MOTIONS OF INSECTS. 315 



the plane of position a . There is a tribe of minute in- 

 sects amongst the Aptera, found often under bark, 

 sometimes on the water, and in various other situations, 

 which Linne has named Podura, a term implying that 

 they have a leg in their tail. This is literally the fact. 

 For the tail, or anal extremity, of these insects is fur- 

 nished with an inflexed fork b , which, though usually 

 bent under the body, they have the power of unbend- 

 ing; during which action, the forked spring, pushing 

 powerfully against the plane of position, enables the 

 animal to leap sometimes two or three inches. What 

 is more remarkable, these little animals are by this or- 

 gan even empowered to leap upon water. There is 

 a minute black species (P. aquatica), which in the 

 spring is often seen floating on that contained in ruts, 

 hollows, or even ditches, and in such infinite num- 

 bers as to resemble gunpowder strewed upon the sur- 

 face. When disturbed, these black grains are seen to 

 skip about as if ignited, jumping with as much ease as 

 if the fluid were a solid plane, that resists their pres- 

 sure. The insects of another genus separated from 

 Podura by Latreille under the name of Sminthurus 

 have also an anal spring, which when bent under the 

 body nearly reaches the head. These, which are of a 

 more globose form than Podura, are so excessively agile 

 that it is almost impossible to take them. Pressing 

 their spring against the surface on which they stand, 

 and unbending it with force, they are out of your reach 

 before your finger can come near them. One of them, 

 S. fuscus, besides the caudal fork, has a very singular 

 organ, the use of which is to prevent it from falling from 

 11 ii. 910. b PLATE XV. FIG. 14. 



