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

In his sermon "Sinners in the Hands of an Angry God," the
Puritan minister, Jonathan Edwards, 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:

readability="11">

...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 metaphor for the congregation continues as the people are told of their weakness
and loathsomeness to God:

readability="10">

...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,
the allusion to the serpent is a comparison of those who are like the serpent in the
Garden of Good and Evil, the penultimate insult.