ANATOMY AND PHYSIOLOGY 139 
The rate of respiration increases or diminishes with the 
activity of the insect and with temperature and other condi- 
tions. In six specimens of Melanoplus differentialis, held be- 
tween the fingers, the thoracic spiracles opened and closed 
respectively 34, 43, 45, 54, 60 and 61 times per minute. Four 
individuals of M. femur- 
rubrum under the same cir- 
cumstances gave 70, 78, 90 
and 92. 
At expansion inspiration 
takes place, and at contrac- 
tion expiration occurs. In 
the grasshopper, the thoracic 
spiracles open almost simui- 
4 
H 
H 
Hy 
FY 
> 
AI 
taneously with the expan- 
sion of the abdomen. Con- 
traction is effected by special 
vertical expiratory muscles 
(Fig. 177), but expansion 
is due to the elasticity of 
UPABITARUOEDL, 
the abdominal wall, abeye cal Diagrammatic cross section of abdomen 
_ 5 oS 4 a A of a grasshopper, Acridium. d, dorsal 
rule ? this 1S the reverse septum, or diaphragm; ex, expiratory mus- 
of what occurs in mam-_ le; f, fat-body; g, ganglion; h, heart; in, 
3 . : inspiratory muscle; v7, ventral septum, be- 
mals, where expiration 1S low which is the ventral sinus. The dorsal 
and ventral septa rise and fall periodically. 
—After GRABER. 
passive and ‘inspiration ac- 
tive. Inspiratory muscles 
are found, however, in Acridiidz, Trichoptera and Hymen- 
optera. 
Though the respiratory movements of an insect may be 
studied with a hand-lens, a more precise method is that of 
Plateau—the chief authority on insect physiology—who made 
use of the stereopticon to project an enlarged profile of the 
insect upon a screen, on which could be marked the different 
contours of the abdomen at its phases of inspiration and 
expiration. 
The way in which the air reaches the finest tracheal branches 
