You generally know where you stand with identifying a creature by counting its legs: eight legs = spider, six legs = insect, four legs = dog. Easy, right?
And then there are centipedes, who raise a middle, er, leg to the whole idea of evolutionarily conserved body plans.
How many legs?
Various species of centipede have between 30 and 354 legs. They get away with this because they basically have three components, a front end, a series of body segments with one pair of legs each, and a back end. In order to change the number of legs, all that needs to happen is for a few extra copies of the body segment to be added and congratulations, you have 177 pairs of legs! This way to the shoe shop.
Centipedes can be distinguished from their cousins the millipedes because millipedes having two pairs of legs on each body segment rather than one like centipedes. Together these groups are known as the myriapoda. The oldest fossil of a land animal ever found was a myriapod, so these guys may have been the first creatures to move onto land from the primordial swamp around 425 million years ago!
Although the name centipede means 100 legs, but centipedes cannot have exactly 100 legs because they always have an odd number of pairs. Despite what some people say, you cannot age a centipede based on how many legs it has. Some groups hatch with the full complement of legs, whereas others gain a few legs at each moult, and you really don’t want to be getting up in their legs enough to be counting anyway, because they will mess you up!
Centipedes are venomous nocturnal predators. They spend their days hiding under rocks and logs because they don’t have the waxy, protective coating that most insects have, so they can desiccate easily. At nighttime, especially in rain, they venture out looking for invertebrates to eat.
They hunt using their speed, and venom which they inject using their ‘poison claws’ which are the modified first pair of legs. If the term ‘poison claws’ isn’t enough to keep you at a distance, some centipedes also secrete hydrogen cyanide and benzoic acid from pores on their undersides when they feel threatened. If caught by a predator, they can even detach some legs in order to make a getaway, and regrow them later! Centipedes are seriously tough and long lived – up to 10 years!
This is one invertebrate that is best admired from a respectful distance.