It's raining here in Denver tonight; a welcome divergence from last year's Spring, which saw almost no rain at all. I was just now observing the earthworms crawling around in my yard, escaping to the surface after a rainstorm, as is their wont.
Several competing hypotheses have been advanced to explain this curious behavior. The first holds that rainwater percolating through the soil becomes enriched in carbon dioxide from the respiration of soil microorganisms. The carbon dioxide dissolves in the water, creating carbonic acid which the worms find irritating. But since the immediate soil surface pH is not likely to differ markedly from that just underneath the surface, this theory appears to me largely discredited.
A second, popularly held notion has it that the worms rise to the surface to breath, since the subterranean tunnels they would otherwise inhabit have become waterlogged by the rains, causing them to drown if they remain below. This too seems rather implausible to me. Earthworms do not have lungs; they breath by gaseous diffusion directly through their skin. They can in fact survive quite comfortably under water, provided the quantity of dissolved oxygen is sufficient to maintain cellular respiration. It's possible that, during a heavy thunderstorm, the in-falling rainwater displaces enough of the soil's natural porosity to render the air supply inadequate; but it's raining very lightly tonight, and in the highly aerated upper soil regions where they live, this seems an unlikely possibility.
The third hypothesis states that the worms are drawn out by the rhythmic vibrations of the raindrops impacting the soil. This is no doubt an atavistic trait shared with their cousins, Shai-Hulud, the great desert sandworms of Arrakis. But seriously, professional worm harvesters (yes, there is such a thing) regularly employ vibrations to lure worms from their underground hiding places. This is accomplished by wriggling a garden rake or other multi-pronged instrument against the ground. A good worm hunter can gather an entire bucket of worms in a few hours this way, without a single drop of rain having fallen.
But there still remains to be explained why the worms behave in this fashion. I believe they are simply taking advantage of the wet conditions in order to get out of the house and move around for a while. Worms die quickly if they are exposed to sunlight or dry conditions. Thus, a nice damp night is the perfect time to hit the town, scout out some new territory, and scope out the ladies. Well, earthworms are hermaphroditic, so they can't really avoid finding the ladies; but then again, they can't avoid finding the gentlemen, either. All they have to worry about is finding another worm. This androgynous sex life ensures that mating opportunities are frequent, if not exactly enjoyable by our standards.
When we look at life in its simpler manifestations, we see how little it differs from the inorganic processes of geochemistry. The worm doesn't merely inhabit the soil; it is the soil, modulated and stabilized into this form by long eons of experience. The worm's boundless appetite for leaf litter and clay particles, its impressive faculty of digestion, its slow, dream-like pulsations propelling it through the abyss of night, these are nothing more than the mighty rotations of the earth, the timeless beating of wind and rain upon the rocks, the deep and pregnant rumblings of the planet that thrust up the mountain ranges and cleave the ocean basins, all intensified and focused and united into a gestalt. A will speaks forth out of the bare earth, a will to exist and to eat, a will to mate with like flesh and perpetuate the form. It bespeaks of a cosmic mind impelling the unfolding of the world through endless ages of ages. Soil has found a voice, and that voice says "I am Worm."
Each one of these miserable lives is a microcosm of creation, with drama and tragedy of its own. They are wasted by the millions, they go down like stalks of wheat before the thresher, but they endure. In this we see the way of all flesh: Dust thou art, and unto dust shalt thou return. Every living organism, every plant and animal and single cell, is a seed planted by the Creator in the deep structure of existence, destined to arise and bear fruit at its appointed time, and then to yield itself up to eternity. Out of this churning mass springs forth the frame of nature, painfully beautiful and severe. The flesh has its requirements; it must do what it must do. But all things are ennobled by the struggle, and we need only play our roles and play them well.
The human race is unique, for here mere flesh is elevated to the rank of spiritual dignity. In the Incarnation we learn that God himself has received our flesh into His heavenly abode, and in turn imparted us with something of His own nature. Our bodies, dust though they are, will nevertheless accompany our souls into eternity. Humanity is the bridge between heaven and earth, at once the crown of nature and the dwelling place of nature's God. Could it possibly be otherwise? The abstractions of philosophers, the force-gods of the pagans, these lend no hope for man as such in eternity. Salvation could only come, has come, through the Son of Man. Without Him we are nothing but weak worms of the dust.
Just take my word for it, and never mind that wikipedia article.