SPIDERS AND THEIR RELATIVES The bodies of these animals are divided into two segments: a cephalothorax and an abdomen. Chelicerates (spiders and their relatives) share three characteristics: First, cheliceraes do not have sensory antennae, Second, most have simple eyes, and Third, all chelicerates have a pair of appendages called chelicerae Chelicerae are the anterior-most appendages, modified into pincers or fangs for killing prey and in some species the chelicerae can inject poison. Most live in terrestrial environments. In order to successfully live on land, the organism had to evolve with ways to retain water in their bodies. Terrestrial chelicerates have three structures that aid in water retention: Malpighian tubules, an exoskeleton, and book lungs Malpighian tubules are excretory structures that remove metabolic wastes from blood and return water to the cells. Book lungs provide efficient gas exchange without water loss. They are called book lungs because they look like the inverted pages of a book. Horseshoe crabs are marine animals that belong to the small class Merostomata. There are only four living species of horseshoe crabs and three of these species live off the coast of Southeast Asia with one species of horseshoe crab that lives in the coastal regions of the western North Atlantic and the Gulf of Mexico. The horseshoe crab is not really a crab. Horseshoe crabs belong to an ancient class of marine arthropods that evolved about 400 million years ago. Scientists often refer to the four surviving species of horseshoe crabs as “living fossils” because they closely resemble their fossil ancestors. Horseshoe crabs have both compound and simple eyes and do not have Malpighian tubules. Horseshoe crabs exchange gases in five pairs of book gills which are similar to book lungs and may have evolved into book lungs found in terrestrial arachnids. Horseshoe crabs are among the few chelicerates that eat solid food by feeding on clam worms, soft- shelled clams, and other animals. Arachnids are terrestrial chelicerates that make up the much larger class Arachnida. There are three familiar orders of arachnids: spiders, scorpions, and mites and ticks. All arachnids have a cephalothorax with six pairs of appendages: one pair of chelicerae, one pair of specialized appendages called pedipalps, and four pairs of walking legs. Pedipalps are used for holding food and sense organs. The first order of arachnids includes spiders. QuickTime™ an d a Sorenson Video 3 decompre ssor are need ed to see this p icture . Spiders are divided into two groups based on how they gather food; individuals that hunt prey and individuals that weave webs to catch prey. Hunting spiders usually have strong legs and excellent eyesight. Web weavers generally have long, slender legs and relatively poor eyesight. Although not all spiders weave webs, they all produce silk. Some spiders use silk for movement-as droplines or parachutes. Other spiders use silk for protection-as a covering for their burrows or a canopy under which they sleep. Still other spiders use silk to wrap their food for storage. Spiders paralyze or kill their prey with chelicerae modified into fangs. While holding their prey with their pedipalps, spiders secrete enzymes that digest and liquefy the prey. They then suck the liquefied food into their stomach. QuickTime™ an d a Sorenson Video 3 decompre ssor are need ed to see this p icture . The second order of arachnids includes scorpions. Scorpions have a long segmented abdomen that ends in a poisonous stinger. A scorpion’s pedipalps are modified into large claws for capturing and holding prey. As it curls its abdomen over trapped prey, the scorpion stings and paralyzes it, then crushes it and sucks its internal liquids. Despite their reputation, only a few species of scorpions are harmful to humans. Quic kTime™ and a TI FF (Uncompres sed) dec ompress or are needed t o s ee t his pic t ure. Mites and ticks are classified in a third order of arachinds. Unlike spiders, mites have only one body segment. Some mites use their piercing and sucking mouthparts to drain the contents of plant cells. Other mite species eat bacteria that live in animal hair follicles. Some mites even feed on the dead skin cells of humans. Millions of dust mites live in our houses and are usually harmless but can cause problems for people with allergies. Most ticks are parasites that require meals of blood before they molt. Ticks attach to their hosts with specially adapted mouthparts and their feeding process can take several days, during which they may grow to many times their original size. Ticks differ from spiders in that they have only a single fused body section. Ticks can be dangerous because they can act as hosts to parasites that cause diseases such as Rocky Mountain spotted fever. A common dog tick is easy to see because it is as large as a sesame seed. The deer tick, however, which causes Lyme disease is very small and difficult to see. References Strauss, E. & Lisowski, M. (2000). Biology: The Web of Life, Second Edition. Scott Foresman-Addison Wesley: Glenview. Pictures http://www.richard-seaman.com/ http://www.daylife.com/story/0ca4d6W0kD5kB www.dkimages.com www.dustmitecontrol.com/ Dust-mite_5.jpg www.camden.rutgers.edu/.../ dads_dust_mite.jpg QuickTime™ an d a Sorenson Video 3 decompre ssor are need ed to see this p icture .
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