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. Again,
the big globe of its abdomen is coriaceous and elastic, and is probably
as devoid of sensation as a ball of india-rubber. The insect, being made
fast by hooks and teeth to its victim, all efforts to remove it only
increase the pain it causes; and animals that know it well do not
attempt to rub, scratch, or bite it off, therefore the great size and
the conspicuous colour of the tick are positive advantages to it. The
flea, without the subtlety and highly-specialized organs of the
Ornithomyia, or the stick-fast powers and leathery body of the Ixodes,
can only escape its vigilant enemies by making itself invisible; hence
every variation, i.e. increase in jumping-power and diminished bulk,
tending towards this result, has been taken advantage of by natural
selection.




CHAPTER XI

HUMBLE-BEES AND OTHER MATTERS.


Two humble-bees, Bombus thoracicus and B. violaceus, are found on the
pampas; the first, with a primrose yellow thorax, and the extremity of
the abdomen bright rufous, slightly resembles the English B. terrestris;
the rarer species, which is a trifle smaller than the first, is of a