80 INSECT PHYSIOLOGY 



The melanogenic enzymes can act upon a great variety of chromo- 

 gens: tyrosine (monoxyphenylalanine), 'dopa' (dioxyphenylalanine) 

 and various other polyphenols ; the complex of enzymes concerned 

 being termed tyrosinase. In the tissues and in the cocoons of certain 

 Lepidoptera {Samia cecropia, Eriogaster lanestris, &c.) and in the 

 elytra of Melolontha, the spontaneously oxidizable 'dopa' has been 

 demonstrated; and in the mealworm Tenebrio, dioxyphenylacetic 

 acid has been proved to be one of the chromogens present. It is, of 

 course, possible that more than one chromogen may be concerned in 

 a given insect; and in any case it is not easy to separate this process 

 of melanin formation in the cuticle from the closely allied process of 

 quinone tanning of protein that is responsible for the hardening of 

 the cuticle which is going forward at the same time. Experimentally, 

 the quantity of melanin formed can be widely varied by changes in 

 the external conditions, notably the temperature. In some cases 

 {Habrobracon, Hym.), these effects may be transmitted to the off- 

 spring up to the second generation. 



Melanin is incorporated in the substance of the cuticle; other 

 brown pigments (insectorubins or ommochromes) are deposited as 

 granules in the epidermal cells. Some pigments are in solution in the 

 blood or in the fat droplets of the cells (lipochromes), many appear 

 as inclusions in the epidermis or other tissues or between the layers 

 of the wing scales. 



In some insects the coloration may be strikingly influenced by 

 external factors. Sometimes, notably in the stick insect Carausius, 

 such changes are reversible - the so-called 'physiological colour 

 change'. There are no discrete chromotaphores in these cases, there is 

 simply a migration of pigment granules, a clumping or spreading, in 

 all the epidermal cells. In Carausius this change may be brought 

 about by a great variety of stimuli : temperature, narcotics, osmotic 

 changes in the blood, mechanical pressure, all of which act directly 

 upon the epidermal cells; and optical stimuli and humidity, which act 

 through the eyes and tracheal system respectively, and then, by way 

 of a centre in the brain, lead apparently to the secretion of a hor- 

 mone into the blood, which determines the pigment movements. 

 These pigment changes follow, also, a diurnal rhythm (the insect is 

 dark by night and pale by day) which, though determined primarily 



