How do millipedes control their legs? Millipedes use a direct-wave gait. That means the legs move from the back to the front in a wave pattern, moving the millipede forwards.
Millipedes are invertebrates, which is a cold-blooded animal with no backbone. They have an exoskeleton and they are made of multiple segments. They come from the animal group anthropoda, which are animals with exoskeletons, segmented bodies, and jointed appendages, such as spiders and shrimp. They are not insects. The word “millipede” came into English from Latin where mille means “a thousand” and ped means “foot”. Despite their name, millipedes don’t have a thousand legs. Most millipedes tend to have around 300 legs and the most legs ever recorded on a millipede was 1,300. Illacme plenipes is the millipede species that has the most legs and it is very long and very thin.
Millipedes have many segments. It depends on the species, but they can have from 25 to over 100. The first part is the head. The part behind the head is called the collum (Latin for neck) and has no legs. The next three segments have one pair of legs each. Every segment after that has two pairs of legs. The last segment has no legs and is the anus. Each leg is also broken up into about seven segments.
So, how do millipedes control their legs? Millipedes are invertebrates, which means they don’t have a spinal column in the way that vertebrates do, but they do have a nervous system. A millipede has a brain and a nerve cord that stretches along the length of its body. Every segment also has a ganglion. Ganglia are clusters of nerve cells. We have ganglia in our body and they serve two functions: to make us move automatically and to send sensory information to the brain. They are autonomous units because we can’t afford to think and send a command from our brain for every tiny thing our body needs to do. The majority of our bodily functions need to operate automatically, and this is the job of the ganglia.
In a millipede, the brain sends a signal along the nerve cord to begin walking. Each ganglion receives the signal and starts the two pairs of legs in its section moving. This is the motion function of the ganglion. Secondly, they send sensory data about what they are walking on and when they are walking back to the nerve cord. The millipede needs to make sure that all of its legs move in sequence. If they started to move higgledy-piggledy, the millipede won’t be able to move. To make sure this happens, the millipede has a central pattern generator. This receives the signals from the ganglia in each segment and makes sure that all of them stay on the same rhythm and at the same time. Thanks to this, the millipede can move its legs in a wave from the back to the front without getting out of step.
Evolutionarily speaking, it might seem odd that millipedes have evolved with so many legs. Surely, each leg requires energy and the total energy requirement for the legs must be huge. However, like most things with evolution, if a mutation allows a creature to keep living and to reproduce, it will most likely be passed on. With millipedes, the advantage to having so many legs comes with the way they feed. Centipedes are carnivores and they prey on insects. Millipedes are detritivores, which means they eat dead and decomposing organic matter. To get to that dead and decaying matter, most millipedes have evolved to burrow and tunnel. A lot of leaf matter can be under rocks, in cracks, or under the soil. The hundreds of legs that millipedes have allow them to create a lot of force and push their head into crevices or into the earth. Their legs also push the soil backwards, like a row of people passing buckets of soil from the front to the back, clearing their tunnel as they dig.
One disadvantage of millipede legs is that they dry out very easily. Well, this isn’t necessarily because of their legs. Most creatures with an exoskeleton produce a waxy covering to keep water in and out. They also seal up the openings they breathe through. Millipedes don’t have a waxy coating and they cannot seal up their respiratory openings, so they lose water very easily. They have to live in humid or moist places or they will dry up and die. And this is what I learned today.
Sources
https://www.frontiersin.org/articles/10.3389/fncir.2021.706064/full
https://www.nwf.org/Educational-Resources/Wildlife-Guide/Invertebrates/Millipedes
https://www.terminix.com/blog/education/centipede-millipede-leg-movement/
https://northernwoodlands.org/outside_story/article/walking-with-many-legs
https://www.etymonline.com/word/millipede
https://brill.com/view/journals/ab/65/2/article-p133_4.xml
https://faculty.washington.edu/chudler/invert.html
https://www.verywellhealth.com/ganglia-5104788
https://en.wikipedia.org/wiki/Millipede
https://www.sciencefocus.com/nature/why-do-millipedes-have-so-many-legs/