Marvel: Spider Physiology

Peter Parker (Spider-Man)
Spider-Man possesses the proportionate powers of a spider, granted to him from an irradiated common house spider which bit him. Spider-Man was apparently already mutated from prior exposure to certain frequencies of radiation and received a final, lethal dose during Spider-Man's attendance of the exhibition. The radioactive, complex mutagenic enzymes in the spider's blood that were transferred at the time of the bite triggered numerous body-wide mutagenic changes within him, granting him superhuman strength, speed, toughened flesh, and numerous arachnid-like abilities.

Ezekiel Sims revealed to Spider-Man that his powers were not of scientific origin, but were a sign that he had become a totemic avatar o the Web of Life and Destiny. While Spider-Man initially struggled to accept this, he later came to the conclusion that both were true, and has had his powers mystically augmented on a number of occasions.