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Bug of the Month: "Beach" millipede

side view of a "beach millipede"Common name: "Beach" millipede (this name given by the author of this page)

Latin name: Thalassisobates littoralis

Taxonomy:
Kingdom Animalia
   Phylum Arthropoda
      Class Diplopoda
         Order Julida
            Family Nemasomatidae

Millipedes in general

close up side view of beach millipede legsMillipedes do not have 1000 legs, in fact, most have fewer than 50 pairs! What distinguishes them from all the other very “leggy” arthropods (e.g., centipedes) is that they have TWO pairs of legs per body segment, (versus centipedes with one pair of legs per segment). Millipedes also tend to have much shorter antennae than centipedes.

You can usually find millipedes in the leaf litter or under the soil surface. Almost all millipedes are decomposers and eat decayed plant material. As the plant matter moves through the millipede's body it is broken up into smaller and smaller fragments which, in turn, provide a larger surface area for the colonization of bacteria and fungi. These microbes continue the process of chemical breakdown of the plant matter. Millipedes are generally harmless to humans (they don't bite like centipedes), but some species can swarm in very high numbers and enter houses, and some have been known to feed on crops like potatoes and sugar beets. Many millipedes secrete defensive chemicals to deter predators, and most millipedes can curl up into a tight spiral or ball to protect themselves.

There are estimated to be more than 80,000 species of millipedes world-wide, although only about 10,000 species have been officially described.

stink glands on a beach millipedeHow to identify Thalassisobates littoralis

The beach millipede is just 9-17 mm long (~1/2 inch) and it's body is very slender (0.5 mm, or less than 1/32 inch). It is the skinniest millipede on the islands! The color of the body in life is pale grayish/tan or white with distinct reddish-brown spots (the stink glands) running down most of the length of the body, one spot per segment. At full maturity, the millipede has 40 or more rings (segments with legs). Probably the best way to identify this millipede, however, is by where you find it (see below).

Where to find beach millipedes on the Boston Harbor Islands

These millipedes live on the beach, in gravel and under stones and seaweed. You can find them just above the high tide line and down in the intertidal zone. There is one other millipede species found at the high tide line on the Boston Harbor Islands, Cylindroiulus latestriatus, and it is about the same length as the beach millipede, but twice as broad (and older individuals can be almost black). To date, the beach millipede has been documented on seven of the Boston Harbor islands. (See our database to find out which islands!)

How beach millipedes make a living

head of beach millipedeThere is little information available about the life history of this species. Millipedes in the family Nemasomatidae lay their eggs one at a time. As with all millipedes, the eggs start developing immediately into an embryonic stage and then moult to produce the first instar with four full body rings. More leg-bearing rings are added with each moult. Beach millipedes will go through seven or eight separate moults before reaching full maturity (~40 rings). Most millipedes take between 18 months to 3 years to develop from egg to sexually mature adult.

Beach millipedes are likely feeding on decaying vegetation on the beach, like seaweed, or other organic material. Some millipedes may be eaten by predaceous invertebrates (beetles, spiders) and perhaps by shore birds.

Where in the world Thalassisobates littoralis occurs

The distribution of Thalassisobates littoralis is somewhat puzzling (see Enghoff 1987). It has been found on shores of the Mediterranean, on the British Isles, coastal Sweden, and there are just two previous records from North America—in Virginia and Massachusetts (exact location unknown). Although originally from Europe, the beach millipede may have come to North America without the aid of humans, because it is so tolerant of salt water.

full view of Thalassisobates littoralis

To learn more about Thalassisobates littoralis (and millipedes in general)

On the web:

In print:

    Blower, J.G. 1985. Millipedes—keys and notes for the identification of the species. Linnean Society Synopses of the British Fauna (New Series) No. 35. Kermack, D.M. and R.S.K. Barnes (eds.). E.J. Brill/Dr. W. Backhuys, London. 242 pages.

    Enghoff, H. 1987. Thalassisobates littoralis (Silvestri)--an amphiatlantic millipede (Diplopoda, Julida, Nemasomatidae). Entomologist's Monthly Magazine. 123:205-206.

    Hopkin, S.P. and H.J. Read. 1992. The biology of millipedes. Oxford University Press, Oxford. 233 pages.