THE CIRCULATORY SYSTEM 35 



down and the cells of the fat body are yielding up their protein and 

 fatty contents. The blood must also be regarded as an important 

 reserve of food material: in the larva of Dei/ephila, during fasting, 

 the protein in the blood is rapidly consumed; and during pupal life 

 in the same insect more than half the total energy metabolism is 

 effected at the expense of the blood. 



(v) In metabolism. The haemocytes may also play a part in inter- 

 mediary metabolism; for example, in the conversion of tyrosine to 

 the polyphenols needed for melanin and sclerotin formation, and 

 perhaps in the formation of some of the blood proteins, 

 (vi) In immunity. The phagocytic blood-cells provide perhaps the 

 chief mechanism for protecting the insect from bacterial invasion; 

 but humoral immunity, both natural and acquired, does occur. The 

 discussion of these matters would take us too far afield, 

 (vii) In respiration. We have already seen that oxygen is ordinarily 

 conveyed directly to the tissues by the tracheal system. But many 

 cells are separated from the nearest tracheal tubes by an appreciable 

 space; sometimes, notably in the pupa, organs may be entirely de- 

 void of tracheal supply; and in some aquatic insects the tracheal 

 system is completely filled with fluid. Moreover, the tracheal system, 

 throughout its length, is permeable to oxygen, which must therefore 

 escape into the blood. Under all these circumstances, the blood acts 

 as a carrier of oxygen. There are, also, here and there among insects, 

 special arrangements of the tracheal system which seem to be de- 

 signed to aerate the blood : a convoluted tract in the aorta of the 

 honey-bee is richly supplied with tracheae; the posterior region of 

 the heart in the metapneustic larvae of Nematocera is often sur- 

 rounded by a basket-work of tracheoles; and in the thorax of Nepa 

 is a peculiar organ so richly supplied with tracheae and bathed with 

 blood as to suggest that it may be a kind of tracheal lung. 



In view of these relations the question naturally arises whether the 

 blood of insects contains chemical carriers of oxygen comparable 

 with the haemoglobins and haemocyanins of other animals. With a 

 single exception, this has not yet been proved to be the case. In those 

 insects such as the larva of the honey-bee, in which the properties of 

 the blood have been most carefully studied, it has not been found to 

 take up more oxygen than can be accounted for by physical solution ; 



