Boring Tools.
NEXT in importance to the edged tools which cut, come the pointed tools by which holes can be bored. We have an abundance of such tools, but they can all be reduced to two types, namely, those which, like the Bradawl, are forced between the fibres, and those which, like the Gimlet, cut away the material as they pass through it.
They may, again, be shown to be different modifications of a single principle—i.e. that of the Wedge or Inclined Plane, which, as has already been shown, is identical with that of the screw. The Bradawl is, in fact, a sharp wedge, which is forced through the fibres, sometimes being merely forced between them, and sometimes cutting them, and thus forcing aside the severed fibres.
A natural example of the Bradawl is to be found in various Ichneumon-flies, especially those with very long ovipositors, which are intended for boring into wood.
All the Ichneumons are parasitic, laying their eggs in the larvæ of other insects, mostly those of moths and butterflies. Generally these larvæ exist in the open air, and the Ichneumon-fly has little difficulty in piercing them. But there are some which live either in wood or underground, and, in order to reach their hidden bodies, the Ichneumon is furnished with an extremely long and sharply pointed ovipositor.
This wonderful instrument is not so thick as an ordinary horsehair, although it is composed of three portions, and seems to be utterly inadequate to the task which it has to perform. Ascertaining by its instinct the exact locality of the caterpillar which it desires to pierce, the Ichneumon-fly clings firmly to the tree, bends the body so as to bring the point of the ovipositor against the wood, and, by moving the abdomen backwards and forwards, gradually works the instrument into the wood, sometimes piercing it to a considerable depth.
Mr. Westwood once saw an Ichneumon-fly thus boring its way into a dry post, the wood of which must have been very hard. When she had bored far enough, she partially withdrew the ovipositor, and then re-plunged it into the hole that she had made, as if she were depositing eggs. While engaged in this operation, she stood very high on her long legs, resting only on the extremities of the feet. She belonged to the genus Pimpla.
The principle of the Wedge or Inclined Plane is admirably shown by objects which we pass unheeded every day, and yet afford wonderful examples of the power of the wedge.
Scarcely any vegetable growth is so plentiful as grass, which has been used in that sense by the highest of all authorities, “which to-day is, and to-morrow is cast into the oven.” Grass forces its way everywhere—not only in cultivated grounds, but in the wildest of lands, where there is scarcely any nurture for it. Even among the habitations of mankind the grass will have its way, and clothes deserted housetops with verdure, and forces itself between the stones that pave neglected streets.
Place side by side some of these stones, together with a very young and tender Grass-blade, and it will seem to be impossible that so fragile an object should be able to exert any influence on the solid stone. Let any one try to push a sharp skewer between the stones, and he will find that he has to exert power sufficient to crush a thousand grass-blades. Yet these slight and delicate objects will force themselves between the stones, and sometimes to such an extent as to cover the whole roadway with verdure.
The force which is employed is simply marvellous, and can only be appreciated by those who know the resisting power of earth, however dry and loose it may be. Even sand has so strong a resistance that tents can be pitched in the desert without difficulty. Of course the ordinary tent-peg would be useless, but the desert dwellers can pitch their tents with perfect security. They fasten the tent-rope to a branch or piece of bush, scrape a hole in the sand, put the bush into the hole, cover it up again, and it will withstand almost any strain, though it be only covered with a few inches of sand.
When miners blast rocks with gunpowder, they take advantage of the resisting power of sand. They bore a suitable hole, place a charge of gunpowder at the bottom, and then merely pour loose sand into the hole until it is filled. When the powder explodes, the rock or coal is shattered to pieces, but the sand is not blown out of the hole. This operation is called “tamping.”
Every one, again, knows how firm are gate-posts, and how they resist the weight, jarring, and leverage of a heavy gate, all because they are sunk a little way into the earth.
Considering, therefore, that such fragile things as young grass-blades can force their way through the superincumbent weight, we can but be amazed at the aggregate of active force which is in full operation in every pasture field and garden lawn.
As far as I know, not being much of a botanist, every seed that springs up does so on the wedge principle, though the form of the wedge may be varied.
A terrible example of the force which is exercised by this principle among the vegetables is shown in some parts of the world where the Aloe flourishes in a wild state. In our colder clime the Aloe, though it does live in the open air, is a slow-growing plant. But, in its own land, it shoots up with a surprising vigour, and its sharply pointed and saw-edged leaves are said to grow to the extent of six inches in a single night.
