MUSCULAR SYSTEM. 91 



The otlier four muscles belong to a different group 

 altogether. The first of them (Pis. LYII, LVIII, h I) 

 rises rather behind and outside the ventral attachment 

 of the muscle a h. It then passes upwards and for- 

 wards and is inserted on the lateral wall of the body- 

 immediately above the ventral tube, and about halfway 

 between it and the central line of the back. 



The sixth muscle {m n) rises close to the preceding, 

 between it and the first, and has the same general 

 direction, but it is inserted higher up. 



The seventh (o j;) rises close to c, and consequently 

 further back than either of the preceding, nor does it 

 pass quite so far forward. It is attached not very far 

 from A*, but behind it. 



The last of this group (PI. LYIII, q) rises close 

 to n, and is inserted below n. 



PI. LVII, s t. This is a small muscle which moves 

 the second segment of the spring. 



There are also several other small muscles in the 

 posterior part of the abdomen, and belonging either to 

 the terminal abdominal segment or to the rectum. 



If a Smynthurns be examined after death, the tail 

 will almost invariably be found extended as in PI. 

 LYII. If, moreover, we consider the mode in which 

 the muscles just described act on the spring, we 

 shall see that the most powerful of them tend to draw 

 it forward, and not to extend it. 



When, indeed, we see a Smyntlmrus leap, one is apt 

 to be surprised at the muscular force which it must 

 possess. It would appear, however, that its power of 

 jumping arises from the elasticity of the spring, and 

 not from direct muscular action. This is, doubt- 

 less, a more economical arrangement of force. It cer- 

 tainly may require less strength to pull the spring 

 gradually forwards into position that it would to strike 

 it against the ground with force enough to throw the 

 Smyntlmrus so high up into the air. "We see the differ- 

 ence very well in a crossbow ; the muscular effort 

 required to set the bow is much less than that which 



