The class Diplopoda, or the millipedes, contains about 10,000 described species. The animals have a long distinguished history on our planet, spanning over 400 million year. Their ecological importance is immense: the health and survival of every deciduous forest depends on them, since they are one of the prime mechanical decomposers of wood and leaf litter, especially in the tropics where they replace earthworms. Despite their importance they are very poorly known and have long been neglected in all areas of biological research. Even basic identification of specimens is a challenge.

Millipede Morphology

Millipedes have a body divided into two regions, an anterior head and a long posterior trunk.  The trunk is made up of body rings (Fig. 1). Adult millipedes carry two pairs of legs on most body rings (Figs. 1 and 2). The first body ring right after the head, the collum, is legless (Fig. 1). The following three rings (body rings 2 through 4) carry one leg pair each (Fig. 1). A juvenile millipede often has usually several legless rings at the end of its body. All millipedes have a legless pre-anal ring and anal valves at the end of the body.  The anal valves open during defecation.

Mouthparts (Fig. 1): millipedes have only two sets of mouthparts, the mandibles used for chewing and a plate behind them, the gnathochilarium. All millipedes have antennae, consisting of 6 or 7 visible articles. The last article carries sense cones, usually four of them. The Tömösváry organ is a sense organ located on the head of many millipedes. It forms a raised ring, a horse-shoe, or may be only a small pore. It is found behind the antenna sockets.

Ozopores: In many orders, some trunk segments carry ozopores, the openings of the stink glands. These may be very obvious or hard to see. In most groups that have them they occur along both sides of the trunk, starting at the 5th ring (Fig. 2); in a few groups the pores are located along the dorsal midline.

Millipedes belong to the arthropods and have a hard exoskeleton. In order to grow, they have to molt. The cuticle of most millipedes is calcified and rather hard. The back of each ring of the millipede is covered with a hard plate termed a tergite. Lateral extensions of the tergites are called paranota (Fig. 3).

Many millipedes have "eyes" at the side of the head. These consist of few to many individual ocelli grouped together in an ocular field.  Some millipedes, like the Polydesmida, never have ocelli. Cave-living millipedes of many orders have lost their eyes, even when their above-ground living cousins have well-developed eyes.

Adult millipedes of many groups have distinct sexual organs, which can be easily seen with a dissecting microscope. These sexual organs occur in both sexes, but are most obvious in males. Modified legs occur in males in two body areas, either around the 7th body ring or at the end of the body, comprising the two last pairs of legs. The modified legs on the 7th ring are sometimes withdrawn into a pouch in the body. In such groups the adult male appears to lack legs on the 7th ring. The modified legs of the 7th ring are called gonopods (Fig. 4) and are very important in making identifications. Females have sex organs called cyphopods, found just behind the second pair of legs.

There are about 10,000 described species of millipedes. They are currently grouped in 15 orders and 148 families.

Fig. 1. Body parts of a male millipede of the order Julida. In side view, the anterior legs appear to emerge from the ring in front of that to which they actually belong (after Blower, 1985).

Fig. 2. Structure of a body ring (diplosegment) (after Demange, 1981)

Fig. 3. A member of the order Polydesmida with paranota, dorsal view

Fig. 4. The male gonopods on the 7th body ring, ventral view

For more information on these fascinating animals, the following reference books are recommended.

Blower, J. G. 1985. Millipedes. Synopses of the British Fauna (New Series). No 35. The Linnean Society of London, 242pp.

Demange, J.-M. 1981. Les Mille-Pattes, Myriapoda. Societe nouvelle des editions Boubee. 284pp (in French, with focus on the French fauna)

Hopkin, S. P. & H. J. Read. 1992. The Biology of Millipedes. Oxford University Press, 233pp.

Shear, W. A. 1999. Millipeds. American Scientist, 87: 232-239.

Shelley, R. M. 1999. Centipedes and Millipedes. the Kansas School Naturalist, 45(3), 15pp. Single copies available without charge from Emporia State University

The following web sites provide further details on millipedes and current millipede research.

http://www.myriapoda.org
http://www.ucmp.berkeley.edu/arthropoda/uniramia/myriapoda.html
http://www.life.umd.edu/entm/shultzlab/vtab/
http://www.mnhn.fr/assoc/myriapoda/INDEX.HTM