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46.1 The evolution of jointed appendages has made arthropods very successful.
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• Jointed appendages and an exoskeleton greatly expanded locomotive and manipulative capabilities for the arthropod phyla, the most successful of all animals in terms of numbers of individuals, species, and ecological diversification.
• Traditionally, arthropods have been grouped into three subphyla based on morphological characters but new research is calling this classification of the arthropods into question.
• Like annelids, arthropods have segmented bodies, but some of their segments have become fused into tagmata during the course of evolution. All possess a rigid external skeleton, or exoskeleton.
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1. What are the advantages of an exoskeleton? What occurs during ecdysis? What controls this process?
2. What type of circulatory system do arthropods have? Describe the direction of blood flow. What helps to maintain this one-way flow?
3. What are Malpighian tubules? How do they work? What other system are they connected to? How does this system process wastes? How does it regulate water loss?
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46.2 The chelicerates all have fangs or pincers.
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• Chelicerates consist of three classes: Arachnida (spiders, ticks, mites, and scorpions); Merostomata (horseshoe crabs); and Pycnogonida (sea spiders).
• Spiders, the best known arachnids, have a pair of
chelicerae, a pair of pedipalps, and four pairs of walking legs. Spiders
secrete digestive enzymes into their prey, then suck the contents out.
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4. Into what two groups are arthropods traditionally divided? Describe each group in terms of its mouthparts and appendages, and give several examples of each.
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46.3 Crustaceans have branched appendages.
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• Crustaceans
comprise some 35,000 species of crabs, shrimps, lobsters, barnacles, sowbugs,
beach fleas, and many other groups. Their appendages are basically biramous,
and their embryology is distinctive.
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5. On which parts of the body do crustaceans possess legs?
6. How do biramous and uniramous appendages differ?
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46.4 Insects are the most diverse of all animal groups.
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• Centipedes and millipedes are segmented uniramia. Centipedes are hunters with one pair of legs per segment, and millipedes are herbivores with two pairs of legs per segment.
• Insects have three body segments, three pairs of legs, and often one or two pairs of wings. Many have complex eyes and other specialized sensory structures.
• Insects exhibit either simple metamorphosis, moving
through a succession of forms relatively similar to the adult, or complete
metamorphosis, in which an often wormlike larva becomes a usually sedentary
pupa, and then an adult.
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7. How are millipedes and centipedes similar to each other? How do they differ?
8. What type of digestive system do most insects possess? What digestive adaptations occur in insects that feed on juices low in protein? Why?
9. What is an instar as it relates to insect metamorphosis? What are the two different kinds of metamorphosis in insects? How do they differ?
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