34 THE BIOLOGY OF INSECTS 



resembling thus the leucocytes or " white corpuscles " of 

 the blood of vertebrates. When, as is sometimes the case, 

 an insect's blood is coloured yellow, green, or even red, the 

 pigment is dissolved in the plasma, not, as in the red blood 

 of vertebrates, concentrated in the specialised cells known as 

 *' coloured corpuscles." 



The blood, being the circulatory agent in the body, must 

 be kept moving in a constant and regular flow. The pro- 

 pellant organ for this circulation is the heart (Fig. 12, h), 

 which has already (p. 6) been briefly described as a narrow 

 tube running fore and aft in the middle axis of the insect, 

 just beneath the dorsal body-w^all, while below it a deUcate 

 membrane is stretched from side to side, forming the floor 

 of the shallow pericardial chamber (Fig. 12, pc) in which 

 the heart lies. It will be remembered that this pericardial 

 cavity is a blood-space. The heart is formed of a variable 

 number of chambers arranged one behind the other in a 

 series, agreeing more or less with the segmentation or serial 

 repetition of similar parts that characterises the body of an 

 insect or other arthropod. But while in a cockroach the 

 heart extends the whole length of the body, with thirteen 

 chambers corresponding to the thrte ' thoracic and ten 

 abdominal segments of the insect, in a bee there are only 

 four chambers all situated in the abdomen, while in a few 

 insect types only a single chamber can be recognised. The 

 wall of the heart is muscular, its hinder end is closed and 

 the blood driven through the successive chambers towards 

 the head, the wave of muscular contraction passing on along 

 the tube from behind forwards. Each chamber is some- 

 what wider at its hinder than at its front end, where the 

 constricted portion is provided with a valve that allows the 

 blood to flow forward but not to return towards the tail. 

 Into the hinder widened end of each chamber open a pair 

 of minute slits (ostia) provided with valves which allow the 

 blood to pass into the heart from the surrounding peri- 

 cardial blood-space, but not in the reverse direction. As, 

 therefore, the rhythmic contractions of the tubular heart 

 drive the blood constantly forward towards the head, a 



