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PHYLUM ARTHROPODA This phylum contains four-fifths of all existing, described animal species. The number of individuals is enormous. Many feel that arthropods are the most successful animals. They are found everywhere. Arthropods have jointed appendages and a chitinous exoskeleton. The earliest known arthropods had one pair of appendages per body segment. Arthropods have an open circulatory system and an excretory system made-up of Malphigian tubules. Subphylum: Trilobita This now extinct taxon is known from an extensive fossil record from the Paleozoic era. Trilobites had a single pair of antennae and a body divided into three lobes by two longitudinal furrows (hence the name “trilobite”). Anteriorly the head region is termed the cephalon followed by the thorax and pygidium. Figure Subphylum: Chelicerata. Members of this group lack antennae and the first post-oral appendages are modified feeding structures or chelicerae. Posterior to the chelicerae are a pair of pedipalps. The body is divided into a cephalothorax equipped with four pair of appendages, and an abdomen without appendages. Class: Merostomata. These are the horseshoe crabs, strange, trilobite-like arthropods with a fossil record dating back 500,000,000 years (Cambrian). Today there are only 3 genera and 5 species. All in possession of a cephalothorax with compound eyes located laterally, an abdomen with gills (book gills), and a long, sharp telson. Figure Class: Arachnida. Include in this taxon are the spiders, ticks, scorpions and mites. The body is divided into a cephalothorax with four pairs of walking legs and an abdomen. Find their chelicerae. Arachnids are mostly terrestrial (some secondarily aquatic species exist) an respire by means of book lungs. Most are predatory and have evolved prey-capturing mechanisms such as webs, claws, stingers, fangs and, in some cases poison glands. They feed via sucking mouthparts on the fluids of their prey. Figure 1. Draw an arachnid 10 cm in size and label their anatomy. Subphylum: Mandibulata. Mandibulata possess a pair of mandibles (jaws), one or two pairs of antennae, and one pair or two pairs of maxillae. Taxonomy varies, however, we will adopt the following classification. Class: Crustacea. Included here are lobsters, crayfishes, shrimps and crabs. Most are marine with some freshwater forms and a few terrestrial species. Crustaceans have two antennae and a number of paired appendages on their heads. The head and thorax are combined into a cephalothorax covered by a dorsal carapace. Class: Branchiopoda. Members include marine and freshwater fairy shrimp, tadpole shrimp, clam shrimp and water fleas. They all carry leaf like, setose appendages. The water flea Daphnia is a common freshwater crustacean that is an example of a microscopic crustacean suitable for laboratory study. Enlarged second antennae aid movements through water. Five pairs of thoracic appendages, equipped with setae, remove food items from the water. These are collected in a median ventral groove and carried anteriorly to the mouth. Obtain a Daphnia from the culture and place a water flea in a depression slide, cover and observe the animal first through a dissection scope and then a light microscope. The body is laterally compressed without obvious segmentation. Notice the head, thorax and post abdomen (the abdomen is reduced). The thoracic and abdominal regions are encased in a chitinous, bivalved, carapace. The head is covered by an exoskeleton. Locate the two large compound eyes, a pair of small ocelli posterior to the the first and second antennae, and thoracic appendages used in feeding. A heart can be seen above the anterior part of the darkened digestive system. Figure 2. Draw a 10 cm, Daphnia and label the above-mentioned structures. Subclass: Copepoda. Copepods are minute aquatic crustaceans with elongated bodies and a forked tail. They have a median eye and elongated second antennae extending perpendicular to the body. The antennae are used in locomotion and in feeding. Rowing motion of the antennae cause the copepod to swim in a “jerky” fashion. Figure 3. Draw a copepod 10 cm in size and label their main structures. Subclass: Cirripedia This entirely marine group, known as barnacles, are sessile as adults, spending their lives residing within calcareous plates attached to rocks, pilings and other objects. They feed by means of feathery legs thrust out of their calcareous plates that scope-up planktonic life forms. Barnacles and copepods share similar larval stages. Subclass: Malacostraca. Shrimp, crab, lobsters, pill bugs and beach hoppers are included in this taxon, which contains three-quarters of all known species of crustaceans. Malacostracans have eight thoracic segments and six abdominal segments followed by a telson. Order: Decapoda. Members of this group have five pairs of walking legs and a trunk covered by a carapace. The anterior most pair of walking legs may be modified into claws and not necessarily utilized for walking. The freshwater crayfish, Cambarus sp., will be the focus of your study. External anatomy. The body of the crayfish is divided into a rigid anterior cephalothorax and a flexible posterior abdomen. Notice that the legs are paired and jointed. The thoracic covering is called the carapace that is