EXCRETION 57 



acid must be endogenous, for it is not increased by the artificial 

 introduction of preformed uric acid. In the larva of the mosquito 

 Aedes the cells of the fat body become laden with uric acid in the 

 fasting insect; but as soon as the larva is abundantly fed the cells of 

 the fat body are filled with reserve food substances and the uric acid 

 is displaced. 



It is probable that not a few of the urate cells of insects are of this 

 nature - not true excretory organs but active cells in which uric 

 acid has crystallized out, as it were, by accident. For instance, the 

 epidermal cells of some Hemiptera contain uric acid; but it has been 

 shown that in one case at least, that of Rhodnius prolixus, the de- 

 position of this uric acid takes place only at one stage of the moulting 

 cycle -just before the new cuticle is laid down; that is, when the cells 

 in question are most active in producing chitin - perhaps from the 

 proteins of the food. 



Pigments 



Besides this uric acid, there is in the epidermal cells of Rhodnius 

 a red pigment, which appears at the same time as the uric acid, 

 and then slowly diminishes. Perhaps this substance, also, is a by- 

 product of the synthetic activity of these cells; and perhaps that 

 is the origin of many of the pigments of insects - accidents in meta- 

 bolism, which only secondarily acquire their biological significance. 

 Such substances as the pterin pigments of many types that occur in 

 the wing scales of Pieridae and other Lepidoptera, or beneath the 

 integument of wasps, the anthraquinone derivatives which form the 

 red colour of the cochineal insect, and the remarkable pigments of 

 Aphids (aphins), also complex derivatives of anthracene, are prob- 

 ably of this type. The red and brown eye colours of insects (ommo- 

 chromes) are derivatives of kynurenine, itself an oxidation product of 

 the important amino-acid tryptophane. The green pigments of in- 

 sects (insectoverdins) are usually mixtures of the blue biliverdin 

 (presumably derived from the breakdown of haemoglobin or cyto- 

 chrome) and some yellow carotinoid obtained from the food. While 

 some insect pigments, such as the flavones in the wings of Satyrine 

 butterflies, are wholly derived from preformed substances already 

 present in their food plants. 



