MIMICRY IN INSECTS. 
101 
almost always bright green, with complementary shades of 
yellow, grey, and purple, just to fit them in with the foliage 
they lurk among. Everybody who has ever hunted the green 
tree-toads on the leaves of waterside plants on the Riviera 
must know how difficult it is to discriminate these brilliant 
leaf-coloured creatures from the almost identical background 
on which they rest. Now, just in proportion as the beetles 
and fiies grow still more cautious, even the green lizards them¬ 
selves fail to pick up a satisfactory livelihood ; and so at last 
we get that most remarkable Nicaraguan form, decked all 
round with leaf-like expansions, and looking so like the foliage 
on which it rests that no beetle on earth can possibly detect 
it. The more cunning you get your detectives, the more 
cunning do the thieves become to outwit them. Look, again, 
at the curious life-history of the flies which dwell as unbidden 
guests or social parasites in the nests and hives of wild honey¬ 
bees. These burglarious flies are belted and bearded in the 
very selfsame pattern as the bumble-bees themselves ; but 
their larvae live upon the young grubs of the hive, and repay 
the unconscious hospitality of the busy workers by devouring 
the future hope of their unwilling hosts. Obviously, any fly 
which entered a bee-hive could only escape detection and 
extermination at the hands (or stings) of its outraged 
inhabitants, provided it so far resembled the real householders 
as to be mistaken at a first glance by the invaded community 
for one of its own numerous members. Thus any fly which 
showed the slightest superficial resemblance to a bee might 
at first be enabled to rob honey for a time with comparative 
impunity, and to lay its eggs among the cells of the helpless 
larvae. But when once the vile attempt was fairly discovered, 
the burglars could only escape fatal detection from generation 
to generation just in proportion as they more and more 
closely approximated to the shape and colour of the bees 
themselves. For, as Mr. Belt has well pointed out, while 
the mimicking species would become naturally more numerous 
from age to age, the senses of the mimicked species would 
grow sharper and sharper by constant practice in detecting 
and punishing the unwelcome intruders. It is only in 
external matters, however, that the appearance of such 
mimetic species can ever be altered. Their underlying points 
of structure and formative detail always show to the very end 
(if only one happens to observe them) their proper place in a 
scientific classification. For instance, these same parasitic 
flies which so closely resemble bees in their shape and colour 
have only one pair of wings apiece, like all the rest of the fly 
order, while the bees, of course, have the full complement 
