The Animal World, A Book of Natural History Part 52

This insect lays it eggs in the ground, and the grubs which hatch out from them are called leather-jackets, because their skins are so very tough. They feed upon the roots of gra.s.s, and sometimes do a great deal of mischief in pastures. Indeed, if it were not for such birds as the crow and meadow-lark, which destroy them in enormous numbers, we should find it almost impossible to grow any gra.s.s at all.

The drone-fly really does look rather like a bee; but it only has two wings instead of four, while its body is much more stoutly built, and it has no sting, so that you need not be in the least afraid of it. You may often see it sitting on flowers on sunny days in autumn, and it is especially fond of those of the ragwort.

The grub of this fly spends its whole life buried head downward in the mud at the bottom of some shallow pool--thick, black mud, which is largely made up of decaying leaves--and never comes out of it even to breathe. But at the end of its body it has a long tube, the tip of which rests just above the surface of the water, so that it can draw down as much air as it requires. And this tube is made something like a telescope, so that if a heavy fall of rain should raise the level of the water, all that the grub has to do is to push out another joint, when it can breathe just as easily as before. This grub is often known as the rat-tailed maggot.

HAWK-FLIES, ETC.

As you walk through a wood in summer, you may often see a black and yellow fly hovering in mid-air. If you move, it darts away so swiftly that the eye cannot follow its flight. But if you stop, and remain perfectly still, it will come back again in a moment or two, and hover just as before.

This is a hawk-fly, and it is very useful, for the mother insect always lays her eggs on twigs and leaves which are swarming with aphides. On these insects the grubs feed, so that as soon as they hatch out they find themselves surrounded with prey, and destroy the little insects in great numbers.

The house-fly and the bluebottle fly also belong to the order of the _Diptera_. They are not very pleasant insects, but while they are grubs they are really most useful, for they feed upon all sorts of decaying substances. And another insect, called the flesh-fly, is even more useful still, for it is the parent of from sixteen to twenty thousand grubs: so that if even a single fly finds the carca.s.s of a small animal and leaves her eggs upon it, the little ones that soon hatch out will devour it in a very short time. In a few days all these grubs turn into perfect flies, and in their turn become the parents of thousands of grubs: so that it has been said that three of these flies could devour a dead ox as fast as a lion could!

The last insect that we can mention is a brown and gray fly known as the warble. It is very troublesome indeed to cattle, for the mother fly lays her eggs upon their backs. Then as soon as the grubs hatch, they burrow underneath the skin of the poor animals, and form large swellings there, in which they spend the whole of their lives. When they are fully fed they wriggle out through a hole in the hide, drop to the ground, burrow into it, and turn to chrysalids, from which the perfect flies appear a few months later.

CHAPTER x.x.xIV

SPIDERS AND SCORPIONS

Most people think that spiders are insects. But this is a very great mistake, for they are just about as unlike insects as they can possibly be.

Insects, for example, always have distinct heads. But spiders never do, for their heads are so sunk and lost in their chests that you cannot possibly tell where the one leaves off and the other begins. So that spiders have their bodies divided into two parts only instead of into three, as is always the case in the insects.

Then insects always have six legs; spiders always have eight. Insects have wings; spiders have none. Insects have feelers; spiders have none.

Insects nearly always have a great many eyes, which are six-sided; spiders never have more than eight eyes, which are round. And while insects may have biting jaws, or sucking jaws, or a trunk, or a beak, spiders always have poison-fangs, which no insect ever possesses.

So you see that as far as the outside of their bodies is concerned, spiders are very different indeed from insects. And the differences inside the body are just as great. Insects have no hearts, the only blood-vessel in their bodies being one long tube which runs along the back; but spiders have quite a big heart, and a good many arteries as well. Insects have no lungs, but breathe by means of slender tubes which run to every part of the body; but spiders have quite big lungs, in which the blood is purified just as it is in our own. Insects have no brains, but only bunches of nerves in different parts of their bodies; but spiders have quite big brains. And besides this, while all insects which spin silk produce it through their mouths, spiders always do so by means of organs at the very end of the body. So that inside, as well as outside, there is hardly any respect in which spiders and insects really resemble one another.

The silk-organs of a spider are very wonderful indeed. Remember, in the first place, that the silk, as long as it remains in the body of the spider, is a liquid--a kind of thick gum, which is produced and stored up in six long narrow bags, or glands. Then if you look at the end of a spider's body through a good strong magnifying-gla.s.s--or, better still, through a microscope--you will see several little projections, which we call spinnerets. Now each of these spinnerets is covered with hundreds of tinier projections still, every one of which has an extremely small hole in the middle. And all these holes communicate, by means of very slender tubes, with one of the silk-glands.

