The Structure and Special Physiology of Insects 17 
behind and before it, the posterior one being closed behind arid the anterior 
Fig. 29.—Cells of digestive epithelium of stomach (ventriculus) of crane-fly, Ptychoptera 
sp., showing secretion of digestive fluids, or expulsion of cell-content. (After Van 
Gehuchten; greatly magnified.) 
one extending forward into or near the head as a narrowed tubular anterior 
portion, which is sometimes called the 
aorta. From the anterior open end of 
this aorta the blood, forced by pulsations 
of the heart-chambers, which proceed 
rhythmically from the posterior one 
forward, pours out into the body-cavity, 
proceeding in more or less regular cur¬ 
rents or paths, but never enclosed in 
arterial vessels, bathing all the tissues, 
and carrying food to them. Finally 
taking up fresh supplies of food by bath¬ 
ing the food-absorbing walls of the 
alimentary canal, it enters the chambers 
of the heart through lateral openings in 
these (either at the middle or anterior end 
of each), which thus establish communi¬ 
cation between the body-cavity and heart- 
The blood receives no more oxygen than 
it needs for its own use, and thus does 
not play nearly so complex a function in 
the insect’s body as in ours. And this 
simplicity of function probably explains 
in some degree the extreme primitiveness 
of the make-up of the circulatory system. 
It will be seen that the respiratory 
system, on the other hand, is particularly highly developed, as it devolves 
Fig. 31. 
Fig. 30. —Diagram of circulatory 
system of a young dragon-fly; in 
middle is the chambered dorsal 
vessel, or heart, with single artery. 
Arrows indicate direction of blood- 
currents. (After Kolbe.) 
Fig. 31. —Dissection showing dorsal 
vessel, or heart, of locust, Dis- 
sosteira Carolina. (After Snodgrass; 
twice natural size.) 
