Myriapoda

The Myriapoda has not been widely supported as a natural group (although see Zrzavy et al., 1998b). Indeed, several studies indicate that the centipedes (Chilopoda) and sym-phylans are basal (although not themselves related), while the Dignatha (Pauropoda and Diplopoda [millipedes]) comprise a sister group to the Hexapoda (e.g., Wheeler, 1998; Kraus, 2001). The symphylans are sometimes classified with the Dignatha into a larger group called the Progoneata. Other views on the phylogeny of Myriapoda are presented by Wheeler et al. (1993a), Kraus and Kraus (1994), Borucki (1996), Kraus (1998, 2001), Ax (1999), and Regier and Shultz (2001b). Overall there is little consensus on myriapod phy-logeny, and we have adopted for the time being the conservative, traditional view of relationships (Figure 3.23). The biology of myriapods is summarized by Camatini (1979).

The centipedes (Chilopoda) are, along with the millipedes (Diplopoda), the best-known group of Myriapoda. Centipedes range in size from minute to gigantic and are active, terrestrial predators. Species occur in numerous habitats but are particularly abundant in the tropics, with a diversity of approximately 2,600 species worldwide. The body of centipedes is typically somewhat compressed dorsoventrally, with the first maxillae expanded at their base and forming a ventral cover for the other mouthparts. The number of leg pairs varies from 15 to 177 and centipedes are capable of quick movement, although not owing to so many legs but instead to the fact that the body generally is suspended below the attachments to the legs, allowing them to step over the legs of preceding segments during movement. Centipedes are principally nocturnal and are carnivorous except for species of Geophilomorpha, which are omnivorous and prefer a diet of plant tissue. The appendages of the first trunk segment are developed into maxillipeds, or more commonly "forcipules," and are used to poison prey. The bases of the maxillipeds fuse to form a lower, shovel-like base for the head. Other defining features of the centipedes include the composition of the stemmata of the eyes and the complete loss of median eyes. Centipede phylogeny has been elaborated upon by Shear and

Bonamo (1988), Borucki (1996), Prunescu (1996), Shultz and Regier (1997), Edgecombe et al. (1999), Giribet et al. (1999), Kraus (2001), and Regier and Shultz (2001b). Fossils centipedes are among the earliest terrestrial arthropods known (Jeram et al., 1990; Shear et al., 1998) (e.g., Figure 3.24).

Symphyla are small centipede-like animals with 15-22 segments, 12 pairs of legs, and, like the centipedes, long antennae (Figure 3.25).

Development, like the Pauropoda (below) is anamorphic; juveniles hatch with 6-7 pairs of legs and progressively add appendages until the full compliment is achieved. Defining traits for symphylans include the unpaired genital opening near the anterior end of the trunk, loss of eyes, a pair of spiracles on the sides of the head, second maxillae fused to form a labium-like structure (analogous to the labium of Hexapoda), spermatheca formed as pockets positioned in the mouth, and terminal spinnerets. They occur in moist soil, in decaying wood, in moss, and under stones, and species are herbivorous. Only two fossil records are known for the Symphyla: one in Baltic amber (Bachofen-Echt, 1949) and one in Dominican amber.

The Pauropoda are minute, infrequently encountered myriapods living in moist leaf litter, in the soil, or under stones or bark (Figure 3.26). There are approximately 500 species principally occurring in tropical or warm temperate regions. Juveniles begin their life as hexapods (only three pairs of legs) but add pairs as they mature, ultimately reaching a total of nine pairs on the anterior trunk segments. The heads of pauropods tend to be relatively small, possessing characteristically branched antennae, and they lack eyes. The posterior head segment lacks appendages and is separated from the remainder to form a circular collar (collum), similar

3.23. Phylogeny of Mandibulata, indicating relationships among the major lineages of Atelocerata.
3.24. Fragments of a mid-Devonian centipede from New York. Arthropods are the earliest known land animals. AMNH 411-7-AR97; length 1.5 mm; from Shear and Bonamo (1988).
3.25. Scanning electron micrograph of a Recent symphylan. Length 2.1 mm.

to the millipedes, although in the latter vestiges of appendages are usually present. The trunk is composed of 11 segments. As in the millipedes, each tergal plate covers two body segments (diplosegments); however, the legs are not doubly paired as in most Diplopoda. Aside from antennal structure, the defining features of the class include the reduction and fusion of the second maxillae and the occurrence of an eversible vesicle on the first trunk segment. Little is known about pauropod biology or phylogeny. The sole fossil record of a pauropod is Eopauropus balticus in mid-Eocene Baltic amber (Scheller and Wunderlich, 2001), a species that is quite modern in appearance. The pauropods are likely quite ancient, but their small body size, delicate bodies, and habitat preference precludes fossilization.

Millipedes are generally herbivorous or detritivorous and, like the Pauropoda, tend to live in leaf litter, in the soil, or beneath stones, logs, and bark. They are also the most diverse of all myriapods, with around 10,000 species known worldwide. Individuals are relatively common and occur worldwide. They range in size from 2 mm to an incredible 28 cm (some fossil diplopods of the Arthropleurida exceeded 1.8 meters!) (e.g., Kraus and Brauckmann, 2003). The trunk is composed of diplosegments in most species and bears two sets of legs on each, except for the anterior three trunk segments, which possess a single pair of legs each. The number of legs varies widely with a maximum record of 350 pairs (700 legs!) in Illacme plenipes. Although not equipped with poisonous "fangs" like the centipedes, millipedes produce cyanogenic compounds from repugnatorial glands to protect themselves from predators. Fossils of millipedes extend back at least to the early Devonian and are known from numerous localities from that time period until the present day (e.g., Shivarudrappa, 1977; Dzik, 1981; Shear, 1981; Hannibal, 1984; Donovan and Veltkamp, 1994; Duncan et al., 1998; Schneider and Werneburg, 1998; Grimaldi et al., 2002). Millipede phylogeny has been studied by Enghoff (1984, 2000), Regier and Shultz (2001b), and Sierwald et al. (2003). Hopkin and Read (1992) have summarized millipede biology.

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