EXCRETION 55 



insects, it rapidly appears in the Malpighian tubes, which are un- 

 questionably excretory organs. In the primitive Apterygota it ap- 

 pears, also, in the lower segment or labyrinth of the labial glands, 

 and it is probably correct to regard these also as excretory organs, 

 homologous perhaps with the excretory glands of Crustacea. That 

 is all the information obtainable with indigocarmine. 



But if the acid dye ammonia carmine (which, in vertebrates, is 

 filtered through the glomerulus of the renal tubule) is injected into 

 insects, it is not excreted by the Malphighian tubes, but is absorbed 

 and segregated by special cells - notably, by the 'pericardial cells' 

 which lie along the dorsal heart and aliform muscles, by the 'gar- 

 land-like strand' discovered by Weismann (1864) in the Muscid 

 larvae, by various scattered groups of cells, and, in the Apterygota, 

 by the upper segment or saccule of the labial gland. On this evidence 

 Kowalevsky (1889) put forward the view that these cells are excre- 

 tory organs (so-called 'acid excretory organs', because litmus is 

 turned red within them) analogous in function to the kidney 

 glomerulus. 



'Nephrocytes' 



Kowalevsky's experiments were so striking that this theory at once 

 gained acceptance; but it is'doubtful if it will bear close examination. 

 Our ideas on the kidney glomerulus have changed so much in the 

 last eighty years that Kowalevsky 's analogy carries no weight at all. 

 If the ammonia carmine cells are really 'storage kidneys' (nephro- 

 cytes, athrocytes) as the theory implies,. the waste substance which 

 they accumulate during life should at least be visible. But in many 

 insects nothing seems to accumulate within them at all. Often they 

 contain pigmented granules ; but this is the case with many cells - 

 in the fat body, epidermis, tracheal matrix and so forth. Occasionally 

 they may become laden with some pigment from the food ; in Rhod- 

 nius they are filled with blue-green granules of biliverdin derived 

 from the breakdown of traces of haemoglobin that have been ab- 

 sorbed unchanged from the gut; and it cannot be denied that they 

 are then serving as 'storage kidneys' for these particular substances; 

 but this can scarcely be their chief function. Perhaps they play some 

 undetermined part in intermediary metabolism; or they may be 



