“144 NATURAL HISTORY OF WASPS. 
from the radiate type of development. Other than 
respiratory purposes are answered by the peculiar 
form and mode of arrangement of these organs, and 
particularly of the large air cavities. One of these 
is the forcible distension of various parts of the body, 
so as to obtain a better fulcrum for, or change the 
direction of, the action of the muscles. Another, to 
which the large abdominal air vesicles mainly sub- 
serve, is that of altering the external figure, and shift- 
ing the position of the weight inside. The specific 
gravity of the insect is thus also altered, but this,* 
though very important to animals moving in such a 
heavy fluid as water, is of comparatively little con- — 
sequence to those which move in the air. Flight is 
a question of well directed strength, not of weight, 
and the advantage of the dimimution of specific 
gravity is more than compensated by the attendant 
increase of bulk. But the power of trimming the 
body by pushing the heavy stomach to one or other 
side, and of poising the whole body on the wings asa 
centre by lengthening the abdomen, both which can 
be effected by the expansion of the abdominal air 
vesicles, is very important. These reservoirs of air 
act also as springs to equalize the pressure of the 
intermittent respiratory movements, like the air 
chamber in a fire-engine. And they assist m those 
functions of the abdominal viscera which in the 
higher animals require the aid of the diaphragm. 
Further, they have their use in all those physiological 
processes which are accompanied im the higher ani- 
mals by vascular turgescence, but in the insect 
economy by increased afflux of air. All these func- 
* * Reign of Law,’ p. 151. 