So what a spider does when it wants to spin its line is to squeeze a little drop of silk into one of the spinnerets. It then just touches the object to which the line is to be fastened, and draws its body away. And as it does so a delicate thread comes out from every one of the projections on the spinneret; and all these threads unite together into one stout cord. That is why a spider's thread is so strong. It really consists of several hundred separate threads all firmly fastened together. And if the spider wants to spin a stronger line still, it can unite all the threads coming from several spinnerets into one, so as to make a very stout cord indeed.

Spiders use this silk for all sorts of different purposes. In the first place, they use it for snaring insects.

THE GARDEN-SPIDER

Let us take for an example, the web of the common garden-spider. It is to be seen in every garden, resting in the middle of its web; and you may always recognize it by the white cross upon its back. But I don't suppose that you have ever seen it spinning its net. For it always does so very early in the morning, generally beginning before sunrise, so that it may be quite ready for use as soon as the insects begin to fly.

First of all, the spider makes a kind of outer framework of very strong silken cords, and fastens it firmly in position by stout guy-ropes of the same material. Next, she carries a thread right across the middle and fixes it down on either side. Then, starting from the center, she carries thread after thread to the margin, carefully testing the strength of each by giving it two or three smart pulls, and fastening it firmly down. When she has finished this part of her task, the web looks like a badly shaped wheel.

The next thing that the spider does is to spin a little silken platform in the middle of her web to sit upon. And as soon as she has done this she begins to spin the spiral thread. Beginning from the center, she goes round and round and round, fastening the thread down every time that it crosses one of the straight cords--the spokes, as it were, of the wheel--until at last the web is finished. Then she goes to the little platform in the middle, and there remains, upside down, waiting for an insect to blunder into her net.

By and by, perhaps, a bluebottle fly does so. Then she shakes the web violently for a few moments, so as to entangle it more thoroughly, rushes down upon it, seizes it, and plunges her fangs into its body. But if she catches a wasp or a bee she nearly always cuts it carefully out, drops it to the ground, and then patches up the hole in her web. For she knows perfectly well that wasps and bees can sting!

Would you like to know why it is that flies stick to the web as soon as they touch it? The microscope shows us. All the way along, the spiral thread is set with very tiny drops of liquid gum. So tiny are these drops indeed, that there are between eighty and ninety thousand of them in a large web! And would you like to know why it is that the spider does not stick to the web as the flies do? Well, the fact is that only the spiral thread is set with these little gummy drops, and that as the spider runs about over her web she is most careful to place her feet only on the straight threads, and never on the spiral line. Other spiders, however, snare their prey in quite a different way.

THE MARMIGNATTO

This small spider, found on our western plains, is remarkable for feeding on large insects, such as gra.s.shoppers and field-crickets, which it catches in an ingenious manner. It stretches a few silken threads across a narrow path way, quite close to the ground, along which these insects are likely to pa.s.s, and lies in wait just opposite until a gra.s.shopper or a cricket approaches. When it comes to the threads the insect is sure to get at least one of its feet entangled. Then it stops, and tries to shake itself free. The only result of its struggles, of course, is that its other feet become entangled too; and while it is struggling the marmignatto springs upon its back, fastens a silken thread to it, springs down again, and fastens the other end to a gra.s.s-stem close by. Over and over again it does this, and before very long the unfortunate insect is firmly fastened down by hundreds of threads, and is quite unable to break free, or even to move one of its legs. Then the spider leaps upon its back once more, plunges its fangs into its body, and proceeds to suck its blood.

HUNTING-SPIDERS

Perhaps you may have seen little hairy black spiders, with white markings upon the upper part of their bodies, running about in an odd jerky way on sunny fences and walls. These are called hunting-spiders, because they hunt their prey instead of snaring it. You may see them gradually creeping up to a fly, so slowly that they hardly seem to move, and then suddenly leaping upon it when they are about two inches away.

Then spider and fly, locked in one another's embrace, go falling toward the ground together. But they never reach it, for wherever a hunting-spider goes it always trails a rope of silk behind it, and fastens it down at intervals. So when it springs from the fence it is brought up at once by its own thread, and swings in the air till its victim is dead. Then it just climbs up its thread, and so gets back to the fence.

BIRD-SPIDERS

These great spiders of the tropics hunt for prey in much the same way.

Only instead of catching flies on walls they prowl about the branches of trees in search of small birds, springing upon them when they are roosting at night, and killing them almost immediately by a smart bite from their venomous fangs. These spiders, of course, are very large. Indeed, the body of a full-grown bird-spider is as big as a man's fist, while its great hairy legs cover nearly a square foot of ground when they are fully spread out.

