Millipedes

Diplopoda. Spanish: Milpies. Portuguese:

Gongolos, piolhos de cobra (Brazil).

Quechua: Pachac chaqui.

Like centipedes, which they superficially resemble, millipedes gain their name from an abundance of legs. Two pairs arise from most apparent segments, a condition created by embryonic fusion of alternate body somites, each carrying one pair of legs, a pair of tracheal openings, and a ventral nerve cord ganglion, to form double somites (diplosomites). The resulting secondary segment (diplosegment), therefore, has two ganglia and four legs and spiracles, all of which are located in the metazonite, representing the posterior of the fused somites. The prozonite, representing the anterior of the fused somites, lacks appendages and can thus be telescoped into the preceding metazonite, resulting in a more compact body form. The four ante-riormost segments have one leg or no legs, a secondary loss associated with the ability to curl the head backward into a protective posture.

The head of millipedes bears one pair of short antennae, one pair of internal mandibles, used to masticate decaying wood and other vegetation, and a flattened, platelike structure, the gnathochi-larium, which forms the anterior floor of the mouth. The latter consists of several small sclerites whose shape and arrangement differ among orders and are, therefore, taxonomically useful.

The subclass Diplopoda is divided into three superorders, each represented in the Neotropical region, but the vast majority belong to the superorder Helminthomor-pha, containing calcified forms that are generally elongate with cylindrical or flattened bodies. The reproductive tracts open on segment 3, but in males, one or both pairs of legs on segment 7 and occasionally the anterior pair on segment 8 are modified into copulatory structures called gonopods. The gonopods transfer sperm packets to the female, and their configurations are of primary significance at the generic and specific levels. Pairs of millipedes are often encountered enraptured, stretched out or coiled together venter to venter in a many-legged copulatory embrace. After copulation, males of some large "flat-backed" species ride on the backs of females for several days (Heisler 1983).

Nine of the eleven helminthomorph orders occur in the Neotropics, but the Polydesmida, Spirostreptida, and Spirobo-lida are dominant. The Polydesmida are generally characterized by lateral expansions of the dorsum called "paranota," which impart a flattened appearance to the animals, hence the name "flat-backed" millipedes. Seventeen polydesmid families occur in the Neotropics (Hoffman 1979),

Figure 4.9 MILLIPEDES AND CENTIPEDES, (a) Spirobolid millipede (Orthoporus sp., Spiro-streptidae). (b) Polydesmid millipede (Barydesmus sp., Platyrhacidae). (c) Giant centipede (Scolopendra gigantea, Scolopendridae). (d) House centipede (Scutigera coleoptrata, Scutigeridae).

many of which are comprised of minute, cryptic forms. The dominant families with large-bodied forms are Platyrhacidae and Chelodesmidae.

Common platyrhacid genera are Nys-sodesmus (Costa Rica) and Barydesmus (Costa Rica to Peru and western Brazil; fig. 4.9b), whose adults range from 70 to 100 millimeters in length. Only slightly smaller are Psammodesmus (Panama to Peru) and the diverse assemblage of forms usually assigned to Amplinus, Pycnotropis, and Polyle-piscus (Mexico to Peru). In the Chelodesmidae, Chondrodesmus, with around forty species, is abundant from Mexico to Brazil. The Spirostreptida and Spirobolida contain phenotypically similar cylindrical forms of variable length and diameter; they are distinguished by details of the head, exoskeleton, and gonopods. Nearly thirty Neotropical spirostreptid genera, in three families, are currently recognized (Hoffman 1979, Krabbe 1982), but the most widespread is Orthoporus (fig. 4.9a), with around forty species that range from the southwestern United States throughout most of South America to Brazil. Vilcastrep-tus hoguei is a large, dark-bodied form with pinkish legs, seen by thousands of tourists each year in the Incan ruins of Machu Picchu (Hoffman 1988).

Of the five Neotropical spirobolid families. the Rhinocricidae is dominant. These millipedes vary greatly in size and are readily recognized as belonging to the family, but a satisfactory generic arrangement has not been attained.

The Latin American millipede fauna is diverse but poorly known. Hoffman (1969) reported some 470 species from Brazil, and Loomis (1968) reported around 750 from Mexico and Central America. These are the only counts available. The total fauna doubtlessly consists of several thousand species, probably less than 20 percent of which have been discovered, much less named and described.

