant
water-bug would be hardly gracious, and might be ill taken. This
terrific creature is like a monstrous exaggeration of our own
water-scorpion, to which it bears a distorted, but real resemblance,
minus, however, the long ovipositor—the so-called tail behind. Its
appearance is not to be described. Like other bugs, and as are the
aphides and cicadas for less cruel purposes, it is armed with a long,
sharp-pointed beak, through which, having plunged it into the body of
its prey, on whose back it has previously leaped, it sucks the
life-juices, holding on, all the while, with its two curved, claw-like
front legs. Its strength is in accordance with its size, and both are
such that it finds no one in its own circle, so to speak, at all capable
of contending with it. “It is the facile master of the ponds and
estuaries of the tidal creeks and rivers of the Atlantic States,” says
Uhler. “Developing in the quiet pools, secreting itself beneath stones
or rubbish, it watches the approach of a _Pomotis_, mud-minnow, frog, or
other small-sized tenant of the water, when it darts with sudden
rapidity upon its unprepared victim, grasps the creature with its
strong, clasping fore-legs, plunges its deadly beak deep into the flesh,
and proceeds with the utmost coolness to leisurely suck its blood. A
copious supply of saliva is poured into the wound, and no doubt aids in
producing the paralysis which so speedily follows its puncture in small
creatures.”[145]
Another American water-bug of similar build, but much smaller size, has
the same general habits, to which it adds the more special one of
carrying about its eggs on its back, where, in time, they are hatched,
but do not, it would appear—though this seems somewhat out of harmony
with the practical spirit of nature—proceed at once to suck their
parent’s blood, an omission which, as it would be a most moving instance
of unselfish surrender on the part of the latter, is, perhaps, to be
regretted. Possibly the reason is that the eggs are not fixed upon the
right back, so that even were this _dénouement_ to take place, we should
not have an instance of maternal, but only of paternal affection. This,
for some reason, is not so effective as the other, and therefore Nature,
who, as we know, is a consummate artist, may not care to waste her
materials on an inferior situation.
Be this as it may, the fact that the domestic economy of these
water-bugs did not proceed, throughout, upon the lines that might have
been expected may first have led a German observer—Schmidt—to suspect
something unusual, in consequence of which misgiving he looked more
closely into the matter, and found—what had not before been
imagined—that the male and not the female was the egg-bearer. He was
not, however, able to determine how this arrangement was brought about,
or with what feelings the male received and bore his burden. This was
left for Miss Slater, who found that the females in her aquarium
insisted upon laying their eggs on the backs of the males, that the
latter objected to their doing so, which led to a struggle between the
two, often lasting for two or three hours, but ending invariably in the
victory of the female. The male has, then, to bow to necessity, but he
does not do so in a cheerful spirit, nor even without some further
efforts to escape his destiny. “That he chafes under the burden,” says
Miss Slater, “is unmistakable; in fact my suspicions as to the sex of
the egg-carrier were first aroused by watching one in an aquarium which
was trying to free itself from its load of eggs, an exhibition of a lack
of maternal interest not to be expected in a female carrying her own
eggs. Generally the _Zaithas_ are very active, darting about with great
rapidity, but an egg-bearer remains quietly clinging to a leaf, with the
end of the abdomen just out of the water. If attacked he meekly receives
the blows, seemingly preferring death (which, in several cases, was the
result) to the indignity of carrying and caring for the eggs.”[145] This
last, however, is not very explicit, so that, the whole account not
being to hand, I cannot say what precisely happened.
It is curious that the male should be so spiritless, after receiving the
eggs, for this would seem to nullify such advantages as the arrangement
might otherwise offer. The eggs must be laid somewhere, and might be
supposed safer on the back of the male than elsewhere—in which fact,
perhaps, we may see the origin of the instinct. But if the male, sinking
under his burden, is able neither to defend himself nor it, this
advantage seems nullified.
Estuaries and tidal creeks, which, as we have seen, are included in the
habitat of these water-bugs—at least, of the giant one—bring us
gradually to the sea. That there are marine insects we know, but they do
not appear to extend beyond the tidal beach, on the sands of which they
expatiate, when the sea is out, and burrow into them on its return. All
are small, and still smaller is the amount said about them, even in such
works as are precisely those where all that is known on the subject
ought to be stated—systematic works of natural history, for instance,
which take “_Arthropoda_ (Insects, etc.)” in their due order, but do not
so much as tell you whether marine ones exist or not. Yet the date of
such works is after 1895. For these reasons, and another which has been
once or twice before alluded to, I have but one remark to make about
marine insects, and I will make that in the next chapter.
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Footnote 143:
Mémoires du Muséum, vols. 18-20.
Footnote 144:
Entomologist, vol. 14 (1881), p. 82.
Footnote 145:
Quoted in The Insect Book, by Leland O. Howard.
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