Why do worms crawl on the sidewalk after it rains?
Why do worms crawl out onto the sidewalk after it rains?
Because otherwise they'd drown when their holes filled up with water. Who says there are no simple answers?
I had to ask
I think I found something in your column that could be described as a mistake, although I wouldn't use so vulgar a term. You said the reason worms crawl out on the sidewalk when it rains is to avoid drowning when their holes fill with water. A few months ago, after the author of an article in Discover magazine made a similar claim, a scientist who studied worms wrote in to say worms can live underwater as well as in the dirt. They don't breathe as we do but get oxygen in some way that makes drowning an impossibility. He said the reason they come out was mating.
Hm. The wormologist in question is Richard Wahl of Shippensburg University in Pennsylvania. He writes:
Worms do not drown when it rains … Worms of all kinds are highly susceptible to dessication [drying out]. They breed when it rains. They come out of the ground to find each other and to lie side by side in a mating posture, a difficult thing to do in the confines of their burrows. [And we complain about the back of a Ford!] The only time earthworms can safely come to the surface to breed is when the ground is thoroughly soaked … Worms don't have lungs.
It's true worms don't have lungs. They breathe via gas diffusion through special organs in their skin. Cecil knew this. It's also true their skin must be moist in order for this to work, same as with the air sacs in our lungs. Cecil knew this too. But I had the idea if the worm were completely immersed it would drown. Maybe not true. I've consulted with Charles Drewes, professor of wormology (actually, zoology and genetics) at Iowa State University, who tells me worms can survive for long periods underwater. I suppose I might have attempted to confirm this, and spent the afternoon trying to drown worms. However, I decided to take the professor's word for it.
All that having been said, it may be overstating the case to say that all the worms one sees following rain are getting it on with the opposite sex. Vulnerable to dehydration as they are, worms normally find the surface hospitable only at night — that's why they're called nightcrawlers. The one exception during the daytime is when the ground is soaked after a heavy rain. Whatever the worms are doing out there, they're doing it because they can.
No less an authority than Charles Darwin addressed this issue. In 1881 he wrote:
After heavy rain succeeding dry weather, an astonishing number of dead worms may sometimes be seen lying on the ground … It is not probable that these worms could have been drowned, and if they had been drowned they would have perished in their burrows. I believe that they were already sick [perhaps due to parasite infestation], and their deaths were merely hastened by the ground being flooded.
Darwin wrote an entire book about worms, evidently a vast topic. I'm glad somebody did this. I'm glad it wasn't me.
Love among the annelids
About that business of worms mating "with the opposite sex" in your recent column. Surely you know that worms are one of the few truly bisexual critters about. They possess both male and female necessities for producing offspring. They are, however, sexually social animals, and only, er, self-mate when not around other worms.
Gawd. But what the hell, we haven't had an in-depth treatment of animal reproductive habits in a long time. Sure, worms are hermaphroditic, having both male and female organs. Not wanting to sensationalize this, I quote from the encyclopedia: "Two worms mate with their heads pointed in opposite directions." You thought only we humans had a sexual practice like this. Uh-uh, bub. Us and the worms.
Getting back to business: "Both worms secrete mucus, covering each other with a 'slime tube' … Sperm are released and carried in grooves, now formed into tubes by the adjoining slime-covered worm, to the sperm receptacles of the partner. The worms then separate. Later [each worm] secretes a mucous ring, which slides forward over the worm's body, gathering several eggs from the oviducts and sperm from the receptacles as it does. Fertilization takes place within the mucous ring, which slips off the front of the worm, closing at both ends to form a capsule," from which one or two worms hatch a few weeks later.
I recall thinking once: sex is sticky. How little I knew.