The phylum Arthropoda (arthro-, jointed; -pod, foot) was recognized by early systematists as a discrete (monophyletic) group because of the distinctive nature of their jointed appendages. The primitive condition for arthropods is for each segment of the body to bear a single pair of multi-segmented appendages. With the increasingevolutionary specialization of different body regions (tagmosis), the legs have also become regionally differentiated. Arthropod legs have become specialized for, among other things, walking, jumping, feeding, carrying eggs, respiration (aquatic and terrestrial), sperm transfer, and swimming. In some groups (such as Crustacea) these specializations may even be found on the same individual! The terminologythat has developed around identifying homologous elements in arthropod legs is extremely complex. Arthropods show two basic leg configurations: uniramous and biramous. Uniramous appendages are present in insects (Fig. 5.1F), myriopods, chelicerates (Fig. 5.1E), trilobites
Figure 5.1 Comparison of various arthropod limb types (Brusca & Brusca, 1990).
(Fig. 5.1D) and some crustaceans (Fig.5.1C). In uniramous appendages the leg consists of a large, basal coxa, which gives rise to the elongate telopodite. While uniramous limbs may have additional branches, called epipodites, these invariably arise from the coxa. True biramous limbs are only present in the Crustacea (Fig. 5.1A). In biramous appendages the basal segment is again the coxa, but arising laterally from the trochanter is asecond ramus of the leg, the exopodite. To make matters worse, the biramous limbs of the Crustacea may also have epipodites (arising from the coxa), thus appearing multiramous.
Figure 5.2 Body postures in selected arthropods (Manton, 19--).
Trilobites (Fig. 5.1D) present an exceptionally difficult morphology which has been interpreted as either biramous or uniramous. We shall follow Snodgrass(1935) and Brusca & Brusca (1990) and consider the outer ramus of the trilobite limb an epipodite, thus interpreting their limbs as uniramous.
Examine the demonstration of an onychophoran. Note that the integument is entirely flexible and only the claws on the appendages are sclerotized, and that the ringed trunk is not segmented externally. The unsegmented appendages, lobopods, expand andcontract accordian-like when the onychophoran walks.
Examine the demonstration of a pleopod (abdominal appendage) of a crayfish, an example of a biramous appendage typical of Crustacea (Fig. 5.3). Note that there are two branches arising from the second podite: an inner branch (the telopodite) and an outer branch (an exopodite).
Obtain a live daddy-longlegs (Arachnida:Opiliones) and observe its locomotion. In what order are the legs moved? Where is the knee? How is the leg brought forwards and back (protraction and retraction) in relation to the body? How is the body held in relation to the legs? Can the animal move backwards?
Figure 5.3 Crayfish pleopods (Snodgrass, 1935).
Obtain a live millipede (Myriapoda: Diplopoda) and ask the same questions as youdid for the daddylonglegs (the knee will probably not be visible). Note how the waves of leg movement pass down the animal.
Obtain a preserved lubber grasshopper (Romalea micropteryx). Examine the legs and identify the following podites (using Fig. 5.4 for help):
coxa trochanter femur tibia tarsus pretarsus
Figure 5.4 Middle leg of a grasshopper (Snodgrass, 1935).
Move the frontand middle leg with your fingers. Observe how the podites move in relation to each other. At which joints do the following motions occur?
promotion-remotion of the coxa levation-depression of the leg abduction-adduction
Examine the tibia-femoral joint in detail. Locate the arthrodial membrane joining the podites and the articulations (the hinges upon which the tibia swings). Is this a...