Set adrift in the intestine, the eggs of the thorny-headed worm find a warm shelter in the rat’s feces. The eggs’ presence, scientists hypothesize, may actually make the droppings more appealing to insects seeking a meal. With luck—from the worm’s perspective, anyway—the feces just might tempt a certain familiar critter known for disliking light. AVERAGE SIZE: 0.004 inch (0.11 millimeter).