250 NATURE'S TEACHINGS. 



Generally these larvse 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 per- 

 form. 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 impos- 

 sible that so fragile an object should be able to exert any influ- 

 ence 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 



