Once Mary's orgasms subsided she dismounted Jeff and collapsed on the floor next to him.
Jeff lay in the cocoon from Mary's webbing, enjoying the warm afterglow, when his body began to feel strange. His legs felt numb while the sides of his torso felt itchy. With his arms essentially trapped at his sides he couldn't really investigate. Then his arms started to involuntarily push outward, pushing against the webbing which began to strain then slowly start snapping. Had Jeff suddenly gotten stronger or had the webbing gotten weaker?
His arms strained at the webbing even as Jeff felt something strain at his sides, something pushing out of his body. What the hell was going on? First his elbows pushed through the cocoon then the rest of his arms and he clawed at more of the webbing to free himself. Yet even as he clawed at the webbing from the outside he simultaneously felt himself clawing at the inside too. As the webbing tore free, Jeff finally understood: he now had two extra pairs of arms like Mary. That left only Jeff's legs, but try as he might he couldn't sit up. With some difficult, Jeff got his lower arms to claw at the webbing around his waist then he crawled out of what remained.
The view shocked him. Jeff not only had four extra arms, but his legs had also transformed into arms. Eight matching appendages, like a spider. Sex with Mary in her spider form, perhaps combined with the webbing, had changed Jeff into a spider man,. Strange as this transformation was, he felt good.
Jeff rolled over onto to his stomach and stood for the first time on his new eight arm-legs and looked at Mary.