The trees seemed to explode in size as Jeff's body shrank to a slightly-less-terrifying size. He watched as his legs crackled and bent, becoming black and segmented and pointy. Toes, heel, and sole vanished. He could feel four itching, burning bulges on the sides of his torso, likely where the other legs were about to burst out.
Without warning his face slammed into the grass. His dick, or rather the spider's body, now comprised most of his mass. His torso shrank and withered, most of the mass going towards his new legs...or rather, his spider mistress' new legs. He could feel his head swell and contort, his brain and insides reforming and rearranging like soupy slime to become the arachnid's organs.
His mouth and stomach relocated a bit lower, becoming the spider's egg sac and ovaries.
His arms grew pointy and contorted as well, the fingers and thumbs merging and fading away.
Coarse black hairs sprouted all over his tiny body.
Four new arms burst out of his withered torso, lengthening and segmenting.
His nose slid further up his "face," splitting to form both the spinneret and anus of his host.
His "view" faced downward, his eyes becoming little more than the design on the spider's belly. He could see, but his vision was always pointed downward and backward.
The grass slid past his face as the spider finished its meal and began climbing the nearest tree to spin a web. Jeff could feel the roach's juices gurgling and coursing through his...or her...new organs.
As the spider found a suitable spot for a web, it began releasing the webbing out of Jeff's former nose. It was the oddest sensation, watching his former arms bend and weave and stitch together the slimy, sticky string coming out of his face.
It was only after an hour or so into it that Jeff realized that the spinnerets were comprised of what used to be part of his brain, meaning that his memories were oozed out into an intricately-woven web right before his eyes.
Jeff thought in a panic, though in his state as part of the spider's body he could do practically nothing on his own. Deep inside his stomach he could feel unfertilized eggs roiling and shifting around as the spider worked. He could taste her pheromones on his nonexistent tongue. Instinctually he knew she needed to feed; to store nutrients for mating and for laying her egg-sac. He slowly passed out as more and more of his memories oozed out of his head.