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 XXXIV
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-glass—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.