ARTHROPOD TAXONOMY
In terms of sheer numbers, the arthropods represent the most successful of the animal phyla. More than 1,000,000 species of arthropods have now been described. Through the many photographs in the Buschbaum text, you get a sense of the diverse habitats in which members of this phylum exist. In addition to the many kinds of terrestrial and aquatic arthropod species, many types of insects have evolved the capability of flight. (No other invertebrate possesses the ability to fly!)
The combination of physical characteristics that distinguish arthropods are:
Most of our focus will be on the living arthropod classes. Keep in mind the characteristics shared by all arthropods while you examine the modifications that distinguish the various subphyla and classes. The appendages vary the most between classes. Do take note, however, that though they may look and function in different ways, all the appendages are homologous across all the arthropod groups. The appendages of a single individual are referred to as serially homologous. Read Buschbaum pp. 323-324 for a discussion of this topic.
As the name implies, the first pair of appendages of the two classes within Subphylum Chelicerata are modified into pincerlike mouthparts called chelicerae. They have no antennae. The head and thorax are fused into a single cephalothorax bearing all 6 pairs of appendages. Within the Subphylum Chelicerata, members of Class Arachnida include the spiders, scorpions and ticks. The second pair of arachnid appendages are called pedipalps, variously modified for grasping, sperm transfer and sense perception. The horseshoe crab, representing Class Merostomata, has been around for about 200 million years, looking much the same as it does today. Notice the similarity of the larval horseshoe crab on demonstration and the extinct trilobite. This similarity is indicative of the close phylogenetic relationship of these two groups.
Members of Subphylum Crustacea all have appendages that are biramous (two-branched). The first three pairs of appendages are modified to form two pair of antennae and a pair of mandibles (chewing mouthparts). There are several other pairs of appendages that function as accessory mouthparts, as well as several that are modified for walking, swimming and reproduction. There are several classes within Subphylum Crustacea, but we will mostly limit our study to Class Crustacea. This group includes the crayfish and lobster, shrimp, crab, and isopods, all on display in lab. Use the information on lobster in Buchsbaum in your study of the crayfish, as the structures of these two organisms are very similar.
Subphylum Uniramia includes Class Insecta (bees, wasps, beetles, flies, fleas, butterflies, moths, etc.), Class Diplopoda (millipedes) and Class Chilopoda (centipedes). All of these have uniramous (unbranched) appendages, including one pair of antennae and a pair of mandibles.
KINGDOM ANIMALIA: Phylum Arthropoda:Subphylum Crustacea
Class Crustacea (mostly aquatic, few terrestrial)
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