270 COLOUR IN NATURE chap. 



The colours are produced by a modification of the 

 barbules of the metallic feathers. We have already- 

 described the general structure of barbules and 

 noticed that each is divided into two regions — a 

 proximal flattened region which may be called the 

 lamina, and a distal slender region which, from its 

 appearance, may be called the filamentous region. 

 Now as in sun-birds the metallic colours are usually 

 confined in quill-feathers to a lateral stripe, it is 

 obvious that it is possible to obtain a single barb 

 which bears both metallic and non-metallic ba'rbules. 

 If we examine microscopically a non-metallic barbule, 

 we shall find that it exhibits the ordinary structure 

 of a barbule, and shows quite distinctly the division 

 into two regions separated by a twist (see Fig. 2,b). 

 The metallic barbules {iT), on the other hand, are of 

 quite different appearance, being broad, flattened, club- 

 shaped bodies supported on a short stalk, and con- 

 taining abundant dark pigment. Close examination 

 of the barb (a) shows that the metallic and non-metallic 

 barbules are not perfectly sharply defined, but tend 

 to pass into one another. Thus, as we follow the 

 non - metallic barbules upwards, we find that the 

 lamina diminishes in size, while the filamentous region 

 becomes flattened, broader, and larger, at the same 

 time losing its slender processes (c). Finally, the 

 lamina becomes so much reduced as to form only the 

 short stalk oT the metallic barbules, while the distal 

 region becomes modified into the club-shaped body, 

 and is then completely devoid of booklets or pro- 

 cesses (cilia). These club-shaped barbules further 

 exhibit a series of cross bars which, according to 

 Gadow, are a series of compartments overlapping 



