38 BEES AND BEE-KEEPING. 



make clear, when sharp eyes, or weak ones with a 

 lens, will detect rhythmic throbbings, continuing long, 

 and moving from behind forwards, driving the blood 

 towards the head, much as water rises through the 

 throat of a drinking horse. The walls of this heart 

 consist of three layers — an internal cuticle, a central 

 layer of muscular fibres, aoWn. in diameter, and an 

 outer coat of connective tissue. In the worker and 

 queen, the dorsal vessel has five ventricles, or con- 

 tractile chambers, corresponding to the five spiracles 

 on each side. As it nears the thorax, the muscular 

 and internal layer now formed into a conducting 

 tube, bends upon itself three or four times from side 

 to side (Plate I.), by which I imagine the rhythmic 

 beats are converted into a steady and equal dis- 

 charge of blood in the head beyond, where the tube 

 opens near to the brain. The vitalising fluid returns 

 by soakage through the body to the posterior part, 

 where it re-enters the dorsal vessel. The ventricles 

 are in valvular communication, while each one has 

 on its sides two openings (dv, Fig. 8), so contrived 

 that, as the muscular coat is causing a ventricle to 

 dilate, blood enters by them, the valve in front at 

 this time closing, as Plate I. will explain. When 

 contraction begins, an internal fold of the wall of 

 the ventricle closes the side apertures, and drives 

 the blood through the communication into the 

 ventricle in front, and in this manner the forward 

 stream is maintained. The dorsal vessel is braced to 

 the dermal skeleton by surrounding muscles, while 

 beneath runs an extension of muscular plates 

 (d, Fig. 8) of most involved character, forming a 



