THEIR ALLIES, 81 
ed, as seen in the body of the caterpillar. When, how- 
ever we turn to a thoracic segment, the relative size of 
the pieces is very unequal, the side-pieces being much 
larger than the upper or under piece, especially in the 
Dragon-fly, which is ever on the wing. In the Libellula, 
the upper part of the ring is greatly reduced in size, and 
the larger part of the ring consists of the side-pieces. As 
a rule, however, the under piece (sternum) is very small, 
the dorsal or upper-piece (fergum) is well developed, 
while the side-pieces are increased in a still greater ratio, 
as seen in the Wasp, which walks and also flies with ease. 
The side, or limb-bearing part of the ring, is generally 
largest in the running insects, as in the Beetles, of which 
Carabus, the Ground-beetle, is a type. On the other 
hand the dorsal (or tergal piece, the more technical name, 
since the word dorsal is more appropriate in speaking of 
the vertebrates, or animals with a back bone) part of the 
ring is quite small in the Dragon-fly and its allies. In 
these insects, which scarcely ever walk, merely using 
their legs in clinging to plants when resting from their 
long sustained flights, the side-pieces are disproportion- 
ately enlarged over the other parts of the ring, for the 
purpose of broad attachments to the muscles of 
flight. 7 
-To the Be ot tha wpa yes, okt es 
and wings, are attached. In order that the legs may 
move freely on the body, and thus give play to hundreds 
of minute muscles within the legs, these side pieces are 
subdivided into several smaller sections. Were this not 
so, and the crust forming the exterior of the insect un- 
broken, thus forming a continuous series of cylinders, 
we should have the poor victims of this stern law of 
morphology enclosed in jackets of the straightest sort! 
AMERICAN NAT. VOL. I. 11 
