NUTRITION AND METABOLISM 79 



recognise the continued presence of the queen, and by the queen 

 during the mating flight as a sexual attractant for the drones. 



Pigment metabolism 



We have already seen that in the course of metabolism pigmented 

 substances may arise as waste products (p. 57). Pigments may also be 

 absorbed with the food and accumulate within the body: the flavone 

 pigment in the wing-scales of the butterfly Melanargia, for example; 

 and the carotinoids in the blood of the potato beetle Leptinotarsa, 

 and in the epidermal cells of the bug Perillus which preys upon it. 

 But it is remarkable that apparently similar pigments in more or less 

 related insects may be quite unrelated chemically : thus, the common 

 red and orange pigments of many Hemiptera may be carotinoids 

 from the food, anthocyanin-like or flavone-like pigments, perhaps 

 also from the food, pterins synthesized along much the same lines as 

 uric acid, or ommochromes derived from the metabolism of the 

 amino acid tryptophane; and the Pieridae may use their pterin 

 colours to mimic other butterflies the chemistry of whose pigments 

 is quite different. Similarly with the green pigments: in many cases 

 these are synthesized by the insect itself, and have no relation to the 

 chlorophyll of plants, even in phytophagous species; for their de- 

 velopment is unaffected (in Carausius, in locusts, and in many cater- 

 pillars) by rearing them upon food containing no pigment ; nor have 

 they the chemical properties of chlorophyll (p. 57) ; they are com- 

 monly mixtures of the blue biliverdin with some yellow chromopro- 

 tein containing xanthophyll or carotene. The sexual differences in the 

 blood of many caterpillars, which have been ascribed to differences in 

 the utilization of plant pigments, are probably due rather to dif- 

 ferences in the composition of the haemolymph proteins. 



The other type of insect pigment is the insoluble black substance 

 melanin, which is formed in the presence of oxygen by the action of 

 an oxidizing enzyme upon some colourless aromatic precursor, the 

 'chromogen'. Melanin-producing enzymes are generally diffused 

 throughout the blood and tissues and secretions of insects, particu- 

 larly near the time of pupation when much black pigment is often 

 formed; the chromogens are usually deposited in restricted areas, 

 and so determine the colour patterns that develop. 



