According to one fan theory, Fly represent Jesse and Todd represent spider. Fly can't be controlled but spider can be.
Here is elaborated detail-
The fly represents the cause of Walt's problems, and I believe that is
Jesse Pinkman. This thread from a few days ago does a good job of
pointing that out here.
The fly is always out there causing problems, and he cannot be
contained by Walt despite all his efforts. There is reason to believe
that Jesse will be causing Walt even further problems in the next
episodes to come. I believe that the only way the fly can be
eliminated is from his natural predator, the spider, Todd. Todd has
consistently taken care of Walt's problems in the past, with the child
and the multiple hits on potential witnesses. Unlike the fly, the
spider is easily contained (kept in a jar) and reliable. Todd's most
recent actions were taking out that sleezebag's crew, who very well
may have caused further complications for Walt. If the buyers from the
Czech Republic who we keep hearing so much about continued to be
unsatisfied with the product, it could cause more trouble for Walt.
Another thread mentioned that Jesse and Todd were opposites. Jesse is
a rude man with a conscious, and Todd is a gentleman without a
conscious. One could argue the fly and spider are opposites, or at
least conflicting forces since flies are prey to the spider. I believe
that we will see some sort of showdown between Jesse and Todd. The
result will cause either trouble or relief for Walt, depending on the
victor. One reason that comes to mind as to why Jess and Todd would
meet is that Jesse makes good meth. Todd may want to lure Jesse into
his web to make better meth in order to keep the satisfaction of the
buyers at a high level.
This theory was old and unaware to the end but still I feel its somewhere partially rite. Jesse looks fly to me too because of his free will, on the other hand Todd looks like a true assistant to me sometime for Walt and other time to his uncle, controllable like Spider which can be contained in jar.