pointed anteriorly to form the rostrum. Locate the transverse cephalic groove between the head and the thoracic portion of the cephalothorax. Examine the exoskeleton covering the abdomen and notice that it is divided into plates called sclerites The dorsal plate is the tergum while the ventral plate is called the sternum or sternites. In between is the largely inconspicuous pleuron. Next observe the appendages. The two pairs of antennae are separated into a pair of biramous medial pair and a much longer lateral pair. Notice the five pairs of walking legs. The first pair is modified into chelipeds (pincers) and is not used in locomotion. Abdominal appendages are called swimmerets or pleopods. The terminal pair of appendages are the uropods and the single, posteriomedial element is the telson. Determining sex in crayfish is relatively easy. Males have the first pair of swimmerets enlarged to form rigid copulatory organs. Pleopods are uniform in females. The sperm duct openings in the male are located at the base of the fifth walking legs, while in the female the oviduct openings are found at the base of the third walking legs. Another guide to sex (often unreliable) is the expanded tergum in females for holding the eggs and young. Figure Internal anatomy. Carefully remove the exoskeleton from the dorsal side of the body. Cut along the side of the cephalothorax about half way between the dorsal and the ventral sides of the body. Now cut from the posterior edge of the cephalothorax anteriorly to about ½ cm from the anterior edge. Then cut from left to right to expose the organs. Underlying tissues may cling to the exoskeleton so care must be used to prevent damage to various organs. The delicate, diamond-shaped heart is located just under the cephalothorax. Look closely and you may see the ostia (openings in the heart) and the anterior and posterior blood vessels exiting this organ. In the anterior part of the cephalothorax lie the esophagus and the cardiac stomach. Posteriorly the cardiac stomach connects to the pyloric stomach, which connects to the intestine passing posteriorly under the heart, ending as the anal opening in the telson. On each side of the cardiac stomach lie the digestive glands. Open the dorsal surface of the cardiac stomach and examine the gastric mill consisting of three grinding “teeth”. The excretory organs consist of the spherical green glands that lie ventral to each eye and anteriolateral to the esophagus. The green glands open to the outside through an excretory pore at the base of each antenna. Beneath the heart lie the reproductive organs or gonads. In the gravid female the ovaries are filled with orange masses of eggs. An oviduct leaves each ovary and exits at the base of the third walking leg. The white testes of the male are fused to form a three lobed sac. The conspicuous vas deferens lead to the fifth walking legs. The brain can be found between the green glands. From the brain two nerves go around the esophagus to connect posterior to the esophagus forming the circumesophageal commissure. To reach the ventral nerve cord remove the tergum and abdominal musculature. Notice that it is a paired ventral nerve cord with ganglia (a swollen bundle of nerves) at each segment. Find the lateral nerves exiting each ganglion. Class: Diplopoda Millipeds appear to have two pair of appendages per each segment, however each of the apparent segments are actually double segments. Often called “thousand leggers”, in reality millipeds have far fewer legs. Their head is small and carries a pair of antennae. Class: Chilopoda Centipeds (“hundred leggers”) have a single pair of appendages per segment. The legs of centipeds are longer than those of millipeds and the first trunk appendage is modified into a pair of venomous claws. The head carries a single pair of antennae. Class: Insecta This class is considered to be the most important arthropod group both in terms of the number of species and their economic impact on human welfare. Three-quarters of all known animals are in this class and two-thirds of all flowering plants are dependent upon insects for pollination. Pollinators include bees, wasps, butterflies, moths, beetles and flies. Mosquitoes, tsetse flies, fleas, lice and houseflies are all vectors in diseases affecting humans, domesticated animals and cultivated crops. Some attack the wooden frames of our homes. Overzealous use of pesticides (poisons) employed in controlling these pests has lead to the evolution of pesticides resistant insects and the accumulation hazardous environmental contaminates. The grasshopper is a good representative of this class showing typical insect characteristics and is found almost everywhere. Romalea microptera or “lubber grasshopper” from the Southern United States is the species selected for studying the external characteristics of an insect. Obtain a specimen and notice the hard exoskeleton composed of chitin. Using your knowledge of the crayfish, locate the tergum, sternum and pleura on your specimen. Notice the three major body divisions; head, thorax and abdomen. The head carries a pair of antennae and a pair of compound eyes. Three pairs of legs and two pairs of wings are found on the thorax. The anterior pair of wings are modified as protective wing covers while the posterior pair are employed in flight (flight wings). On the first abdominal segment hidden under the wings is an organ of hearing or tympanum. Respiratory openings or spiracles can be found on the lateral side of each segment. Reproductive structures can be seen on the terminal abdominal segment. Locate the mouth of the grasshopper. The labrum (upper lip) is attached to the dorsal clypeus. Below the labrum is the dark, harden mandibles. Maxillae lie posterior to the mandibles, and fusion of a second pair of maxillae form the labium. Figure 1. A frontal view of the grasshopper’s face 3 cm across. Label the above mentioned structures. Figure 2. After dissecting out the eye of the grasshopper and cleaning it, make a wet mount slide and examine with the light microscope. The individual lenses are termed facets. Draw the compound eye 3 cm across at 40X. The honeybee, Apis mellifera, was domesticated thousands of years ago and its biology is well understood. Using prepared slides accomplish the following: Figure 3. Draw the third leg 5 cm in length and label the structures as seen in the diagram. Figure 4. Draw the honeybee stinger 5 cm across. Label as directed. Study Questions 1. 2. 3. 4. 5. 6. 7. Identify the similarities between crustaceans and insects. On what body division are the legs and wings found in insects? Order Orthoptera includes grasshoppers and what other insects? What is the function of spiracles? Compare the exoskeleton of a grasshopper and a honeybee. How does a grasshopper hear? How does a grasshopper smell? There are approximately 29 orders of insects. These taxons are based on the structure of the wings and mouthparts, on the pattern of metamorphosis and on a number of other features. Zoology 5 students will be required to learn the following orders of insects. Order: Odontata (tooth) These are the predaceous flyers known as dragonflies and damselflies. The order is characterized by long, narrow, net-veined wings, large eyes and chewing mouthparts. Dragonflies have stouter bodies while damselflies are slender and delicate. Order: Isoptera (equal wing) Termites are social insects living in colonies of winged kings and queens, and wingless workers and soldiers. Often referred to as “white ants” (although not true ants), these little insects destroy wooden structures by feeding on cellulose. Order: Orthoptera (straight wing) Grasshoppers, katydids, crickets, cockroaches, preying mantis and walking sticks are members of this insect order. They have large heads with strong chewing mouthparts. Many have enlarged hind legs adapted for jumping. Winged forms have leather-like forewings protecting the more delicate membranous hind wings. Feeding habits include carnivores and herbivores. Order: Demaptera (skin wing) Earwigs are elongated insects with forceps-like cerci (tail). The first pair of wings are short and leathery and the second pair, if present, are membranous. They are scavengers with chewing mouthparts. The term “earwig” is derived from and old superstition that these insects enter people’s ears. This belief is without foundation. Order: Hemiptera (half wing) These are the “true” bugs (stinkbugs, water striders, milkweed bugs and assassin bugs), all with piercing mouthparts and a beak arising from the front portion of the head and extending ventroposteriorly. The membranous forewings are thickened distally and at rest overlap the abdomen, forming a triangular pattern on the back. The hind wings are entirely membranous. Order Homoptera (same wing) This taxon includes cicadas, leafhoppers, aphids and scale insects. Although related to the Hemiptera, these herbivorous insects have entirely membranous forewings. It is common for them to hold their wings in a tent-like position over the body. Equipped with piercing and sucking mouthparts, that damage many cultivated plants and are regarded as serious agricultural pests. Order: Diptera (two wings) True flies, horseflies, gnats, midges and mosquitoes are found in this order. The forewings are employed in flight, and the hind wings are reduced to knob-like, balancing organs called halteres. Mouthparts are utilized for piercing and sucking plant and animal fluids. Order: Lepidoptera (pretty wing) The large wings, soft bodies and appendages of butterflies and moths are covered with pigmented scales. Adult mouthparts are modified as a coiled proboscis used for sucking the nectar of flowers. Many adults feed little or not at all. Lepidoptera larvae (caterpillars) have chewing mouthparts that can prove destructive to numerous cultivated plants. Order: Coleoptera (sheath wing) This order includes beetles and weevils. A famous scientist once remarked that God must have had an inordinate fondness for beetles because 40% of all known animals are beetles. The body is very hard, as are the modified forewings or elytra that cover the membranous hind wings. In some species the elytra fuse making flight impossible. Food habits include dung eating; scavenge ring, active predation and plant eating. Order: Hymenoptera (membrane wing) Include in this order are the ants, bees, bumble bees and wasps; many species exhibit solitary, semi-social or social life styles. They have chewing mouthparts often modified for sucking or lapping. Winged and wingless species exists and many have reproductive structures modified as stingers at the end of their abdomens.