Zoology Exercise #13: Chelicerata Lab Guide Arthropods are diverse phylum that includes chelicerates (spiders, scorpions, mites, ticks), crustaceans, myriapods (millipedes & centipedes), and hexapods (insects). These animals have jointed appendages. Similar to annelids, their body is considered an extended series of repeated segments. Remember, this design is called metamerism. But, unlike annelids, this design has been, through fusion, reduced and specialized. Now we have distinct, functional units called tagmata. Ex. Insects have a head, thorax, and abdomen while spiders and ticks have a cephalothorax and abdomen. They also share, with the previous phylum, features such as they are triploblastic (made up of 3 distinct tissues), have a true coelom, have bilateral symmetry, cephalization, and all organ systems are accounted for. They have striated muscle for rapid movement, an exoskeleton or cuticle containing a tough polysaccharide called chitin for support and protection. Gills and a very efficient tracheal system for gas exchange and greater specialization of body organs. Especially the appendages. They now have a hemocoel filled with blood, called hemolymph, which circulates through their open circulatory system. Exercise 13A Chelicerate Arthropods (Horseshoe crab and Garden spider) The chelicerates include horseshoe crabs, sea spiders, spiders, scorpions, mites, and ticks. They do not possess mandibles (jaws) for chewing. Instead, their first pair of appendages, called chelicerae, are feeding appendages adapted for seizing and tearing. Most have a two part body which consists of a cephalothorax (prosoma) and abdomen (opisthosoma). There are no antennae. Horseshoe Crabs Horseshoe crabs are not really crabs. They are marine, bottom dwellers that feed on molluscs, worms, and dead fish. They live along the Atlantic coast from Nova Scotia to Mexico s Yucatan peninsula. Their eggs are a very important food source for migratory shore birds. External Features The entire body is covered with a tough, leathery exoskeleton that contains chitin, a tough polysaccharide. As a horseshoe crab grows, it must shed (molt) its exoskeleton through a process called ecdysis. The cephalothorax (prosoma) is covered by a hard, horseshoe shaped carapace which is concave below and convex above. A pair of lateral compound eyes and a pair of median simple eyes are on the dorsal side. On the ventral side are 6 pairs of appendages, located around the mouth. The first pair called chelicerae are small and used to detect and manipulate food. The second pair are called pedipalps. The next four pairs are walking legs. All appendages except the last pair of walking legs and the pedipalps of the male are chelate (means to bear pincers or chelae). All, except the chelicerae, have spiny masticatory structures called gnathobases on the basal segments. Imagine how these gnathobases would tear up food and move it toward the mouth. The chelae of the appendages pick up food and pass it to the gnathobases. Remember, the last pair of walking legs does not have chelae, instead each has four, movable, bladelike structures, one of which is tipped with a pair of spines. These legs are used to push against the sand to help in forward movement and in burrowing. Between the last pair of walking legs is a small pair of rudimentary appendages called chilaria. The abdomen (opisthosoma) bears six pair of spines along the sides and, on its ventral side, six pair of flat, plate like appendages. The first of these forms the genital operculum, on the underside of which are two genital pores. The other five abdominal appendages are modified as gills. Because of their arrangement, they are often called book gills. Exchange of gases between blood and water takes place in the lamellae. Movement of the gills not only circulates water over them but also pumps blood in and out of the lamellae. The blood contains hemocyanin, which in addition to carrying oxygen, also contains a powerful amebocyte to kill bacteria. Beating of the abdominal flaps can also be used in swimming and may aide the animal in burrowing by creating a water current that washes out mud and sand posteriorly. There is a long, slender telson or tail spine. This is used to anchor the animal when it is burrowing or plowing through the sand. Or, in righting itself when turned over. The anus is located under the proximal (center of the body) end of the telson.
