ANATOMY AND PHYSIOLOGY 05 
with the fulcrum at p, it is easy to understand how the con- 
traction of muscle ec raises the wing and that of muscle d low- 
ers it. These muscles are shown diagrammatically in Fig. 
75, B. Besides these, there are certain muscles of flight which 
act indirectly upon the wings, by altering the form of the 
thoracic wall. Thus 
the muscle ze (Fig. 75, 
B) elevates the wing 
by pulling the tergum 
toward the sternum; 
and the longitudinal 
muscle 7d depresses the | 
wing indirectly — by ) || 
a= 
N 
arching the tergum of NG 
the thorax. 
Though up and 
down movements are 
all that are necessary 
for the simplest kind 
of insect flight, the 
process becomes com- 
plex in proportion to 
the efficiency of the 
flight. Thus in dragon 
flies there are nine 
; A, diagram to illustrate the action of the wing 
muscles to each wing: muscles of an insect. B, diagram of wing mus- 
_ , cles. a, alimentary canal; cn, muscle for con- 
five depressors, three 
tracting the thorax, to depress the wings; d, de- 
elevators and one ad- pressor of wing; e, elevator of wing; ex, muscle 
for expanding the thorax, to elevate the wings; 
ductor. id, indirect depressor; ie, indirect elevator; /, leg 
Abdomen. ase Al h e muscle; p, pivot, or fulcrum; s, sternum; ¢, ter- 
gum; wg, wing.—After GRABER. 
chief functions of the 
abdomen are respiration and reproduction, to which should be 
added digestion. The abdomen as a whole has undergone less 
differentiation than the thorax and presents a simpler and more 
primitive segmentation. 
Segments.—A typical abdominal segment bears a dorsal 
6 
