28 INSECT PHYSIOLOGY 



insect is at rest, is at atmospheric pressure (Odonata) or even less 

 (Dytiscus, Apis). In the more soft-bodied insects, the general pressure 

 may be raised (to 40 or 50 mm. of water in the larva of Aeschna, or 

 even more during exertion)by the muscular tension of the body-wall. 

 The pressure may be increased, also, during the ventilation of the 

 tracheal system; and during those special acts which are brought 

 about by the displacement of the blood, such as the expansion of 

 the wings in emerging adults (p. 10), the general pressure may be 

 maintained for a prolonged period at a high level (75 mm. of water 

 in the dragon-fly). 



But, like the blood in the vena cava of mammals, the blood of 

 insects is always aspirated into the heart, during diastole, under a 

 negative pressure: a pressure less than that existing in the general 

 body cavity. This force is due partly to the elastic muscular walls of 

 the heart itself, partly to the elastic traction of the dorsal diaphragm 

 which is tied to the lower wall of the heart, and partly to the contrac- 

 tion of the muscle fibres (the aliform muscles) which commonly occur 

 in the substance of this diaphragm. During systole, a weak positive 

 pressure develops and the blood is driven forwards. 



In some insects, such as the larvae of Nematocera and the Ephe- 

 meroptera, the valves of the ostia have become so modified as to 

 form interventricular valves; these divide the heart into a succession 

 of chambers through which the blood can flow only forwards. But 

 this arrangement is the exception ; in most insects the heart is patent 

 throughout its length and acts as a whole, the blood being carried 

 along by peristaltic waves. As a rule, such waves start at the hind 

 end of the heart and pass forwards ; but in many insects the direction 

 of beat may be periodically reversed, and the blood then escapes into 

 the abdominal cavity through the relaxed ostia. Sometimes beats 

 may arise at a number of points simultaneously and extinguish one 

 another when they meet. 



Thus the circulation of the blood is secured primarily by the work 

 of the heart, which aspirates it from the abdominal cavity and pumps 

 it forwards to the head. In the thorax, accessory pumps are often 

 present. These aspirate blood from the thoracic cavity, through cer- 

 tain of the wing veins, and return it through connecting vessels 

 either to the aorta itself (Dytiscus, Aeschna larva, Sphinx) (Fig. 6, A) 



