Among the many symbols for evil, the image of the mosquito or gnat designates the lowest form.
Because this creature takes and does not give, while the holiness is that which offers, influences, and creates.
The obscure ecology of the spiritual process involves the fact that the shell, which often has a kind of fascination, does provide something even if it takes (steals) value from the holy and gives back less than it takes.
The mosquito is part of the shell, part of the framework of nature, while the sinner is not necessarily a part;
he is only a possibility. He is the result of a choice, made as a direct result of man's free will.
In the animal world, even the wild beasts, who may cause injury to man, are part of the manifestation of God's will.
(As an aside, it should be mentioned that wild animals rarely, if ever, prey on man. Man is simply not part of their ecological balance.)
–Rabbi Adin Steinsaltz
From The Long Shorter Way by Rabbi Adin Steinsaltz