Major Types of Arthropods Help (page 2)
Introduction to Major Types of Arthropods
There are several major types or categories of arthropods. The first question a person might ask, however, is: “The arthropods are all invertebrates. So, why weren’t they included within Chapter 10, which described the invertebrates?” [ Study suggestion: Take a careful look at Figure 11.2. What particular feature really sticks out at you? How do you think this feature explains why arthropods are placed in a separate chapter of their own?]
Using a traditional approach to taxonomy, there is a single phylum Arthropoda (ar- THRAH -pah-da), subdivided into five smaller classes. These are the Crustaceans (krus- TAY -shuns), Arachnids (ah- RACK -nids), Chilopods ( KY -luh-pods), Diplopods ( DIP -luh-pods), and Insects. In Figure 11.2, the relative size of each oval shape representing these arthropod classes visually pictures the number of known species belonging to each one.
Ancient Trilobites And The Modern Crustaceans
Let us begin our examination of the major arthropod classes with the crustaceans – invertebrates whose bodies are surrounded by hard “shells” (crustace). In the modern world, the crustaceans include crabs, lobsters, crayfish, and shrimp. Besides their hard shells (rigid exoskeletons), these crustaceans have jointed bodies and legs, as well as gills that allow them to exchange their hemolymph gases with the external gases in the surrounding water.
There are now about 40,000 known species of crustaceans, most of them living within either freshwater or saltwater environments. Yet, biologists have repeatedly observed an early ancestor (long extinct) of the modern crustaceans in the Fossil Record. This extinct group of arthropods is the trilobites ( TRY -loh-bites). The trilobites consisted of about 4,000 species, and they were very abundant in the sea during the Paleozoic Era (Chapter 3). The trilobites had bodies comprised of “three” (tri-) vertical “lobes” (lob) subdivided into many horizontal segments (Figure 11.3). Sprouting from either side of the body segments were numerous jointed legs. But unlike the modern lobster and other crustacea, these ancient marine invertebrates had appendages running along either side of their body that showed very little difference or specialization. For instance, they lacked the protective pincers of present-day crabs and lobsters.
The trilobites rather suddenly became extinct about 250 million years ago, near the end of the Paleozoic Era (shortly before the dinosaurs appeared during the Age of Reptiles). Because they never reappear at later stages in the Fossil Record, the limited time period of trilobite existence strongly supports the concept that once a group of animals becomes extinct, they generally stay extinct! There is a definite Biological Order or recognizable sequence of body forms and patterns that seem to precede or follow one another during the long time-scale of evolution.
The Arachnids: Spiders And Their Relatives
The arachnids are literally “spiders or webs” (arachn). Besides the spiders, however, the arachnids include scorpions, ticks, and mites as well. The arachnid group includes about 57,000 species. The basic anatomy of the arachnids is displayed within Figure 11.4. The spider body has two segments – head and thorax (cephalothorax). Situated farthest in the front of the head are the pedipalps ( PED -uh-palps) and chelicera (kuh- LIS -er-ah). The pedipalps are a pair of “feeling” (palp) “feet” (pedi) – small foot-like appendages that help sense when prey is present. The chelicera or “claws” (cheli) then come into play. The two chelicera serve much like fangs, injecting poison from a gland just below the four eyes. The chelicera inject digestive juices, as well as poison, into captured prey, which softens the attacked body as well as killing it. The spider finally sucks the liquefied food into its digestive system.
Along each side of the cephalothorax, spiders have four pairs of walking legs. Other special characteristics include a stack of book lungs , as well as a silk gland with spinnerets ( SPIN -uh-rets) – small organs used for spinning webs.
Centipedes And Millipedes: “so Many Legs!”
The Chilopod class literally consists of arthropods with “lip” ( chil ) “feet” ( pod ), obviously named for their appearance. There are approximately 2,000 species in this group. The main Chilopods are the centipedes ( SEN -tuh-peeds) or organisms with a “hundred” ( centi -) “feet” ( ped ). Centipedes are thin, worm-like arthropods with segmented bodies having many pairs of legs (but certainly not 100!). The centipede has very long antennae and poison claws that catch smaller insects to eat (Figure 11.5, A).
The Diplopod class includes those creatures with “double” ( dipl ) “feet” ( pod ). About 7,000 species of Diplopods exist. True to their Class name, each body segment bears two legs with feet. The primary Diplopods are the millipedes ( MIH -luh- peeds ) – translated to mean arthropods with a “thousand” ( milli -) “feet.” The millipede body, of course, has far fewer than 1,000 feet (Figure 11.5, B)! Millipedes feed on moss, decaying leaves, and a variety of other types of plant matter.
Insects: The Swarming Herd
Of all the arthropods, the insects represent by far the hugest number of species – at least one million known (and maybe another million or so insect species still undiscovered, mainly within tropical forests)! This means that there are more species of insects than species of all other types of animals, combined!
Yet, despite their overwhelming numbers and diversity, insects share a common body plan (Figure 11.6). The word insect actually comes from the Latin for “cut” (sect) “into” (in-). The reason for this name is that the body of most insects is “cut” (subdivided) into three major segments. Using the example of a grasshopper, there is a head, a thorax, and an abdomen. The insect head bears two antennae and a pair of compound eyes. In their adult stages, many insects have three pairs of jointed legs and either one or two pairs of wings, all attached to the thorax.
Practice problems for these concepts can be found at: Arthropod Test