Milli-PEET: Millipede Body Organization

There are over 12,000 described species of millipedes placed in 16 orders. On this website, the orders, their suborders and families are listed under Taxonomy and their possible relationships are illustrated in Millipede Systematics in the Tools & Resources section. The basic parts of a millipede body and the terms used to describe them are explained in the Introduction to Millipedes section.

The body of a millipede is divided into two distinct parts, the head and the trunk. The head is at the front of the animal, and is easy to identify because it houses the eyes, antennae and mouthparts. The main functions of the head are sensing the environment around the millipede using the eyes and antennae, and eating using the mouthparts. The eyes of the millipede are composed of ocelli, or simple eyes. There are usually multiple ocelli that compose an eye, but certain millipede orders lack eyes entirely. The trunk is composed of several body rings. The first body ring is called the collum and is found directly behind the head. The collum segment is legless. The next four body rings behind the collum each have only one pair of legs. All remaining body rings, except the very last few have two pairs of legs. The number of these apodous body rings, or rings that lack legs, can vary. Many millipedes have ozopores, or defense glands, on the sides of most of their body rings. The millipede produces a chemical defense that is exuded through the ozopores to deter predators or other curious animals.

Anadenobolus arboreus (Spirobolida, family Rhinocricidae)

Many male millipedes use modified legs to place their sperm into the vulvae of the female. The paired female vulvae are located on or behind the base (coxa) of the second leg pair. The position of these sperm-transferring organs in males differs in the various orders and suborders of millipedes. In the small bristle millipedes (order Penicillata) males do not have modified legs; they deposit a sperm packet, or spermatophore, on the ground. The female picks the sperm up herself. In the pill-millipedes and related groups, either one leg pair (orders Glomeridesmida and Glomerida) or two leg pairs (order Sphaerotheriida) at the posterior end of the males are strongly modified for mating and sperm transfer. In the Helminthomorpha, which contains all the remaining 12 orders of millipedes, the legs of the 7th and/or 8th body ring are strongly modified into gonopods. In some groups only one of the leg pairs may be modified, while in others one leg pair of the 7th ring might be reduced. The gonopods may be carried in a pouch inside the body.

The two links below present a schematic overview of the different configurations of gonopods in the helminthomorph orders. The Colobognatha (orders Platydesmida, Polyzoniida, Siphonocryptida, and Siphonophorida) carry 8 walking legs in front of their leg-like gonopods. The posterior leg of the 7th body ring and the anterior legs of the 8th body ring are modified to gonopods. In the remaining helminthomorph orders, there are 7 walking legs in front of the gonopods. The anterior, and in many groups also the posterior, legs of the 7th body ring are modified. In the order Chordeumatida, the legs of the 8th ring are modified as well.

The schematic drawings are purely descriptive and do not imply homology. For example, the structure of the gonopods in Callipodida and Siphoniulida is very different. These illustrations show only which legs are modified to gonopods, not how they are similar or different in both groups. The distinction between the sperm-transferring gonopod (indicated in black) and the accessory gonopods (indicated in grey) was inferred by the presence or absence of the sperm groove in the sperm-transferring gonopod. Very little is known about the function of gonopods (see the Bibliography in the Tools & Resources section for literature on this topic).

Body plan of the four suborders of the order Chordeumatida

Body plan of the remaining helminthomorph orders