APTERA. 213 



state this organ is kept folded under the abdomen, where it 

 is concealed in a groove. The pieces of whicli it is com- 

 posed are articulated together in such a manner as to admit 

 of their being rapidly unbent by the action of its muscles, 

 the whole mechanism conspiring to produce the effect of a 

 powerful spring, by which the body is propelled forwards 

 to a considerable distance. In some species, this flexible 

 tail has a flattened form, for the purpose of enabling the in- 

 sect to leap from the surface of the water, an action which it 

 performs with apparently as much ease as if it sprung from 

 a solid resisting plane. 



The Lapisma leaps by means of moveable appendages, 

 placed in a double row along the under side of the body, and 

 acting like springs. There are eight pairs of these members, 

 corresponding in situation and structure to the false feet of 

 the Crustacea, and, like them, terminating in jointed fda- 

 ments. 



The Juhis and the Scolopendra, which compose the fa- 

 mily of the Myriapoda, so called from the immense num- 

 ber of their feet, undergo, to a certain extent, a kind of me- 

 taphorphosis in the progress of their development. When 

 first hatched they have often no feet whatever, and resem- 

 ble the simpler kinds of worms. Legs at length make their 

 appearance; but they arise in succession, and it is not until 

 the later periods of their growth that these animals acquire 

 their full complement of segments, with their accompanying 

 legs. The Jidus ierrestris, for example, (Fig. 143) has, at 

 ^ its entrance into the world, only 



jiii0^(4^^^^^j^^^^^^ eight segments and six feet; but 

 ^^^^^Hll!lliP\ acquires in the course of its deve- 

 lopment, fifty segments and about two hundred feet. The 

 anterior legs are directed obliquely forwards, and the rest 

 more or less backwards. The mandibles have the form of 

 small feet; as we have seen is frequently the case in crusta- 

 ceous animals. 



