Poulton, Mimicry and Natural Selection. D 
~ 
Another admitted fact of wide application is the tendency 
of mimetic resemblance to appear in the female rather than the male. 
Thus female butterflies of many species are associated with non- 
mimetic males while the converse relationship is almost unknown. 
The non-mimetic male in the species referred to maintains the 
ancestral appearance which has been lost in the female, although 
distinct traces of it can nearly always be recovered by the care- 
ful study of individual variation, and comparison with allied species. 
This is a remarkable reversal of the ordinary rule that when male 
and female differ the latter is the more ancestral. This striking 
exception is quite unintelligible except under the theory of Na- 
tural Selection which offers the convincing explanation, long ago 
suggested by Alfred Russel Wallace, that the slower flight 
of the heavier females and their exposure to attack during ovi- 
position render it especially advantageous for them to resemble 
conspicuous distasteful species in the same locality). 
Another aspect of Mimicry affords, in my opinion, perhaps 
the most powerful argument of all in favour of an interpretation 
based on the theory of Natural Selection. If these resemblances 
are attained by selection because they are advantageous in the 
struggle for life we should expect to find that they are pro- 
duced in a great variety of ways; for one species would reach 
the beneficial end by one path pointed out to it by the structure 
it possessed at the beginning and by the trend of its variation, 
while another species with a very different initial structure would 
reach the same end by a widely different path. Thus many 
Diptera, for example species of Ceria, gain a superficial resem- 
blance to wasps by a narrowing in the anterior abdominal region 
which suggests the characteristic peduncle of a Hymenopterous 
insect. On the other hand Longicorn beetles of the genus Oberea 
gain the same effect by a patch of white which obliterates the 
anterior abdominal region with the exception of a small linear 
remnant representing the peduncle. In brilliant illumination the white 
is not seen as part of the insect. The resemblance of the Locustid 
Myrmecophana fallax to an ant is produced in the same manner. 
The Homopterous family Membracidae are characterized by an 
enormous growth of the dorsal region of the pro-thorax which 
spreads backwards and in many species covers the insect like a shield. 
In the American species which mimic ants this shield, and not the 
insect beneath it, becomes ant-like. Some of the larval Membracidae 
are laterally compressed, becoming in the dorsal region as thin as a 
leaf, and the body is green like a leaf, while the. head and legs 
are brown. The whole appearance is singularly like that of the 
tropical American ant Oecodoma cephalotes carrying its leaf 
vertically in its mandibles and thrown over its back so that the 
I) Trans. Linn, Soc, Lond., Vol. XXV, 1866, p. 22. 
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