Although deserticolous forms exist, these are creatures mostly of dank, humid habitats. They live in rotten logs, among leaf litter, in the soil, under stones and loose tree bark, in caves, and are a major component of the forest ground fauna; a few are symbiotic with army ants (Loomis 1959).

Millipedes are harmless creatures, protected primarily by their hard exoskeleton. However, most produce droplets of caustic or noxious secretions from a series of lateral pores on most segments (Eisner et al. 1978). The liquid usually oozes out, but some large millipedes, such as the Peruvian platyrhacid Barydesmus (orig. obs.), the Costa Rican Nyssodesmus python (Heisler 1983), and Rhinocricus lethifer, a Jamaican rhinocricid (Loomis 1936), can forcibly squirt the fluid 1 to 4 decimeters or more.

The active chemicals in these secretions are diverse. Most orders have single-chambered glands that produce benzoquinones, cresols, or aldehydes that can blister or tan human skin (Burtt 1947). Polydesmids give off hydrogen cyanide from the outer part of two-chambered glands, from nontoxic mandelonitrile precursors in the inner compartment (Woodring and Blum 1963). Diplopods also adopt a defensive posture by coiling with the vulnerable head tucked in the center. Two Neotropical polydes-moid families can roll up into balls or spheres, an adaptive feature that has evolved independently in other polydes-moid families and another subclass in other parts of the world.

Members of the class harbor a diverse microflora in the gut which functions to release nutrients and reduce toxins from ingested plant matter (Sakwa 1974).

Discussions of the anatomy and general biology of millipedes are available in several general textbooks on arthropods (Kaestner 1968: 389-429). Refer to Loomis (1968) and Hoffman (1979) for bibliographies and taxonomy. Important regional taxonomic works have been written by Loomis (1936, 1964, 1968) and Jeekel (1963).

References

Burtt, E. 1947. Exudate from millipedes with particular reference to its injurious effects. Trop. Dis. Bull. 44: 7-12. Eisner, T., D. Alsop, K. Hicks, and J. Mein-wald. 1978. Defensive secretions of milli-peds. In S. Bettini, ed., Arthropod venoms. Springer, Berlin. Pp. 41—72. Heislf.r, I. L. 1983. Nyssodesmus python (milpies, large forest-floor millipede). In D. H. Janzen, ed., Costa Rican natural history. Univ. of Chicago Press, Chicago. Pp. 747-749. Hoffman, R. L. 1969. The origin and affinities of the southern Appalachian diplopod fauna. In P. C. Holt, ed., The distributional history of the biota of the southern Appalachians. Virginia Polytech. Inst., Blacksburg. Pp. 221-246. Hoffman, R. L. 1979. Classification of the

Diplopoda. Mus. Hist. Nat., Geneva. Hoffman, R. L. 1988. A new genus and species of spirostreptoid millipedes from the eastern Peruvian Andes. Myriapodologica 2: 29-36.

Jef.kel, C. A. W. 1963. Diplopoda of Guiana (1-5). Nat. Stud. Suriname Nederlandse Antillen 4(27): 1-157.

Kaestner, A. 1968. Invertebrate zoology. Vol. 2. Wiley Interscience, New York.

Krabbe, E. 1982. Systematik der Spirostrep-tidae (Diplopoda: Spirostreptomorpha). Natur. Ver. Hamburg Abh. (n.f.) 24: 1-476.

Loomis, H. F. 1936. The millipedes of Hispa-niola, with descriptions of a new family, new genera, and new species. Mus. Comp. Zool. (Harvard Univ.) Bull. 80: 1-191.

Loomis, H. F. 1959. New myrmecophilous millipeds from Barro Colorado Island, Canal Zone and Mexico. Kans. Entomol. Soc. J. 32' 1-7.

Loomis, H. F. 1964. The millipedes of Panama (Diplopoda). Fieldiana: Zoology 47: 1-136.

Loomis, H. F. 1968. A checklist of the millipedes of Mexico and Central America. U.S. Natl. Mus. Bull. 266: 1-137.

Sakwa, W. N. 1974. A consideration of the chemical basis of food preference in millipedes. Zool. Soc. London Symp. 32: 329-346.

Woodring, J. P., and M. S. Blum. 1963. Anatomy and physiology of repugnatorial glands of Pachydesmus. Entomol. Soc. Amer. Ann. 56: 448-453.

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