Taking advantage of this rapid, and, at the same time, powerful growth, the natives, when they want to punish a man with more than ordinary severity, tie him hand and foot, and bind him to the earth just over a sprouting aloe plant, and leave him there. In twenty-four hours the man is nearly certain to be dead, the aloe-leaf having forced itself completely through his body. Or, if he be not actually dead, he lives in frightful tortures, which are continually increased by the flinty point and notches forcing themselves slowly, but surely, through the body.
For an example of the Gimlet we may take the ovipositor of the Sirex, an insect which I believe has no popular name. It is coloured much after the same manner as the hornet, and is often mistaken for that insect by those who are not versed in entomology. And, as its long and straight ovipositor is generally taken for a hornet’s sting, the insect assumes a double terror to the ignorant.
Now, the real fact is, that in its larval stage of existence the Sirex feeds upon the wood of the fir-tree—a diet which, to our ideas, is about as unsatisfactory as can well be imagined. In order that the young Sirex may be within reach of food, the egg must be introduced deeply into the body of the tree, and, for the egg to be so received, a channel must be cut for it.
This is done by means of the marvellously formed ovipositor. Many admirable descriptions have been given of the head of this instrument and its boring powers, but I am not aware that any one has noticed the secondary cutting blades that are set along the shaft of the principal borer, and which answer exactly the same purpose as the spiral cutting edge of the gimlet or auger.
Not being desirous of repeating my own observations in different words, I transfer to these pages a short account of the ovipositor of the Sirex, as examined by me when writing my work on British Insects, entitled “Insects at Home,” and published by Messrs. Longmans and Co.:—
“I very strongly recommend any of my readers who may obtain a female Sirex to disengage the actual borer from its two-bladed sheath, and examine it with the aid of a microscope. A half-inch object-glass will give quite a sufficient power.”
“It is straight, stiff, and elastic, as if made of steel, and, if bent, will spring back to its proper form with the elasticity of a Toledo rapier.
“But the borer possesses an auxiliary cutting apparatus which places it far above the rymer in point of efficacy. Even with an ordinary magnifying lens, it is easy to see that the end of the borer is developed into a sharp head, very much resembling that of a boarding-pike, and that the outline of the shaft is broken into a series of notches.
“The half-inch glass, however, discloses a marvellous example of mechanical excellence. The head of the borer is then seen to be armed with long, sharp teeth, slightly curved inwards, and acting just as does the carpenter’s ordinary centrebit.
“So much for the head of the borer: we will now turn to the shaft.
“It appears that, in order to make a clean-cut hole for the reception of the egg, the shaft of the borer has to finish the task which the head begins. Accordingly, it is armed on each of its sides with a series of hard, sharp-edged ridges, running diagonally across it, and acting exactly as do the sharp ridges of a coffee-mill.”
In point of fact, the ovipositor of the Sirex is the natural type of the improved gimlet of the present day. Instead, however, of having a single, spiral, sharp-edged groove running along the whole length of the shaft, it has a series of small, sharp blades, set exactly in the same line as is taken by the spiral groove, and acting in exactly the same manner—i.e. by cutting out successive portions of wood, and, by the diagonal position of the blades, throwing out the debris as fast as it is cut.
I cannot but think that, if any modern tool manufacturer could take as his model the saw-like ovipositor of the Tenthredinidæ, and the auger-like ovipositor of the present insect, he would produce a series of most valuable implements, possessing powers far beyond those of ordinary tools.
These short blades are arranged just like the “studs” on modern shells, and very much resemble them in shape, though not in material.
The Auger finds also a natural representative in the ovipositor of an insect.
That of the common Gad-fly (Œstrus bovis) is most beautifully constructed. It is tubular in form, and is of a telescopic nature, consisting of four tubes of different sizes, the smaller fitting into the larger just as is done with the joints of a common telescope, or those of a Japanese fishing-rod.
The end of the ovipositor is developed into little projections, some of which are armed with hard, sharp points, which act exactly like the cutting edge of the auger. This elaborate appliance is necessary on account of the thick, tough skin of the ox, which the Gad-fly has to penetrate before it can deposit its eggs. Perhaps the reader may be aware of the fact that the modern system of cutting channels in stone with the diamond point, as was so well exemplified in the Mont Cenis Tunnel, is but an imitation, and an imperfect one, of the method adopted by the Gad-fly. We shall soon recur to this instrument.