Kirby has remarked (and I have observed the same fact) that the
anterior tarsi, or feet, of many male dung-feeding beetles are very
often broken off; he examined seventeen specimens in his own
collection, and not one had even a relic left. In the Onites apelles
the tarsi are so habitually lost, that the insect has been described as
not having them. In some other genera they are present, but in a
rudimentary condition. In the Ateuchus or sacred beetle of the
Egyptians, they are totally deficient. There is not sufficient evidence
to induce us to believe that mutilations are ever inherited; and I
should prefer explaining the entire absence of the anterior tarsi in
Ateuchus, and their rudimentary condition in some other genera, by the
long-continued effects of disuse in their progenitors; for as the tarsi
are almost always lost in many dung-feeding beetles, they must be lost
early in life, and therefore cannot be much used by these insects.
In some cases we might easily put down to disuse modifications of
structure which are wholly, or mainly, due to natural selection. Mr.
Wollaston has discovered the remarkable fact that 200 beetles, out of
the 550 species inhabiting Madeira, are so far deficient in wings that
they cannot fly; and that of the twenty-nine endemic genera, no less
than twenty-three genera have all their species in this condition!
Several facts, namely, that beetles in many parts of the world are very
frequently blown to sea and perish; that the beetles in Madeira, as
observed by Mr. Wollaston, lie much concealed,
until the wind lulls and the sun shines; that the proportion of
wingless beetles is larger on the exposed Dezertas than in Madeira
itself; and especially the extraordinary fact, so strongly insisted on
by Mr. Wollaston, of the almost entire absence of certain large groups
of beetles, elsewhere excessively numerous, and which groups have
habits of life almost necessitating frequent flight; these several
considerations have made me believe that the wingless condition of so
many Madeira beetles is mainly due to the action of natural selection,
but combined probably with disuse. For during thousands of successive
generations each individual beetle which flew least, either from its
wings having been ever so little less perfectly developed or from
indolent habit, will have had the best chance of surviving from not
being blown out to sea; and, on the other hand, those beetles which
most readily took to flight will oftenest have been blown to sea and
thus have been destroyed.
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