Why does Jonathan Edwards in "Sinners in the Hands of an Angry God" compare sinners to spider and serpents?

In his sermon
"," the Puritan
minister, , preaches a fire and brimstone sermon in order to frighten
the
congregation into being more righteous. As part of his scare tactics, Edwards seeks
to
awaken the unconverted people by convincing them that theirs is a tenuous
situation:

...and your own care and
prudence, and best contrivance, and all
your righteousness, would have no
more influence to uphold you and keep you out of hell, than a
spider's web
would have to stop a fallen rock....

This
use of the spider  for the congregation continues as the people
are told of their weakness and
loathsomeness to God:

...The God that holds you over the
pit of hell,
much as one holds a spider, or some loathsome insect over the fire, abhors
you
...He looks upon you as worthy of nothing else but to be cast into the
fire

In his Puritan zeal, Edwards also
continues to express the
congregations' insignificance as they are only
worthy of being cast into the fire.  And, if this
is not enough, Edwards
continues to explain how lowly God considers them in contrast to the more

abominable serpent,

you are ten thousand times
more
abominable in His eyes than the most hateful venomous serpent is in
ours.  You have offended Him
infinitely more than ever a stubborn rebel did
his prince.

Of course, theto the
serpent is a comparison of those who are like the serpent in the
Garden of
Good and Evil, the penultimate insult.