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Millipedes are relatively common litter and soil animals that occur in most parts of the world. The word 'milli' is latin for a thousand and 'pede' is for foot so a millipede ought to be a thousand footed animal. However whoever coined the name was guessing because although some millipedes have a lot of legs, none actually have a thousand. In fact the record holder is Illacme plenipes which has an amazing 750 legs or 350 pairs. Most millipedes have far less than this though, normally 100 to 300. Millipedes are distinguished from all other Myriapods because they have two pairs of legs per body segment. This is because each segment is actually two segments fused together. These special segments are called 'Diplosegments' . Millipedes use their legs to push themselves into the soil, leaf litter or rotting wood, and the more legs you have the more you can push so it makes sense to have plenty of legs. Millipedes do not really need that many body segments though so the fusing of two segments into one functional unit whilst maintaining both pairs of legs allows millipedes to generate a lot of push without becoming impossibly long. Though it is not visually obvious millipedes do have a what is called a thorax and an abdomen. The thoracic segments are the 1st 3 body segments, the rest are the abdominal segments. The 1st thoracic segment is apodous (without legs) and the 2nd and 3rd have only one pair of legs each. In all Millipedes except the Colobgnatha the anterior (front) pair of legs on the 4th body segment is also missing, hence if you want to know how many legs a millipede has you can count the number of body segments, multiply by 4 and subtract 10 unless it is a member of the colobgnatha in which case you only subtract 8. Millipedes always look pretty clean and when healthy they are in fact very clean animals, spending a great deal of time cleaning and polishing all the various parts of their bodies. They have special brush like group of hairs on their 2nd or 3rd pair of legs which are used in cleaning the antennae. They are also noted for being meticulous about cleaning the gonopods after sex.
There are 4 main types of millipedes in the world, Cylinder Millipedes, Plated Millipedes, Pill Millipedes and Bristly Millipedes.
The long cylinders can be further divided into Bulldozers, (Julids or Iulids) and Borers (Polyzoniida).
Bulldozers are your typical millipede, they are long and thin and use their flattish heads as a bulldozer blade. Their numerous legs supply the oomph which allows them to push themselves through the soil and leaf-litter.
Borers are the Colobgnatha, smarties called sucking millipedes, they are to Julids similar but have narrow pointed heads with each successive segment getting slightly larger, they use this shape to winkle themselves into the the substrate they are trying to penetrate. They have small legs which are invisible from above. Colobgnath millipedes have reduced mandibles and likely feed on rotting vegetation, they are also often blind.
Plated millipedes, sometimes known as Wedgers are the Polydesmida. They have the upper plate of each segment expanded out from the centre of the body into a keel or paranota, thus giving them a less rounded appearance. They work their way into the substrate by wedging in the first plate and then lifting up the body to widen the space, then they push in a little more and lift again.
Pill millipedes are members of either the Glomeridae or the Sphaerotheriida, they are also bulldozers by nature but are better known for their ability to role into a ball
Bristle millipedes are bark dwellers, they do not burrow, are covered in bristles and are very small being only a few mm long Polyxenus lagurus 2-3mm is the only British species. These are atypical millipedes and many of the generalisations made in this paper do not apply to them.
Plated millipedes, Pill millipedes and Bristle millipedes all have far fewer segments than the long thin cylinder types.
Most millipedes live in the leaflitter zone or burrow into the top soil but some specialise in living under the bark on dead trees or else in rotten wood and some can even be found climbing living trees. Millipedes are important agents of nutrient fluxes in many habitats, in fact in some tropical areas they are of more importance than earthworms. Their main role is one of comminution of plant material, meaning they breakup dead plant material into small pieces thus increasing the surface area. This is important because the surface is the only part the bacteria and microfungi can reach and they are the main agents of recycling in the soil. Millipedes when they eat rotting vegetation are really only digesting the fungi and bacteria and the plant material they have already broken down. An american Biologist called F. H. Colville once calculated that the millipedes in an un-named forest in USA contributed 2 tons of manure per acre per year to the forest floor.