THE CIRCULATION IN INSECTS GENERALLY. 637 



resistance to the flow of blood. Again, the circulation through 

 the wings is dependent on the efferent channels of the nervures 

 communicating with sinuses in front of the mesophragma ; 

 whilst the afferent channels which return the blood to the 

 thorax communicate with cavities behind it. The circulation 

 is thus maintained by the arrangement of the peritracheal 

 passages in which it flows, and not by the existence of subsi- 

 diary hearts. These definite sinuses resemble the lymphatic 

 capillaries of Vertebrates, and under certain circumstances the 

 course of the circulating fluid may be arrested or reversed in 

 direction, but in general its direction and course are constant 

 and uniform in all the large tissue spaces or peritracheal sinuses 

 of the insect. 



The Dorsal Vessel consists of two parts, the ventricles, situated 

 in the pericardial cavity in the abdomen, and the aorta, which 

 is not contained within a pericardium and which traverses the 

 thorax to the head. The whole may be described as a cylindrical 

 muscular tube, which is dilated into a series of ventricles, one 

 behind the other, in the abdomen, and narrowed into a uniform 

 cylinder, which extends from the anterior part of the anterior 

 abdominal ventricle to the head. In some insects the number 

 of ventricles reaches eight, but there are usually fewer. Gene- 

 rally the anterior ventricle is the largest. 



The Pericardial, or Auricular Sinus. This cavity is separated 

 from the general cavity of the abdomen by a horizontal fibro- 

 muscular septum, perforated by numerous pores, by which the 

 blood flows into it from the abdominal cavity. The septum is 

 termed the pericardial septum. 



The Alar Muscles. The pericardial septum is largely formed 

 by a series of fan-shaped muscles, which arise by narrow 

 origins of a tendinous character from the integument of the 

 dorsal region on either side of the middle line, usually one pair 

 from each abdominal ring. The muscles spread out fan-wise 

 and unite with each other in the middle line of the pericardial 

 septum. 



By their contraction they cause the septum to descend, thus 



