Kayla didn't hear her boyfriend Tommy come home. He found wearing a latex bodysuit and her examining herself in the mirror. He really liked how her ass looked in the costume. Kayla always had a noticeable booty, but now it was a particularly plump ass. He walked up behind her and wrapped his arms around her, kissing her neck and fondling her breasts. They seemed bigger and firmer too.
Kayla was pleasantly surprised by her boyfriend's arrival, and even moreso by his reaction to the costume. She could feel his erection pressing against her ass. Kayla backed against Tommy, slowing pushing him toward the bed even has he continued to grind against her and kiss and a caress her. Tommy was having fun, though he couldn't help but think Kayla's ass was somehow slowly swelling.
When Tommy had finally backed into the bed, Kayla gave an extra shove with her butt and knocked him onto his back. She turned around, then quickly unfastened and removed his pants. Kayla climbed onto of Tommy, shoving her breasts - now definitely D's instead of C's - in his face, to his delight. She spread her legs wide, somehow almost straddling the bed. She had the overwhelming desire to mate. Without doing anything to the costume she lowered herself onto Tommy's erection and felt him slide into her tight, wet, rubbery vagina.
Kayla began pounding away, driving herself onto Tommy. He tried to grab her ass, but it had expanded behind his ability to grasp. He settled for pulling her into a kiss, which Kayla returned hungrily. She was thrusting at a rate Tommy would have never been able to without hurting her. She drove him deeper into her until finally she let out an unnatural scream of pleasure.
With her first orgasm, Kayla's legs, already more flexible than they ever could be normally, each split into four smaller legs. The eight legs instinctively gripped the sides of the bed. More orgasms followed as Tommy climaxed deep within her. She clung to him tightly, desiring nothing so much at that moment as to be filled by her mate.
When Tommy finished, Kayla let out a contented sigh and slowly slid off his softening shaft before plopping down on the bed next to him. This gave Tommy a chance to see what had happened. He saw Kayla's legs and huge latex ass...or abdomen, if he remembered biology class.
"Kayla, what was that costume?"
"Black Widow" she answered dreamily.
"Not the kind of black widow I think you were looking for you. You're some kind of half-spider now." His brain recalled something from a D&D manual. "You turned into a drider. A black widow drider." After a moment to think about the implications, Tommy added: "You're not planning to eat me, are you?"