44 FORM AND COLOUR. 



sing or chirp. Song is mainly concerned with sex and is possibly one of 

 the few means by which the male captivates or pleases the female ; song 

 may also be connected with the fact that the male is not burdened with 

 the chief care of life, the satisfactory deposition of the eggs, and so 

 utilizes his superfluous energy in song. Another manifestation of sexual 

 difference may perhaps be found in the luminous insects. 



Colour. 



All insects which live in the open air are coloured in a more or less 

 complex manner ; the scheme of colouring is in many species variable 

 within certain limits, but generally is uniform and fixed in all individuals 

 of one or both sexes of the same species. These colour schemes are 

 evidently important to the welfare of the insect, and attempts have been 

 made to elucidate the general principles that underlie them ; no two 

 species have precisely the same form and colour, but large numbers have 

 a similar colour scheme, differing in detail in each species but agreeing in 

 the general effect. 



We have seen above that some insects, such as stick insects, are so 





FIG. 61. 

 Moth on Bark of Tree. An instance of cryptic marking. 



formed as to closely resemble their surroundings and so escape notice ; 

 this is associated with colouring, and the conjunction of cryptic form 



