MOTIONS OF INSECTS. 



247 



move in this way, which is not improbable, from the 

 difficulty of attending at the same time to the movements of 

 so many members, is not easily ascertained. 



The dog-tick (Ixodes Ricinus), if when young and active 

 it moves in the same way that it does when swoln to an 

 enormous size with blood, seems to afford an exception to 

 the mode of walking just described. It first uses, says Ray, 

 its two anterior legs as antennae to feel out its way, and then 

 fixing them, brings the next pair beyond them, which being 

 also fixed, it takes a second step with the anterior, and so 

 drags its bloated carcass along. 1 Kedi observes that when 

 scorpions walk they use those remarkable comb-like processes 

 at the base of their posterior legs to assist them in their 

 motions, extending them and setting them out from the body, 

 as if they were wings : and his observation is confirmed by 

 Amoreux, who calls them ventral swimmers. 2 I have often 

 noticed a millepede (Julns terrestris), frequently found under 

 the bark of trees,, and where there is not a free circulation 

 of air, the motions of which are worthy of attention. Ob- 

 served at a little distance, it seems to glide over the surface, 

 like a serpent, without legs ; but a nearer inspection shows 

 how its movement is accomplished. Alternate portions of 

 its numerous legs are extended beyond the line of the body, 

 so as to form an obtuse angle with it ; while those in the 

 intervals preserve a vertical direction. So that, as long as 

 it keeps moving, little bunches of the legs are alternately 

 in and out from one end to the other of its long body ; and 

 an amusing sight it is to see the undulating line of motion 

 successively beginning at the head and passing off at the 

 tail. The motion of centipedes (Scohpendra), as well as that 

 of this insect and its congeners, is retrogressive as well as 

 progressive. Put your finger to the common one (Lithobius 

 forficatus), and it will immediately retrograde, and with the 

 same facility as if it was going forwards. This difference, 

 however, is then observable — it uses its four hind legs, 

 which, when it moves in the usual way, are dragged after it. 

 Almost all the other apterous insects, as well as many of 



' Hist. Ins. 10. 



2 Rcdi, Opusc. i. 80. Amoreux, 44. 

 R 4 



