Class Diplopoda

Class Diplopoda

The diplopods include arthropods commonly referred to as millipedes and get their formal name, Diplopoda, from what appears to be two pairs of legs on each trunk segment. Their common name, millipede, implies that they have a thousand legs. It's a reputation they don't live up to, and around 375 pairs is one of the highest number of legs counted.

Diplopods are the most abundant myriapod, with 10,000 terrestrial species living in leaf litter, rotting logs, and under stones and debris. About 30 fossil species have been identified. Millipedes are usually several centimeters long but there are species as small as 2 centimeters and tropical species are up to 28 centimeters long. The body is composed of two tagmata. The head is a fusion of the usual six segments found in atelocerates (tracheates), and appendages include antennae, mandibles, and the first pair of maxillae maxillae fused to form a gnathochilarium. Externally, the second pair of maxillae is missing. All but the first four trunk segments are fused in pairs forming diplosegments, each with two pairs of legs, the origin of their name Diplopoda-two-footed. Evidence for the fused segments is easy to find; internally each diplosegment has two pairs of ostia in the heart, two pairs of spiracles, and two pairs of ganglia in the ventral nerve cord. The first of the four anterior segments of the trunk is modified into a large, legless, wedge-shaped collum; there is some evidence that the collum may be the missing second maxillary segment, making it a part of the head instead of the trunk. Each of the three segments behind the collum has its own pair of walking legs, and diplosegments follow.

The body is covered with a cuticle with an inner procuticle hardened with calcium salts, and an outer epicuticle without waterproofing waxes; millipedes, like their centipede cousins, are restricted to moist environments. Gas exchange occurs through spiracles along the length of the body and trachea carry oxygen directly to the tissues. Millipedes can't close their spiracles; it's another reason they're restricted to moist environments. There are no respiratory pigments in the hemolymph, and the circulatory system is open, with a dorsal tubular heart located in a pericardial cavity. Hemolymph enters the heart through segmentally arranged ostia along its length and is pumped toward the anterior end of the animal. Malpighian tubules filter the hemolymph and remove metabolic wastes, usually as uric acid.

Most millipedes are herbivores feeding directly on plants or decomposing vegetation; there are only a few carnivorous species. Food is manipulated by the mandibles and placed into the mouth followed by the esophagus and tubular gut. The gut is lined with a protective peritrophic membrane, and final digestion is extracellular with nutrients passing across the gut to the hemolymph that bathes the digestive system. Millipedes are slow moving and use their legs to plow and push their way through the substrate. Although slow moving, they aren't defenseless, and repugnatorial glands at the base of the coax secrete a variety of defensive chemicals. The nervous system consists of a three-segmented brain: protocerebrum, deuterocerebrum, and tritocerebrum, and the subesophageal ganglion is connected to a paired ventral nerve cord with segmentally arranged ganglia. Sensory structures include antennae, from 2 to 80 simple eyes, and sensory setal hairs; compound eyes aren't found in the group

The sexes are separate, and the fused female gonad divides into two oviducts that open into invertible pouches, vulva, on the ventral surface of the third segment. The testis in males is paired and also opens on the third segment. The spermatophore is transferred to the female by either using the mandibles or a modified pair of legs on the seventh trunk segment. Sperm is stored in the female's seminal receptacle, and eggs are fertilized as they are oviposited into a nest made from surrounding soil that may be reinforced with fecal material. The eggs are well provisioned with yolk, development is direct, and the small millipedes that emerge usually have about seven trunk segments and only three pairs of legs; additional segments are added as the animal molts.

Chilopoda, centipedes, and Diplopoda, millipedes, have been treated here as two separate classes. Some classification schemes combine the two with the Symphyla and Pauropoda in the class Myriapoda making it a sister group to the Hexapoda (Insecta). The position of the myriapods and insects within the Arthropoda is still debated; especially how they fit with the onychophorans, considered by some to be the ancestor to all the uniramous arthropods.

  

Click for larger picture Millipedes are herbivores and detritivores with a head protected by an enlarged segment, the collum. They also have a cluster of simple eyes rather than the typical arthropod compound eye.

Houseman, BIODIDAC

 

Click for larger picture Millipedes, in this case Narceus americanus, have two defensive strategies: either roll up in a ball or secrete defensive chemicals. Take a close look of the enlarged version of this picture and you can see the secretion on the anterior surface of the animal.

Onuferko, BIODIDAC