122 AN INTRODUCTION TO ENTOMOLOGY 



The pulsations of the heart. — When a heart consists of several 

 chambers, they contract one after another, the wave of contraction 

 passing from the caudal end of the heart forwards. As the valves 

 between the chambers permit the blood to move forward but not 

 in the opposite direction, the successive contraction of the chambers 

 causes the blood received through the ostia to flow toward the head, into 

 the aorta. 



The aorta. — The cephalic prolongation of the heart, the aorta 

 (Fig. 139, a), is a simple tube, which extends through the thorax into 

 the head, where it opens in the vicinity of the brain. In some cases, 

 at least, there are valves in the aorta. 



The circulation of the blood. — The circulation of the blood can be 

 observed in certain transparent insects, as in young naiads, in larvae 

 of Trichoptera, and in insects that have just molted. The blood flows 

 from the open, cephalic end of the aorta and passes in quite definite 

 streams to the various parts of the body-cavity and into the cavities 

 of the appendages. These streams, like the ocean currents, have no 

 walls but flow in the spaces between the internal organs. After 

 bathing these organs, the blood retiuns to the sides of the heart, 

 which it enters through the ostia. 



Accessory circulatory organs. — Accessory piilsating circulatory 

 organs have been described in several insects. These are sac-like 

 structures which contract independently of the contractions of the 

 heart. They have been found in the head in several Orthoptera; in 

 the legs of Hemiptera, and in the caudal filaments of Ephemerida. 



VI. THE BLOOD 



The blood of insects is a fluid, which fills the perivisceral cavity, 

 bathing all of the internal organs of the body, and flowing out into the 

 cavities of the appendages of the body. In only a comparatively 

 small portion of its course, is the blood enclosed in definite blood- 

 vessels; these, the heart and the aorta are described abo^-e. The 

 blood consists of two elements, a fluid plasma and cells similar to the 

 white corpuscles of the blood of vertebrates, the leucocytes. 



The blood of insects differs greatly in appearance from the blood 

 of vertebrates, on account of the absence of red blood-corpuscles. In 

 most insects the blood is colorless ; but in many species it has a yellow- 

 ish, greenish, or reddish color. In the latter case, however, the color 

 is not due to corpuscles of the type which gives the characteristic 

 color to the blood of vertebrates. 



