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 barbules. 
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 (d), 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 of the metallic barbules, while the distal 
region becomes modified into the club-shaped body, 
and is then completely devoid of hooklets 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 
