194' NATURAL HISTORY. [cH. XI. 



moved from one extremity of the body to the other 

 during the progress of an insect through its different 

 stages of life. 



Its last metamorphosis into a winged fly is 

 attended with curious circumstances. When nature 

 has prepared the insect to change its element, instead 

 of lying rolled up on the surface of the water, it. 

 stretches out its body, and by some mechanism, 

 puffs up its corslet so that it splits between the stig- 

 mata or the breathing-horns. As soon as the fissure 

 is sufficiently enlarged to make way for it, the head 

 of the gnat appears in its perfect shape ; but this is 

 the most critical period of its whole life ; up.to this 

 time it was an aquatic animal ; now it has nothing 

 to dread so much as the water. It has, moreover, 

 the use neither of leg nor wing; these members are 

 as yet soft, moist, and bound up, and it only pro- 

 trudes itself from its skin, by means of a wriggling 

 action given to its body. If at this critical juncture 

 the water should happen to touch its corslet or ab- 

 domen, the gnat would inevitably and instantly 

 perish. In such circumstances, then, it requires the 

 prudence of an old gnat, at least, to escape the dan- 

 gers which surround the young one. Nature, how- 

 ever, has conferred upon the insect an instinct suit- 

 able to the emergency. As soon as it puts out its 

 head, it elevates it above the water ; and Avorming 

 itself out always perpendicularly, supported only by 

 the inequalities of the skin which it is about to cast 

 off, with no power to balance itself, surrounded by 

 an unfriendly element, it literally becomes a canoe, 



