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cally more slender and laterally compressed. Both pairs of antennae are usually well developed and long. The legs are variously modified, some of the anterior often as grasping devices (raptorial) and always with three pairs specialized for walking or hopping (uropods). The exo-skeleton may be thin, mineralized, or occasionally heavily sclerotized. It is generally well pigmented and often pitted or otherwise microsculptured. The dorsal plates often have lateral, winglike flanges.

Like the isopods, amphipods require a moist or watery environment, not only for survival but for reproduction as well. They always return to water to deposit their eggs. There are several aquatic larval stages, the animal moving to land only with the last molts.

The Talitridae are known everywhere and normally encountered burrowing in damp sand, particularly that beneath beach-stranded seaweed and other debris. The dominant Central and South American genus is Hyale, with many species from all shores (Barnard 1979). The widespread I sand flea Orchestia platensis (fig. 4. le) is only

! common on Caribbean seashores, there be ing no members of the family on mainland South America in spite of the fact that the family is otherwise cosmopolitan.

This is a neglected group in the region. Much more is to be learned regarding species present and their biology in Latin America.

Reference

Barnard, J. L. 1979. Litorral gammaridean Amphipoda from the Gulf of California and the Galápagos Islands. Smithsonian Contrib. Zool. 271: 1-149.

arachnids_

Arachnida

Arachnids comprise the majority of the chelicerates and are predominantly terres trial (Besch 1969, Cloudsley-Thompson 1958, Savory 1977).

Chelicerates are defined by the structure of the anteriormost appendages (chelic-erae), which are made up of a bulky basal segment with an apical movable finger. The chelicerae are modified to form varied organs, such as the venom apparatus (fangs) in spiders, piercing stylets in parasitic mites, or masticating jaws of scorpions.

The several orders are quite diverse. In all, the head is undefined, being fused with the thorax (cephalothorax); in some, the abdomen is also fused into a single body complex (mites).

Arachnids are an ancient stock, and they are diverse today, though poorly studied in Latin America. A great many species, especially among the spiders and mites, are yet to be discovered.

Arachnids exhibit a strongly climate-dependent distribution, mainly in two directions. There are those in moist habitats (Uropygi, Amblypygi) and deserticolous forms (Solpugida). Some transcend these divisions and are widespread and broadly adapted (Araneae, Scorpionida); other minor groups have specialized niches, such as ectoparasitism.

Several anatomical characteristics distinguish the arachnids. The head and thorax (leg-bearing portion) are fused into a single unit, the cephalothorax (prosoma). The abdomen (opisthosoma) may or may not be distinct. Mouthpart appendages are composed of a single pair of pinching or piercing chelicerae, with a stationary base and movable finger. A pair of segmented sensory appendages (pedipalps) precede the four pairs of legs. Some spiders and scorpions are capable of audible stridu-Iation (Lucas and Bticherl 1972).

Arachnids are protected mainly by their secretive, commonly nocturnal habits and camouflage. Besch (1969) noted the predominance of green in the coloration of spiders and other Arachnida in South America. Many are capable of dropping

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appendages to escape capture (appendoto-my) (Roth and Roth 1984). Quite a few also produce noxious chemicals, some of which are very potent in repelling enemies.

Most arachnids are predaceous and feed by ejecting enzymes onto the prey from a preoral cavity and siphoning the resultant liquids, or by piercing the prey's skin and sucking blood or lymph. A few are plant feeders, using elongate sucking mouth-parts to take sap or other liquids from the host.

References

Besch, W. 1969. South American Arachnida. In E. J. Fittkau, J. lilies, H. Klinge, G. H. Schwabe, and H. Sioli, eds., Biogeography and ecology in South America. 2: 723—740. Junk, The Hague. Cloudsley-Thompson, J. L. 1958. Spiders, scorpions, centipedes and mites. Pergamon, New York.

Lucas, S., and W. Bücherl. 1972. Aparelhos estriduladores do escorpiáo, Rhopalurus iglesia-si dorsornaculatus (Prado 1938), e da aranha caranguejeira, Theraphosa blondi (Latreille) 1804. Cien. Cult. 23: 635-637. Roth, V. D., and B. M. Roth. 1984. A review of appendotomy in spiders and other arachnids. Bull. Brit. Arachnol. Soc. 6: 137-146. Savory, T. 1977. Arachnida. 2d ed. Academic, London.

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