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 tothe 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 
