Earthworms move through the soil by a combination of flexing their muscles and using bristly hairs to brace themselves.
Hairy little worms
Do you think you could use their hair to get around? No, probably not. Yet this is precisely what earthworms do. We may not think of them as hair creatures, but earthworms are actually covered in lots of little bristly hairs known as setae.
These setae aren’t designed to keep the earthworms warm, or even to help them attract a mate, instead they are the closest things an earthworm has to arms and legs.
To explain the bizarre type of locomotion that earthworms use may at first seem a little complicated. When we think of an earthworm we perhaps think only of a only and squiggy tube, without arms or legs. They don’t seem particularly able to go anywhere, apart from being able to wiggle from side to side.
Yet, these tiny creature can bore through densely packed soil as if they were propelled forward by some kind of internal engine.
The trick is all to do with muscles, hairs and pressure.
How do they move through the soil?
As you may know earthworms are made up a series of segments, around 100 to 150 in most species. Each of these sections contains a circular muscle that wraps around the body of the worm, and longitudinal muscles that extend the length of the body.
Below the muscles sits the worm’s skeleton. While the idea of a small worm skeleton on display may be enticing, these skeletons are actually made of fluid rather than bone like our own skeletons. Hydrostatic skeletons can be found across many soft-bodied animals, such as slugs. They function through the use of pressurised liquid held within cavities within the animal’s body.
When a worm wants to move it releases both it’s circular and longitudinal muscles, which allows it to stretch forwards. The setae (bristly hairs) are arranged in pairs. These are then used to stick into the soil around it. Normally they remain inside the body. Having ancored itself in place the worm contracts its muscles, which pulls the rest of the worm’s body forwards.
This movement can then be repeated again and again. This gives the worms movement it’s distinctive rippling motion.
When it comes to how they move through the soil part of this is due to their shape. With a pointed head they can push through the soil, and use their pressurised bodies to almost create a drill, making their own cavity in the earth.
While many worms will spend most of their time moving horizontally through the soil profile, many species also move vertically. While it may seem logical to us that these worms would simply slip back down the hole behind them, their setae grabbing hold of the earth and helping them to climb towards the surface.
Does the early bird get the worm?
If you’ve ever watched a bird trying to pull a worm from the soil, it can truly seem like a David and Goliath type of battle. Despite the fact the bird is so much bigger the worm seems able to hold its own, often getting away.
If the worm was to be surprised then the bird might be luckier, because if the worm has dug its setae into the soil then there’s little the bird can do to budge the stubborn creature.
We may think of worms as wiggly but most of the time their movements are much more purposeful. Their reasons for wiggling when uncovered are in part to scare and confuse their attacker. Additionally, in these moments they usually have less to grab on to, and wiggling may move them into a better position to escape.
Where to go
Now that the earthworm can move the important thing is where to go. Earthworms spend most of their time looking for food. Luckily for them their food is largely readily available, from leaflitter to animal waste.
Amazingly, some earthworm species actually go to the surface to retrieve leaves, grabbing hold of them and pulling them down into their burrows. Talk about determination. Others spend their time in high nutrient environments such as compost heaps, where there’s plenty of food available at all times.
Those that spend their time heading through the soil to find food can use their sensory organs to locate what they need. Their sensory organs can help them sense what is around them, in the form of touch. This can help them decide which root to take through the dark world around them.
Their ability to pick up chemical signals is also important, allowing them to pin down where food may be found, but also avoid dangerous underground predators.
A worm’s method of moving may not be the fastest, but it is incredibly effective. Afterall, could you imagine being able to force your head through the soil using only your toes? Perhaps not.
While they might not be in the running for the speediest animal on Earth, they really aren’t in a rush. Afterall, everything they need is directly around them.