ANATOMY OF INSECTS INTERNAL 33 



extends the length of the body along the median line just beneath 

 the notum. In the abdomen of adult insects this tube is divided 

 into several chambers, each of which has a valve at either side, 

 allowing the blood to flow into it but preventing its escape. The 

 chambers are also separated by valves which allow the blood to 

 flow forward but prevent its backward passage. The abdominal 

 part of the tube, the heart proper, pulsates and drives the blood 

 toward the head, while the forward part is a simple bloocl vessel, 

 called the aorta, which usually divides in the head, where it ends 

 abruptly, allowing the blood to flow into the body cavity. Thus the 

 blood is admitted to the heart by the lateral valves, is forced forward 

 to the head, and thence flows in more or less defined currents 



FIG. 38. Portion of a trachea of a caterpillar, with its branches 

 (After Leydig, from Gegenbauer) 



throughout the body, bathing all the organs. The pulsation of the 

 heart and the flow of the blood may be observed in many thin- 

 skinned larvae and nymphs. 



The blood consists of a watery fluid, the plasma, or serum, 

 and the white corpuscles, or leucocytes. Usually colorless, it is 

 often yellowish or greenish. The blood has almost nothing to do 

 with the aeration of the tissues, that being done by the respiratory 

 system, as described below, its chief function being to nourish the 

 tissues with the food materials that it carries. 



Respiratory system. Insects have no lungs, but breathe through 

 a system of tubes, called trachea, which extend to all parts of the 

 body, bringing fresh air to the tissues and carrying off the carbon 

 dioxide. On each side of two thoracic segments, and on all the 



