652 EXTERNAL ANATOMY OF INSECTS 



celestial purity of its azure wings : our native coppers also, 

 Lyccena dispar*, Virgaurecc, &c., are remarkable for 

 the fulgid colour of these organs; in Argynnis the upper 

 side of their wings is tawny, spotted with black, while the 

 under side of the secondary ones is very often adorned by 

 the appearance of silver spots. How this remarkable effect 

 of metallic lustre, so often reflected by spots in the wings 

 of butterflies, is produced, seems not to have occupied 

 the attention of Entomologists. M. Audebert is of opi- 

 nion that the similar lustre of the plumes of the humming 

 birds (Trochilus) is owing to their density, to the polish 

 of their surface, and to the great number of little minute 

 concave mirrors which are observable on their little 

 beards b . But these observations will not apply to the 

 scales of the wings of butterflies, which are always very 

 thin and generally very flat: in some instances, as in 

 Morpho Menelans, there appears more than one very slight 

 channel upon a scale; but this takes place also in others 

 diat reflect no lustre. Their metallic hues must there- 

 fore principally be occasioned by the high polish of their 

 surface and the richness of their tints. It is the purity 

 of the white, in conjunction with their shining surface, 

 contrasted with the dull opaque colour of the under side 

 of the secondary wings, that causes the spots that decorate 

 those of the Fritillaries ( Argynnis) to emulate the lustre 

 of silver. In Papilio the Trojans are distinguished by 

 the black wings with sanguine spots, and the Greeks by 

 the same with yellow spots ; but these have proved in 

 some instances only sexual distinctions c . In the Danai 

 candicli L. the colour of the tribe may be described as 



* Plate III. Fig. 1. k x. j) wt , d'Hist. Nat. viii. 257. 



e See above, p. 303. 



