The Structure and Special Physiology of Insects 1 7 

 behind and before it, the posterior one being closed behind and 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. 30. 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.) 



