34 INSECT PHYSIOLOGY 



unknown : they increase in number during moulting, like the plasma- 

 tocytes; and they collect around foreign bodies in a zone peripheral 

 to the plasmatocytes themselves. 



Finally, in certain Hemiptera, there are blood-cells laden with fat 

 globules, 'adipocytes', and others charged with wax; but the relation 

 of these to the types described above is uncertain. 



The functions of the blood 



(i) Mechanical. The blood plays an important part in transmitting 

 pressure from one region of the body to another : the hatching of 

 many insects from the egg (p. 10), the rupture of the old skin at 

 moulting (p. 9), the expansion of the wings in the newly hatched adult 

 insect (p. 10) and many other movements are brought about by the 

 localized pressure of the blood. 



(ii) In blood coagulation. In some insects, such as the larva of the 

 honey-bee and in Rhodnius and many other Hemiptera, the blood 

 does not clot; but in most insects it does, and thus plays an impor- 

 tant part in the closure of wounds. In the haemolymph of most 

 insects, besides the haemocytes mentioned above, there are certain 

 'hyaline haemocytes', indistinguishable in ordinary stained prepara- 

 tions but easily recognized with the phase contrast microscope. 

 When the blood coagulates these cells extrude thread-like pseudo- 

 podial expansions or produce a fine precipitate in the plasma around 

 them. 



(iii) In connective tissue formation. The neutral mucopolysaccharide 

 inclusions of the plasmatocytes are discharged during the course of 

 moulting on to the basement membranes and connective tissues 

 which separate the tissue cells from the haemolymph. What propor- 

 tion of these membranes is contributed by the blood cells and what 

 proportion by the tissue cells themselves is uncertain and surely 

 varies from one tissue to another. 



(iv) In nutrition. The blood conveys the nutrient materials and hor- 

 mones to the tissues, and the waste products to the excretory organs. 

 The precise part that it plays in these functions is not really under- 

 stood, but it is probable that the composition of the blood may vary 

 enormously with the state of nutrition and, notably, during moulting 

 and metamorphosis when many of the larval tissues are breaking 



