King David, lying on his couch one evening, let his thoughts wander through the corners of creation he could not make sense of.
"Of what use is the spider in this world?" he asked himself. "It increases only dust and cobweb, makes places unsightly, and brings annoyance to anyone who cleans a corner."
Then he thought of a man he had passed that day, a person whose mind had been broken. "How unfortunate such a being is. I know all things are ordained by God with reason and purpose, yet this is beyond me. Why should men be born idiots, or grow insane?"
As he lay turning these questions, a whine rose beside his ear. Mosquitoes had come in through the window. David slapped and cursed and thought, "And what is the mosquito good for? What purpose does the world gain by its biting existence? It only disturbs our rest."
Years later, David would discover that each of these creatures — the spider, the mosquito, the man who appeared to be mad — had saved his own life at a crucial hour. A spider spun a web across the mouth of a cave where he hid from Saul, fooling the pursuers into thinking no one could have entered. A mosquito bit Saul's foot at the moment David reached to cut his robe, distracting the king from turning. And when David fled to the court of Achish of Gath, he feigned madness by scratching on the gate and letting his spit run down his beard — and Achish let him go, thinking, "Do I lack lunatics that you must bring me this one?"
The sages are teaching that when a human being questions creation's usefulness, he is usually asking too early. The spider he scorned and the mosquito he swatted were holding his life in reserve.
And then there is the other half of the teaching, also preserved here: "Wisdom resides with the aged, and understanding in length of days." (Job 12:12) A young disciple once freed a slave only after the slave had given him the counsel of years. Experience is the one coin that does not shrink with handling.
(From the 1901 Hebraic Literature anthology, drawing on Midrash Tehillim 34.)