Chelicerates lack antennae and typically have six
pairs of appendages: the chelicerae (pointed appendages used to
grasp food); the pedipalps (or leglike
appendages in horseshoe crabs), which have lobes that serve as jaws;
and six pairs of legs. Usually the body has two divisions: the
prosoma (or cephalothorax) which bears the
chelicerae and anterior portion ("head") of the animal, and the
opisthosoma (or abdomen).
Arachnids have four pairs of legs, and two pairs of additional
appendages, the chelicerae and the pedipalps (used for sensing and
locomotion). There are 11 recognized orders, and are by far the
largest and most important class of chelicerates, with over 65,000
described species (8,000 in North America).
Spiders are divided into two suborders: the Mesothelae and
Opisthothelae. The Mesothelae are restricted to Japan and Southeast
Asia, and retain features of primitive spiders (the order can be
traced back to the Devonian) such as segmentation of the abdomen and
ventral spinnerets. The suborder Opisthothelae is broken into two
infraorders, the Mygalomorphae (large spiders such as tarantulas)
- Family Theridiidae - Cobweb Spiders: These spiders have
abandoned the orb web in favor of a mesh sheet or
three-dimensional cobweb, although some have adopted a
kleptoparasitic lifestyle. They are usually eight-eyed (rarely
six-eyed or blind) with a high clypeus (sclerite on lower part
of face: think "upper lip area"); they may have a
stridulatory organ above the pedicel between the carapace and
abdomen. Theridiids may have a comb of serrated setae on the
fourth tarsus. The tarsal comb is used to wrap prey in silk
before envenomation. They rarely have teeth on the retrolateral
margin of the chelicerae; teeth on the promargin occur, but are
also rare. The paracymbium, if present, is fused to the
retrodistal part of the cymbium; the palpal tibia never has
apophyses. The colulus is reduced or absent in some theridiids.
This family includes the widow spiders (Latrodectus),
known for their potent venom. Also represented among the 250 or
so North American theridiid species are social spiders (Anelosimus)
and kleptoparasites (Argyrodes).
- Family Araneidae - This is a diverse family, nearly all of which
construct an orb web. Included in this group are the distinctive
day-active spiny orb-weavers of the genera Gasteracantha
and Micrathena, the garden spiders of the genus Argiope,
and the Bolas spiders (Mastophora). Bolas spiders have
abandoned the orb web; they use aggressive chemical mimicry to lure
male moths, which they disable by striking them with a globule of
sticky silk swung on the end of a line. There are 163 araneid
spiders widely distributed in North America.
- Family Thomisidae - These somewhat flattened spiders lack
scopulae and claw tufts. The lateral eyes are often larger than
medians and positioned on tubercles. The chelicerae are almost
always without teeth, or have both margins toothed. Legs I and II
are longer and thicker than legs III and IV. One common species in
this group is the goldenrod spider (Misumena vatia), which
is white or yellow with a red band on either side of the abdomen.
This spider is a sit-and-wait predator, often occupying a flower and
attacking pollinators. This spider can change color, over a period
of a few days, depending on the color of the flower. There are over
140 species of thomisids in North America.
- Family Salticidae - Jumping Spiders: These stout-bodied,
short-legged spiders have a distinctive eye pattern, with the
anterior median eyes by far the largest. The body is rather hairy
and is often brightly colored or iridescent. Some species are
antlike in appearance. Jumping spiders forage for prey in the
daytime, approach their prey slowly, and then suddenly leap onto it.
They can jump many times their own body length. Before jumping, they
attach a silk thread dragline, which they use to climb back to their
starting location. Salticids are the world's most diverse spider
family, with over 330 representatives in North America.
- Family Theraphosidae - Tarantulas: These distinctive,
conspicuously hairy two-clawed spiders are among the largest U.S.
species (8-34 mm). They build open burrows, although these may be
closed during the day by a thin sheet web or plugged with soil
during the winter. Despite their reputation, the venom of North
American tarantulas is not dangerous. However, they can produce
clouds of urticating hairs by rubbing their abdomen with their hind
legs. These hairs can irritate the mucous membranes of mammals, such
as the eyes and respiratory passages. There are 57 species in the
Order Ixodida (Acari)
Ticks and mites (Acari) constitute a very large group of animals,
with more than 30,000 described species. The body is usually oval,
with little distinction between the two regions. There are both
aquatic and terrestrial forms, and many are parasitic on both
vertebrates and invertebrates. Many free-living forms are
predaceous, and some of these prey on undesirable arthropods. Many
are also scavengers and plant feeders.
Ticks (Ixodida) consist of two families in North America, and are
parasitic. Those attacking humans are annoying pests, and some
species serve as disease vectors.
- Family Ixodidae - These ticks are the hard ticks that attack
dogs, deer, cattle, other livestock, and humans. They may serve
as vectors for Rocky Mountain spotted fever, Lyme disease,
relapsing fever, tularemia, Texas cattle fever, and Colorado