The Polydesmida is the largest order of millipedes, also known as flat-backed millipedes. While there is often confusion between centipedes and millipedes, polydesmidae will all have two pairs of legs per body segment and will often lay motionless when disturbed. Where alternatively, centipedes have only one pair and become active and hide when disrupted. Centipedes are also predatory whilst millipedes are herbivores. Unfortunately, it is often not possible to identify the exact species of Polydesmus millipedes without examining the female epigonal structures or the male gonopods. Furthermore immature Polydesmidae cannot be identified until they develop. When trying to identify the species I found, I was only able to narrow it down to Polydesmus sp., moreover there are four species in this order found across the UK, including Norfolk, that could potentially be the organism I found. This blog will cover these four species; Polydesmus angustus, Polydesmus denticulatus, Polydesmus inconstans, Polydesmus coriaceus.
Polydesmida- Habitat and External Anatomy:
This order of millipedes includes thousands of species and can be found across the globe, excluding Antarctica and the higher Arctic. They mostly eat decaying vegetation and fungi, so they are often found in woodland areas with lots of leaf litter. They are commonly known as ‘flat-backed’ millipedes due to their general shape. Most Polydesmida are covered in structures called paranota, which are flat, wide extensions on their backs. However, some Polydesmida have either reduced paranota or none at all, instead they are cylindrical shaped. There legs are also slightly longer than in the other orders of millipedes. The name ‘Polydesmida’ refers to these paranota, which seem to act as short pieces that bond the body together. The word polys means ‘many’ in Greek, while desmos means ‘bond’. The actual structure of the external anatomy is far more complicated than this. It it made up of diplosegments which often has the lateral extensions of paranota. Each ring is made up of an anterior prozonite with a smaller diameter, a larger posterior metazonite and a waist where they join. The prozonite will slot into the metazonite of the next ring. Sometimes the suture, a narrow line, can be seen on each segment which shows the join between the prozonite and metazonite.
Polydesmus angustus:
This species is very common and can be found across the UK, it is frequently spotted in woodland habitats and is found in almost any area occupied by millipedes. Recently, through examination of recording scheme habitat data, there has been evidence to show an increasing preference for heath and moorland with peat, non- calcareous soils. This millipede has also been introduced to Austria, North America and Mexico. This millipede is known as a ‘wedger’, to push through soil or under rocks/ wood they use their head (flat-headed) to push open cracks in leaf litter or soil. They have longer legs than other millipedes and use this to wedge their body into the openings. They can be spotted throughout the year, however they are nocturnal so are more active at night. While they have a lifespan of 2-3 years, both sexes will die after reproduction so sights of these millipedes are rarer over summer.
Polydesmus denticulatus:
Similarly to P. angustus, this species is also widespread across the UK however it is sparser in northern Scotland and Ireland. 55% of records show this species frequently resides in woodland, however there is also a preference to wetland areas; this may be due to its ability to survive inundation, which competing species may not be able too. However, in countries further east, such as Germany and Switzerland, this millipede is more commonly seen in grassland and arable land. Furthermore, in the UK this species is often found in suburban sites. This shows the diversity and adaptability of this species. They can also be found throughout the whole year, but are commonly seen from April- September.
Polydesmus inconstans:
This specie has an Atlantic distribution but surprisingly it is only sparsely seen across south west England, south west Ireland and Wales. It is more commonly seen cross the rest of the UK. One thing that makes this millipede less likely to be the species I found, is that it is often sited in sand dunes and grassland, often avoiding woodlands. The species I found was underneath a log, in a grassy area next to a woodland and lake. On the other hand, there has been records of this species in damp river-side grasslands in Oxfordshire. Like Polydesmus a., both sexes will die after reproduction so they are harder to spot in late summer and are more frequent in spring, April- June.
Polydesmus coriaceus:
This species is very common in Southern England and Ireland, however is very uncommon in northern England with only one recording in Scotland. P. coriaceus. are distributed across the Atlantic, mainly residing in southland such as Belgium, western France, southern England, the Azores and Madeira. It is found in almost any habitat that millipedes reside in, but has a preference for areas of cultivated land, grassland and waste grounds. Furthermore, there has been increasing evidence that this millipede may also favour slightly damper habitats. The habitats they are found in have clay soils with a strong negative association to non-calcareous and sandy soils. They may be present on other soils, if the soil has low drainage creating damp conditions. They are most frequently spotted in spring, but are present throughout the year.
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