MOTIONS OF JNSECTS. 313 



alighted upon a rail on which was its station. It keptcrawl- 

 ing under the rail till it arrived at the part opposite to 

 the fly, when stealing up it would attempt to leap upon 

 it. If it discovered that it was not perfectly opposite, it 

 would immediately slide down again unobserved, and at 

 the next attempt would come directly upon the fly's 

 back. Did the fly happen not to be within a leap, it 

 would move towards it so softly, that its motion seemed 

 not more perceptible than that of the shadow of the 

 gnomon of a dial. If the intended prey moved, the 

 spider would keep pace with it as exactly as if they 

 were actuated by one spirit, moving backwards, for- 

 wards, or on each side without turning. When the fly 

 took wing, and pitched itself behind the huntress, she 

 turned round with the swiftness of thought, and always 

 kept her head towards it, though to all appearance as 

 immovable as one of the nails driven into the wood on 

 which was her station : till at last, being arrived within 

 due distance, swift as lightning she made the fatal leap 

 and secured her prey a . I have had an opportunity 

 of observing very similar proceedings in Salticus see- 

 nicus. 



But the legs of insects are not the only organs by 

 which they leap. The numerous species of the elastic 

 beetles (Elater), skip-jacks as some call them, perform 

 this motion by means of a pectoral process or mucro. 

 These animals having very short legs, when laid upon 

 their backs, cannot by their means recover a prone 

 position. To supply this seeming defect in their struc- 

 ture, Providence has furnished them with an instrument 

 which, when they are so circumstanced, enables them 

 * Evelyn, quoted in Hooke's Microgr. 200 



