I would argue that the one of the primary reasons for the title is that it refers to the web in which the main character, Valentin, is drawn. Valetin shares a cell with the homosexual/transvestite Molina. Though he tries to resist his/her "poison," eventually Valentin succumbs to her/her charms.
Though he "mocks and insults" Molina for "his sexual orientation, and even physically abuses him for his resistance to traditional masculine behavior, Valentin, however, eventually grows to love Molina, as a friend and then as a lover. Valentin's softening from an aggressively masculine man to the point of loving another man forces him to question and then reevaluate his attitude toward homosexuality."
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Also, it is referenced near the end when Valentin suggests that Molina be a panther woman, and Molina doesn't like that idea because he wants to kiss Valentin. Valentin then suggests the spider woman, and Molina agrees and says he likes it. Later at the end, the imagery of the spider woman in Valentin's drug/torture induced dream reoccurs.