80 METAMOKPHOSES OF MAN 



posed of about twelve rings, is provided with a very 

 imperfect buccal apparatus, is without a vestige of 

 limbs, and its dense tuberculated body is like a piece 

 of saturated parchment. It may be found in almost 

 all our ponds, where it swims about somewhat in the 

 manner of a leech. It can only breathe by air, and it 

 does so in a very peculiar way. In the last segment 

 of the body, which is much elongated, two very large 

 tracheal canals, which pass from end to end of the 

 larva, terminate in a single orifice ; this orifice is sur- 

 rounded by a bunch of branching filaments ; and by 

 the approximation of these latter it is kept constantly 

 closed. As soon as the creature wants fresh air, it 

 ascends to the surface, expands this filamentous or- 

 gan, which thus constitutes a sort of float, and then, 

 head downwards, it allows the atmospheric air to enter 

 through the orifice, pass along the trachea, and so fill 

 the entire body. 



Reaumur has not told us how long the stratiomys 

 lives in the larval condition. Invariably about the 

 beginning of summer we find some of them twisted 

 in a zigzag manner, and quite motionless and stiff; 

 and if one of them be carefully opened, the fully- 

 formed pupa may be seen in its interior. At the 

 moment of its metamorphosis the stratiomys detaches 

 itself as completely from the larval skin as occurs in 

 the case of other insects; but instead of emerging 

 from it and throwing it away, it still remains within it, 

 thus saving itself the trouble of preparing a chamber 

 in the earth or spinning a cocoon. In fact this skin 

 forms a kind of house for the pupa, which in its 

 present condition it is unable to fill completely. In 

 assuming its nymph form, its body is so much con- 



