TRACHEATES 1 89 



down the sides, which stand at the same angle to the 

 length of the body as the side-ribs of a leaf do to the 

 central rib. This colouring gives great protection to the 

 caterpillars, as it divides their body into sections just as 

 the ribs do the leaf. 



In the pupa stage, when the insect cannot fly, it is 

 especially important to have protective colouring ; and 

 it is, as a fact, very common amongst pupae. On the 

 other hand, a flying insect cannot have protective 

 colouring because of the constant change of the 

 animal's background ; moreover, a flying object is 

 always easy to see. Hence it is that the upper 

 surfaces of the wings of many butterflies are of a 

 light colour, so that the marks for recognising the 

 species are well in evidence. But the insects are not 

 always flying ; they often rest, and could then, especi- 

 ally when they are asleep, be surprised by enemies. 

 They therefore need a protective colouring, though this 

 is only necessary at the parts that are seen when the 

 animal is at rest. 



From this we can understand why nocturnal butterflies 

 (or moths) and diurnal butterflies have protective colour- 

 ing at different parts of the body. The day-lepidopters 

 fold their wings over them when they are at rest, so that 

 we only see the under-side. This alone, therefore, has 

 a protective colour in their case. We have a striking 

 example of this in the tortoise-shell butterflies. In the 

 day peacock's-eye and the black butterfly the very light 

 colours of the upper-side of the wings disappear when 

 they are folded. The dark brown of the under-side 

 now makes the creatures hardly distinguishable in the 



