Hoffman, Richard L. Department of Recent Invertebrates, Virginia Museum of Natural History, Martinsville, Virginia.
- Classification and distribution
- Additional Readings
A class of tracheate, terrestrial, oviparous arthropods (millipeds), characterized by a compact head with a coalesced labrum, a pair of eight-segmented antennae, bisegmented mandibles (typically with a series of pectinate lamellae), and a subbuccal gnathochilarium (lower lip) composed of two pairs of embryonic maxillary elements. The diplopod (milliped) body (Figs. 1 and 2) is not differentiated into thoracic and abdominal regions. Instead, it is composed of a variable number of similar, subcylindrical units (segments). In most species, the components of these segments coalesce into rigid cylindrical units, with each being formed by the fusion of two embryonic somites into apparent segments, most of which carry two pairs of walking legs (which is the source of the class name, Diplopoda). Several of the anteriormost segments have only one pair of legs, with the second pair being reduced to vestigial elements. The body wall is chitinous; in most species, it is heavily impregnated with calcium carbonate. The anterior part of each segment typically telescopes into the preceding one, and most millipeds are capable of coiling into a spiral—in some cases, a compact sphere with the head innermost—as a protective measure (Fig. 3). In the majority of species, each segment posterior to the fourth segment is provided with a pair of glands (ozadenes), which produce a variety of volatile defensive secretions. Gas exchange occurs through a system of profuse fine trachea that open through stigmata located on the sternal elements adjacent to the leg bases. The digestive system is typically a straight tube divided into esophageal, gastric, and intestinal regions; no proventriculus (dilation of the foregut) is present. Excretion is through nephridial tubes located in the hemocoelic cavity and discharging into the hindgut. The anal opening is in the last segment and is closed by two tightly fitting anal valves (paraprocts). Water recovery is accomplished in the subterminal rectal region. See also: Arthropoda
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