ral enemies, but under the artificial conditions brought about by
the settling and developing of any district great changes come about. It
very often happens that these changes are favorable to the development
of the noxious insects and unfavorable to the development of their
enemies.

A striking example and one to the point is afforded in the introduction
of mosquitoes into Hawaii. Up to 1826 there were no mosquitoes on these
islands. It is supposed that they were introduced about that time by
some ships that were trading at the islands. Indeed it is claimed that
the very ship is known that brought them over from Mexico.

Once introduced they found conditions there very favorable to their
development, plenty of standing water and few natural enemies to prey on
them, so they increased very rapidly and gradually spread over all the
islands of the group. This was the so-called night mosquito, _Culex
pipiens_. Much later another species, _Stegomyia calopus_, just as
annoying and much more dangerous was introduced and has also become very
troublesome. We have a few species of top-minnows (Fig. 88) occurring in
sluggish streams in the southern part of the United States that are
important enemies of the mosquitoes of that region. A few years ago some
of these were taken over to Hawaii and liberated in suitable places to
see if they would not help solve the mosquito problem there. The fishes
seem to be doing well. Already they are destroying many mosquito larvae,
and there are indications that they are going to do an important work,
but of course can be depended on only as an aid.

[Illustration: FIG. 86--Salt-marsh mosquito (_Ochlerotatus
lativittatus_); male.]

[Illustration: FIG. 87--Salt-marsh mosquito (_O. lativittatus_);
female.]

[Illustration: FIG. 88--Top-minnow (_Mollienisia latipinna_). (From
Bull., 47 U.S. Fish Com.)]

[Illustration: FIG. 89--Dragon-flies. (From Kellogg's Amer. Insects.)]

On account of the various habits of both the larvae and adults it will
never be possible for any natural enemy or group of natural enemies
effectively to control the mosquitoes of any region, but as certain of
them are important as helpers they deserve to be mentioned.

ENEMIES OF THE ADULTS

Birds devour a few mosquitoes, the night-flying forms being particularly
serviceable, but the number thus destroyed is probably so small as to be
of little practical importance.

The dragon-flies (Figs. 89, 90, 91) or mosquito