CHAPTER XXIV
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 isthewater-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 wasone 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 ofapparentlyuseless 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 trulyridiculous 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 femaleHydrophilusbetake 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 thereal 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 hismétierto 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 loosethe 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 asGyrinus,Dytiscus,Hydrobius,Donacia, etc., “and letthemspeak 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. Itsstrength 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 aPomotis, 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 thisdénouementto 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 theZaithasare 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 wholeaccount 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.