EXTERNAL ANATOMY OF INSECTS. 651 
of opinion 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*. 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 Menelaus, there appears more 
than one very slight channel upon a scale; but this 
takes place also in others that reflect no lustre. Their 
metallic hues must therefore 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 Fritil- 
laries (Argynnis) to emulate the lustre of silver. In 
Papilio the Trojans are distinguished by black wings 
with sanguine spots, and the Greeks by the same with 
yellow spots; but these have proved in some instances 
only sexual distinctions®, In the Danaz candidi L. the 
colour of the tribe may be described as sacred to the 
day, since every shade, from white or the palest yellow 
to full orange, is exhibited by them. The yellows pre- 
vail also in those Noctuid@, the trivial names of which 
Linné made to end in ago, as Xanthia Fulvago, Citrago, 
&c. I must not conclude this part of my subject with- 
out noticing one of the most striking ornaments of the 
wings of Lepidoptera, the many-coloured eyes which de- 
corate so large a number of them. Some few birds, as 
the Peacock and Argus Pheasant, have been decked by 
* N. Dict. d'Hist. Nat. viii, 257. » See above, p, 302. 
