72 Colouration in Animals and Plants. 



of the wing. Even in the so-called whites (Pieris) this part of the 

 wing is dusky, and instances are numerous on Plate IV. 



The scales, which give the colour to the wings, deserve more than 

 a passing notice. They are inserted by means of little stalks into 

 corresponding pits in the wing-membrane, and overlap like tiles on a 

 roof; occasionally the attachment is a ball and socket (Morphince), in 

 which case it is possible the insect has the power of erecting and 

 moving its scales. The shapes are very numerous, but as a rule they 

 are short. To this there is a remarkable exception on the wings of 

 the males of certain butterflies, consisting of elongated tufted 

 prominences which appear to be connected with sense-organs. They 

 are probably scent-glands, and thus we find, even in such minute parts 

 as scales, a difference of function emphasized by difference of orna- 

 mentation, here showing itself in variety of forms ; but, as we have 

 said, ornamentation in form is often closely allied to ornamentation 

 in colours. In some butterflies, indeed, these scales are aggregated 

 into spots, as in Danais, and have a different hue from the surround- 

 ing area. 



The scales are not simple structures, but consist of two or more 

 plates, which are finely striated. The colouring matter consists of 

 granules, placed in rows between the striae, and may exist upon the 

 upper surface of the upper membrane (epidermal), or the upper 

 surface of the under or middle plate (hypodermal), or the colour may 

 be simple diffraction colour, arising from the interference of the light- 

 waves by fine striae. 



Dr. Haagen, in the admirable paper before mentioned, has 

 examined this question thoroughly, and gives the results set forth in 

 the following table : — 



Epidermal Colours. 



Metallic blues and greens 



Bronze 



Gold 



Silver } Persistent after death. 



Black 



Brown 



Red (rarely) 



