vii THE COLOURS OF THE LEPIDOPTERA 149 



microscope a wonderful play of colours which is 

 quite invisible to the naked eye. He suggests 

 reasonably enough that it is in no respect im- 

 probable that these colours may be visible to other 

 insects although not to us except by the aid of 

 optical instruments, and that therefore they may 

 quite alter the appearance of the insects as seen by 

 other butterflies. Such observations are of interest 

 as tending to exercise a check upon the purely sub- 

 jective treatment of questions of colour-resemblance. 



The Pigments of the Pierid^e 



As an introduction to the discussion of the pig- 

 ments of butterflies we may take Mr. F. Gowland 

 Hopkins's laboriously patient work on The Pigments 

 of the Pieridce. 



The Pieridae include a large number of butterflies, 

 among which the most familiar are our common 

 Cabbage Butterflies or Garden Whites ; they are 

 very widely distributed, and are said to exhibit in 

 a striking manner the phenomena of mimicry. In 

 America especially they are said to mimic very 

 closely the (by hypothesis) well-protected Heliconidse. 

 The Pieridae exhibit a relatively simple plan of colora- 

 tion ; optical colours are absent or little developed ; 

 and we shall see later that this greatly limits the 

 possible colouring. In fact we find that the colours 

 are mainly white, yellow, and black. The yellow, 

 in accordance with a rule which is exceedingly pre- 

 valent among butterflies, may deepen into orange or 

 red, this occurring most frequently in species from 



