118 THE ALIMENTARY AND EXCRETORY SYSTEMS [CH. 



sections, usually only from three to five cells are met with in the 

 case of the Dragonfly, though six or more appears to be the usual 

 number in most insects. The fine chitinous intima, which bounds 

 the lumen in some insects, has so far not been seen to exist in the 

 Dragonfly. 



2. Physiology of Excretion. 



It is the function of the Malpighian tubules to extract nitrogenous 

 and other waste products from the blood, in the form of uric acid 

 and other compounds. Griffiths [59] has conclusively shewn this in 

 the case of the Dragonfly, both by obtaining the crystals direct from 

 the tubules, and also by the more delicate murexide test. It is 

 supposed that the crystals are got rid of simply by being passed 

 down the tubules into the hind-gut. The passage even of very 

 minute crystals down the irregular canals, in the case of the Dragonfly, 

 would be a matter of considerable difficulty. 



The fat-body plays a secondary part in the physiology of 

 excretion, by reason of its general function as a store-house for 

 excess products. When the blood is so overloaded with impurities 

 that the Malpighian tubules cannot adequately deal with them, 

 the fat-body comes to the rescue by storing up the excess in certain 

 of its cells. These impurities are later on given out again, when 

 the tubules are able to deal with them. 



Voinov [187] has shewn that the cell- wall of the heart can remove 

 foreign matter from the blood, and pass it to the haemocoele. Such 

 an action does not really rid the blood of impurities, but only serves 

 to clear the passage of the heart. 



The Fat-body. 



The fat-body is more conspicuous in the larva than in the imago. 

 In neither, however, does it form so large a mass as in the majority 

 of insects. It is a lobed mass of a whitish or yellowish colour, 

 forming a kind of sheath or envelope for the various internal organs. 

 It is especially noticeable in th region of the heart, the alimentary 

 canal, the large tracheal trunks, and more particularly the gonads. 



The fat-body of the Dragonfly appears to consist of only two 

 kinds of cells, known as adipose cells and pericardial cells. The 

 uric-acid cells and bactericidal cells found in Orthoptera are absent, 

 or at any rate have not yet been noticed. 