TRAP-DOOR SPIDERS

These famous spiders are found more or less commonly in all warm countries. They all live in tunnels in the ground, which they dig by means of their fangs; and as they do not want the situation of their nest to be discovered, they carry the earth away to a little distance as fast as they dig it up, and carefully hide it. Very often the hole which they dig in this way is eighteen inches or two feet deep. And at the bottom it always turns sideways for an inch or two, so that the general shape of the burrow is very much like that of a stocking.

This hole is always dug in the side of a bank, so that when there is a heavy fall of rain the water may run away without flooding it.

When the burrow is finished, the spider lines it throughout with two sheets of silk. The outer sheet, which comes next to the earth, is rather coa.r.s.e in texture, and is quite waterproof, in order to keep the tunnel dry. The inner one is very much finer and softer, so that the little home may be as comfortable as possible.

As soon as the lining process is completed, the spider sets to work on the trap-door. This she makes in the cleverest manner possible. First she measures the doorway most carefully by the aid of her feelers. Then she spins a thin silken pad of exactly the same size and shape. This is sticky on the top, like the spiral thread of the web of the garden-spider: and she sprinkles it all over with very small sc.r.a.ps of earth. Upon this she fastens another silken pad, which she sprinkles with earth in the same way. And then comes another and then another, and so on till the door is sufficiently thick. Finally, she fastens it in position by means of a hinge, which is also made of silk; and she always places this hinge on the upper side of the doorway, so that the door may fall down behind her by its own weight whenever she leaves the burrow. She is rather a lazy creature, you see, and does not want to have the trouble of shutting the door for herself! And if she left it open, every pa.s.ser-by would find out where she had made her home.

The door always fits most wonderfully into its place, and the spider carefully covers the top with little bits of moss and small sc.r.a.ps of earth and stone, so as to make it exactly like the surface of the ground all round it. Indeed, unless one happens to see the spider push it open, it is almost impossible to find it.

When one of these spiders is in her burrow, she always fastens about half a dozen silken threads to the inner side of the door, carries them down to the bottom, and sits with one of her feet resting upon each. No one can then try to force her door open without her knowledge, and as soon as she feels the least pull upon the threads she rushes up the burrow, clings to the walls with her hind feet, seizes the door with her front ones, and pulls it downward with all her might. And if the door is forced open in spite of her efforts, she slips into a sort of side tunnel which she always makes near the top of her burrow, and stays there until the danger is past.

THE RAFT-SPIDER

There are several spiders which live on or in the water. One of these is the raft-spider, which is found in the fen districts of England. If you should happen to meet with it you can recognize it at once, for all round the upper part of its body is a narrow band of yellow, and inside this is a row of small white spots.

This spider is about an inch long, and owes its name to the fact that it actually makes a little raft on which to go out searching for water-insects. Collecting together a quant.i.ty of little bits of leaf and cut gra.s.s and reeds, it fastens them firmly together with silken threads, just as s.h.i.+pwrecked sailors might lash planks together with ropes in order to escape from a sinking vessel. In this way it makes a small floating platform, perhaps a couple of inches in diameter.

When the raft is finished, the spider gets upon it, pushes off from the sh.o.r.e, and allows the current to carry it along. By and by, perhaps, it catches sight of some water-insect floating at the surface, or of a drowning fly which has fallen into the stream. Then it leaves its raft, runs along over the surface of the water, seizes its victim, and carries it back to the raft to be devoured. And if it should be alarmed, or think itself in danger, it gets under the raft and clings to the lower surface, so that it cannot be seen from above.

THE WATER-SPIDER

More curious still is the water-spider, which actually makes its nest under water. This spider, which is almost black in color, and has a very hairy body and legs, is common in ponds and ca.n.a.ls, and spends almost the whole of its life beneath the water. Its little silken nest is shaped like a thimble, with the mouth downward, and is placed among weeds, to which it is firmly fastened down by guy-ropes, also of silk.

And when it is finished the spider fills it with air. She does this in a most curious manner. Rising to the surface, she turns upside down, pokes her long hind legs out of the water, and crosses the tips. Then she dives again, carrying down a big bubble of air between these hairy legs and her equally hairy body as she does so. She next gets exactly underneath the entrance to her nest and separates her legs. The result is, of course, that the air-bubble floats up and occupies the upper part. Another bubble is now brought down in the same way, and so the spider goes on, fetching bubble after bubble, until at last her little nest is completely filled with air. Then she gets inside it, and watches for the grubs of water-insects to swim by.

In this wonderful nest the spider lays her eggs and brings up her family. When the little ones have been hatched, of course, the air in the nest very soon becomes too impure to breathe. Then the little spiders cling tightly to the walls, while the mother gets outside and tilts the whole nest sideways, so that all the exhausted air floats up in one big bubble to the surface. Then she pulls the nest back into position, hurries up to the top of the water and brings down a bubble of air, and then another, and so on until the nest is filled with air all over again.