Colouration of Animals. 
6 53 
To proceed : Mr. Lewis contends, as we understand him, 
that the colouration of animals, especially of inserts, is due 
not to physiological processes carried on within the body, 
but to the mechanical surface adtion of the light-rays and 
of the invisible heat-rays. Thus he holds that the most 
brilliant, and especially the iridescent, colours appear in 
animals, or in parts _ of animals, upon which the rays of 
light fall in constantly varying directions. As an instance 
he referred to the varied play of colour so often observed on 
the necks of pigeons, a part which is almost constantly re- 
ceiving the sun’s rays in a varying direction, whether the 
bird is engaged in its usual circular flights, or walking about, 
or is even sitting still — the neck being kept almost constantly 
in motion. Many other instances might here be given which 
seem to support this notion. Thus the tail and the neck of 
the peacock undergo a very varied exposure to light. So 
does the entire plumage of most humming-birds, and the 
wings of the diurnal Lepidoptera as they flit up and down 
in their constantly wavering flight. But there are other 
cases of brilliant colouration in creatures of very sedentary 
habits. Few groups of birds are more splendidly coloured 
than the trogons, which sit, dull and sullen, upon boughs 
in the shady parts of woods. Not a few ground-beetles, 
which very rarely fly, and which creep mostly on flat sur- 
faces, are also splendidly coloured. 
Mr. Lewis further calls attention to the circumstance that 
inserts which creep upon the ground have their underside 
black or colourless ; such as fly, or as creep upon leaves 
and flowers, are often gaily tinted beneath. As an affirma- 
tive instance he mentioned the genus Calosoma. C. syco - 
phantciy notwithstanding the splendour of its elytra, is black 
on its underside. But according to Mr. Bates a certain 
tropical species of Calosoma — which Mr. Lewis did not 
name — passes its life upon trees, and is brilliant upon both 
sides. Yet there is a British species of the same genus, 
C. inquisitor , which is chiefly found upon oak trees, but is 
below of as sombre a tint as the terrestrial species. 
Mr. Lewis may appeal with confidence to the difference in 
the colouration of the upper and lower surfaces traceable in 
two closely allied groups, the Cicindelidse and the Carabidse. 
The former — of which our common tiger-beetle (C. campes- 
tris), to be seen on any fine day during the spring and early 
summer at the upper extremity of Epping Forest, is a 
typical example — are during all the sunny hours of the day 
in rapid and unceasing motion, taking short flights, alighting 
for a moment on the heath or the gravel, and then again 