These animals will congregate in shallow water to reproduce. A male will clasp a female s carapace with his larger, modified pedipalps and be carried around by the female while she lays eggs in a depression she creates in the sand. He sprays sperm on the eggs as she lays them. After several weeks, the eggs will hatch as trilobite larvae. This larvae looks like the adult, but lacks the tail spine and has only two of the five pairs of book gills. As it molts, it will add appendages until the young animal reaches adult form. Garden Spider Spiders are distributed in all kinds of habitats, such as forests, deserts, mountains, swamps, land and water. The garden spider will build its orb web in sunny places in gardens and tall grass. It is found throughout the United States. In most cases, the males will be about ¼ the size of the female. They are rarely seen. This spider will spin a symmetrical orb web and prefers to hang head downward in the center, holding its forelegs and hind legs close together. They will capture insects in its web net and bite the prey to paralyze it, securing it with a silken thread. Spiders secrete enzymes to begin digestion outside of the body. Most spiders have poor vision (except jumping spiders), but are covered with sensory hairs and are very sensitive to touch and vibration. External Features The chitinous exoskeleton is hard, thin, and somewhat flexible. Sensory hairs project from all parts of the body. The tagmata of the arachnid include the anterior cephalothorax and the posterior abdomen joined by a slender waist (pedicel). Its cephalothorax has 6-8 eyes on its anterior, dorsal surface. Spiders do not have compound eyes, all are simple ocelli. Look for the paired chelicerae, which are vertically oriented on the front of the face. The terminal segment of a chelicerae is a fang. This is how the spider injects poison from its poison glands. The pedipalps are six jointed and used for gripping prey.
In males, the pedipalp is modified as an organ for transferring sperm to the female. The base of the pedipalp is used to squeeze and chew food. There are 4 pairs of walking legs, each made up of 7 segments: coxa, trochanter, femur, patella, tibia, metatarsus, and tarsus. The tarsus has claws and a tuft of hair at its terminal end. The abdomen will have two lateral, slit like openings that mark the location of the book lungs. These book lungs are fashioned very much like the book gills of the horseshoe crab except that they are enclosed internally in pockets. The inner walls of these pockets are folded into long, thin plates (leaves of a book) and are held apart by bars, so that there is air spaces between them. Gas exchange occurs between blood circulating inside the lamellae and air flowing in the spaces between the lamellae. These air spaces connect with a small air chamber in each lung that opens to the outside through a slit like opening called a spiracle. Between the spiracles, the epigynum, which conceals the female s genital pore, is visible. Preserved specimens are most likely all female. Posteriorly on the abdomen, just in front of the spinnerets, is a small tracheal spiracle. This is an opening into a small chamber from which tracheal tubes extend into the body. These tracheal systems are similar to those of insects, but are less extensive. Garden spiders have both book lungs and tracheal systems, but some spiders will only have one type of respiratory organ. There are 3 pairs of spinnerets on a raised surface. The middle pair is small, but the other two pairs are rather large, conical, and readily movable. The end of the spinnerets have a variety of tiny silk spouts, each producing a particular kind of silk. The silk is secreted as a fluid by the silk glands and hardens with exposure to air. A small fleshy papilla just posterior to the spinnerets bears the anus.
Procedure Exercise 13A 1. Place a preserved horseshoe crab in a dissecting pan. SKETCH and make observations of the following parts: cephalothorax, abdomen, chelicerae, carapace, compound eye, simple eye, pedipalps, walking legs, chelae, gnathobases, chilaria, genital operculum, genital pore (if possible), book gills, telson, anus SKETCH FUNCTION OF EACH PART 2. Place a preserved garden spider in a dissecting pan. SKETCH and make observations of the following parts: sensory hairs, cephalothorax, abdomen, waist (pedicel), simple ocelli, chelicerae, fang, pedipalps, walking legs, coxa, trochanter, femur, patella, tibia, metatarsus, tarsus, spiracle, epigynum, tracheal spiracle, spinnerets, anus SKETCH FUNCTION OF EACH PART Analysis 1. Think carefully about the two animals you observed. What are characteristics would you use to support your claim that both animals should be classified as Arthropods? As both being Chelicerates. (PLEASE address both!)
2. The terms bilateral symmetry and cephalization were used in this lab. Explain each and then contrast these two characteristics with an animal you have already studied. 3. Fill in the following classification scheme for both the horseshoe crab and garden spider. Level of Classification Horseshoe Crab Garden Spider Kingdom Phylum Class Order Family Genus Species 4. Horseshoe crabs are often called living fossils. Why? 5. Horseshoe crabs are important for biomedical research. Explain. 6. Garden spiders are often referred to as orb spiders. Explain why. 7. Explain how web building occurs in most spiders. Be specific about the mechanics. There are many parts to a typical web, so there is a method to its madness. 8. Chelicerates, like spiders, will secrete enzymes to begin the digestion of their food. Explain how other members of this group, such as mites, ticks, and scorpions will capture and digest their prey. 9. Most spiders have poor vision. What do they use to compensate for this?
10. There are two dangerous spiders in the United States. The Brown Recluse and the Black Widow. Describe how you would recognize each and then discuss the type of venom each will produce as well as the symptoms if you are bitten.