SIR JOHN LUBBOCK ON THYSANURA. | 289 
With patience and spirits of wine, however, 1 have been able to make out the principal 
muscles pretty clearly. 
Muscles of the Spring.—These muscles (Plate XLV. figs. 9-11) are nine in number on 
each side of the body. The stoutest of all (Plate XLV. fig. 10, ab) rises on the ventral 
side of the body, close to the middle line, and immediately in front of the reversible base 
of the saltatory apparatus. It passes upwards, expanding gradually, and is inserted by 
two heads on the side-wall of the body (Plate XLV. fig. 10, à). 
The second muscle, which is much smaller (c d), rises behind the first, and somewhat 
further from the middle line, being separated from the corresponding one on the other 
side by the central thickened part of the reversible base of the spring. It passes straight 
upwards, without altering in diameter, and is attached to the skin at d, behind but on 
the same line as the preceding muscle. 
The third muscle (ef) rises close to the preceding, but a little outside it, and, passing 
upwards and forwards, is attached close to the posterior branch of the first muscle, by 
which, therefore, in Plate XLV. fig. 10, it is necessarily concealed. 
The fourth (gh) lies parallel to, and rather behind the second; it is, however, rather 
shorter, and consequently does not reach so far up towards the back. 
The fifth (ij) rises close behind the last three. It is shorter and broader, and passes 
straight backwards to the posterior wall of the body, where it is inserted between the 
spring and the small terminal abdominal segment. The other four muscles belong to 
a different group altogether. The first of them (Plate XLV. fig. 10, 41) rises rather 
behind and outside the ventral attachment of the muscle a ð. It then passes upwards 
and forwards 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 (mn) rises close to the preceding, between it and the first, and has 
the same general direction, but is inserted higher up. 
The seventh (op) 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 £, but 
behind it. 
The last of this group (gr) rises close to n, and is inserted below o. 
Plate XLV. fig. 10, st. 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 Smynthurus be examined after death, the tail will almost invariably be found 
extended as in Plate XLV. fig. 11. 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 Smynthurus 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. I presume 
that this is a more economical arrangement of force. It certainly may require less 
strength to pull the spring gradually forwards into position than it es y strike it 
Q 
