56 INSECT PHYSIOLOGY 



excretory organs in the sense of synthesizing the waste products that 

 are to be eliminated by the Malpighian tubes. They are highly 

 capricious in their absorptive properties: they will take up various 

 proteins; they will absorb some dyes, such as trypan blue and am- 

 monia carmine, but not others; and it has not been possible to recog- 

 nize any common feature in molecular structure or physical or 

 electrical properties to account for these differences. These proper- 

 ties are doubtful evidence of an excretory function. In vertebrates 

 these same properties are shown by cells of the reticulo-endothelial 

 system, and the pericardial cells of insects are commonly regarded as 

 being equivalent to these. We have noted already their function of 

 producing a cardiac accelerator in the cockroach (p. 31). 



Urate cells 



As was observed by Milne Edwards and later by Fabre (1863), there 

 are cells in the insect body which become laden with crystalline 

 spheres of uric acid ('urate cells') ; and they surely can be regarded as 

 storage kidneys, for uric acid is an undoubted excretory substance. 

 In many cases this view is probably correct; throughout larval life 

 in the social Hymenoptera, uric acid collects in solid form in special 

 urate cells scattered throughout the fat body, and is not transferred 

 to the Malpighian tubes (of the adult) until the end of pupal life; in 

 Lepisma, Dermaptera and many Orthoptera, this state of affairs 

 persists throughout life, and the Malpighian tubes discharge very 

 little uric acid. But even here the observations must be interpreted 

 with caution. For uric acid is the end product of protein katabolism; 

 and when protein breakdown is occurring very actively within a cell, 

 it is likely that the rate of formation of uric acid may, on occasion, 

 exceed the rapidity with which it can diffuse from the cell and be 

 earned away by the blood; particularly if conditions in the cell be- 

 come unduly acid. The uric acid will then crystallize out, and its 

 subsequent solution and removal will take place very slowly. That 

 indeed is what appears to happen in the cells of the fat body in 

 Muscidae during metamorphosis: granules of uric acid ('pseudo- 

 nuclei') crystallize out within the 'albuminoid' deposits, and not 

 until late in pupal life is the uric acid transferred to the Malpighian 

 tubes. The same thing happens in Lepidoptera; and here the uric 



