Entomologists regard the flea as a degraded fly. Certainly it is very
much more degraded than the bird-borne Ornithomyia, with its subtle
motions and instinct, its power of flight and social pastimes. The poor
pulex has lost every trace of wings; nevertheless, in its fallen
condition it has developed some remarkable qualities and saltatory
powers, which give it a lower kind of glory; and, compared with another
parasite with which it shares the human species, it is almost a noble
insect. Darwin has some remarks about the smallness of the brain of an
ant, assuming that this insect possesses a very high intelligence, but I
doubt very much that the ant, which moves in a groove, is mentally the
superior of the unsocial flea. The last is certainly the most teachable;
and if fleas were generally domesticated and made pets of, probably
there would be as many stories about their marvellous intelligence and
fidelity to man as we now hear about our over-praised "friend" the dog.
With regard to size, the flea probably started on its downward course as
a comparatively large insect, probably larger than the Ornithomyia. That
insect has been able to maintain its existence, without dwindling like
the Leptus into a mere speck, through the great modification in organs
and instinct, which adapt it so beautifully to the feathery element in
which it moves. The bush-tick, wingless from the beginning, and
diverging in another direction, has probably been greatly increased in
size by its parasitical habit; this seems proven by the fact, that as
long as it is parasitical on nothing it remains small, but when able to
fasten itself to an animal it rapidly developes to a great size.
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