Major Types of Arthropods Help

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.

Fig. 11.4 The amazing body of a spider.

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.

Fig. 11.5 Centipedes and millipedes: So many legs!

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.

Fig. 11.6 The grasshopper: An insect body plan.

Practice problems for these concepts can be found at: Arthropod Test