We are all familiar with the bathroom-spider. The one spider that stays quietly creeping in the corner near the ceiling. Or perhaps it is a new spider every day, the first begetting the next and so-forth so that there is a constant flow of closely related bathroom-spider buddies. Whatever your guess, whether winter or spring, we have all been haunted by these pesky spiders that climb in through small cracks in the window or get carried in from other rooms in the home.
To a homeowner, the bathroom seems like the farthest place any insect would want to be. It's nowhere near the kitchen and no amount of human-food has ever passed over its threshold. Despite all of the unattractive qualities of the full bathroom, spiders seem to love to make it home. And, to those familiar with the biological needs of insects and bugs, we know this is no accident.
What bathrooms provide to spiders is a water-park paradise. Spiders and insects have what is called a poor surface-area-to-volume ratio. The small size of spiders keeps there volume very low, while their surface area is still significant. This makes them prone to evaporation, removing great amounts of water through their exoskeleton into the air. Like all creatures, spiders need water to survive. Because they cannot risk becoming dehydrated, living near a source of water helps them to survive.
So despite our paranoia that spiders choose to lurk in bathroom corners to strike us when we are most vulnerable, they are likely equally displeased to see us. Each time we enter the bathroom, we replenish their fountain of life so that they can live another day. And as we leave, they can take advantage of the drops of water we have left behind and sweep up other insect visitors into their webs to satiate their hunger.