36 INSECT PHYSIOLOGY 



but it must be admitted that such investigations have not yet been 

 very extensive. The single exception is in the larvae of certain Chiro- 

 nomidae, which contain haemoglobin in the blood in free solution - 

 that is, not in corpuscles. The affinity of this haemoglobin for oxygen 

 is, however, very different from that of mammals. At all ordinary 

 tensions of oxygen it remains fully saturated; and it only begins to 

 liberate its oxygen when the pressure of this gas is reduced to 1 per 

 cent, of an atmosphere or less; the precise level varying in different 

 species. In other words, it is capable of acting as a carrier only under 

 conditions of extreme paucity of oxygen and seems, in fact, to be a 

 special adaptation to life in poorly oxygenated waters - the larvae 

 that contain haemoglobin being much less susceptible to oxygen want. 

 We have seen, also, that the blood probably plays a considerable 

 part in the carriage of carbon dioxide from the tissues (p. 15). But 

 here again there seems to be no chemical provision for its transport; 

 the bicarbonate and the carbon dioxide capacity of insect blood are 

 both very low. 



Organs and tissues associated with the blood 



The blood itself is an important tissue; but it is not commonly 

 recognized as such because the cells of which it is composed are free 

 and unattached and, with the exception of the 'phagocytic organs' 

 (p. 33), show no constant arrangement. But there are other organs 

 and tissues which are likewise bathed by the circulatory fluid and 

 which perform their functions solely through exchanges with it. 



(i) There are the pericardial cells scattered along the heart, alary 

 muscles and aorta. These have been considered in the past to have 

 an excretory function (see 'nephrocytes' p. 55) ; but nowadays they 

 are regarded as comparable with the reticulo-endothelial system of 

 vertebrates (p. 56). We have already referred to them as a source of 

 a cardiac accelerator (p. 31). 



(ii) The most obvious of these organs is the fat body which is often 

 the most conspicuous object in the body-cavity of the insect. It 

 consists of a loose meshwork of lobes, invested in delicate connec- 

 tive-tissue membranes, so as to expose the maximum of surface to 

 the blood. The whole mass has a fairly regular anatomical arrange- 

 ment constant in each species. The fat body is important in the 



