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Phylum: Chelicerata (Arthropoda) Class: Arachnida Order: Araneae
Spiders are arthropods called arachnids. All arachnids have 4 pairs of walking legs and fangs (chelicerae) adapted for liquid feeding instead of jaws or other types of feeding structures. Mites and ticks, harvestmen (daddy-longlegs), and scorpions are all arachnids. Spiders are all members of the order Araneae and are distinguished from the other arachnids by two unique features:
- All spiders have finger-like appendages at the end
of their abdomen called spinnerets, which secrete silk during all
stages of life. Silk is a fibrous
protein that is light and elastic, but it is stronger than steel fibers
of the same thickness. Although not all spiders spin webs, all spiders
do use silk for many
purposes, including capturing prey, rearing young, moving about, and
making shelter (see Natural History, below). - All spiders use a unique form of mating, in which the male transfers sperm to the female using specially modified appendages near the mouth, called pedipalps. Because of this, it is easy to tell what sex a spider is: Female spiders have pedipalps which look like short legs, whereas male pedipalps look like "boxing gloves".
Spiders
reproduce sexually like most animals. Females lay eggs in batches, wrapping
the batches in silken egg sacs. In some species the female carries the
sac with her and in other species she merely deposits it in a safe place
and abandons it. Basilica spiders (photo at left) lay their eggs sacs
one under the other. The spiderlings hatch within the sac and then leave
the sac to begin feeding. Like other arthropods, spiders must shed their
old external skeleton (molt) in order to grow larger. Most spiders live
only one or two years. They will molt 4-7 times as juveniles, no longer
grow any larger after they mature into adults and so do not molt. However,
the tarantula-like spiders (Mygalomorphs) live many years longer than
most spiders and so must occasionally molt as adults to "renew" their
external skeleton and sensory hairs.
All spiders are predators, and the majority of them
are generalist predators (eating many different kinds of insects and other
small animals). Most rely on venomous bites to subdue their prey. Spiders
in the family Uloboridae do not have venom glands at all, but merely bite
their prey to subdue them. Also, some spiders have become specialized
for feeding on certain types of prey. For example, pirate spiders (family
Mimetidae) specialize on capturing other spiders, and other spiders are
kleptoparasites
,
living in the webs of and stealing food from other spiders. Many spiders
do not use a web at all but subdue prey by pouncing on them (like wolf
spiders, Lycosidae; or jumping spiders (Salticidae).
Crab spiders (Thomisidae), often hide in flowers and deliver a carefully placed fatal bite to bees, butterflies, and other would-be pollinators that often 2 times its size. The flower spider, Misumena vatia, will change the color of its large abdomen, over several days, to match the color of the flower. Run your mouse over the photo to see the difference.
The order Araneae (spiders) is the seventh most diverse
animal order, in terms of numbers of known species, after Coleoptera (beetles),
Hymenoptera (ants, wasps and bees), Lepidoptera (butterflies and moths),
Diptera (flies), Hemiptera (true bugs), and Acari (mites and ticks). It
is also the largest group of animals made exclusively of predators—all
spiders kill and eat other animals for food. 
About 34,000 species of spiders are known to science, belonging to about 105 families. About 10% of these species are known from America north of Mexico (3,500 species; Roth, 1993; Schaefer and Kosztarab 1991). Many spider species are not yet known to science, though, particularly in the tropics. Some arachnologists estimate that only 20% of spider species have been described, meaning that there may be as many as 170,000 spider species sharing our planet with us (Coddington and Levi, 1991).
Some of the most diverse spider families include the familiar hairy, big-eyed "jumping spiders" (Family Salticidae, 4,400 species worldwide); the small "sheet-web spiders" (Linyphiidae, 3,700 species); the "orb weaving spiders" (Araneidae, 2,600 species); the "cobweb spiders" (Theridiidae, 2,200 species); the non-web weaving "wolf spiders" (Lycosidae, 2,200 species); and the "crab spiders" (Thomisidae, 2,000 species; Coddington and Levi, 1991).
About 500 species of spiders have so far been found in Wisconsin (Jass, J., unpublished data; Kaspar, J., unpublished data), but we do not know exactly how many species live in the state. Certainly more remain to be found. As an example, consider the family Linyphiidae, sheet web spiders, which is the most species-rich spider family in Wisconsin. About 97 species of sheet-web spiders are known from Wisconsin, but 185 species are known from the region including Wisconsin and all adjacent states (Iowa, Illinois, Michigan, Minnesota, and the Canadian province of Ontario; data from Buckle et al. 1993). Many, but not all, of these other species undoubtedly also occur in Wisconsin. There are probably between 550-1100 kinds of spider that call Wisconsin home.
Spiders have much more to fear from us than we do from them (Skerl 1997). Many spider species are restricted to very particular habitat types, such as caves, old-growth forests, or fens, bogs or other wetland habitats. Spider species are threatened by loss of habitat caused by human development, resource extraction, and agricultural activities. They are also threatened by the effects of pollution, acid precipitation, and human-caused climate change on their habitats. A few large tarantulas are threatened by overcollecting for the pet trade. Three spider species are certainly going extinct each year, but we know so little about how many spiders there are, and where and when they can be found, that it is difficult to know which species are threatened and which may already be extinct. Four spider species are currently on the U. S. Endangered species list: Three cave spiders (two from Texas and another from Hawaii) and one aboveground species (from the Smoky Mountains of North Carolina and Tennessee).
Biodiversity Topics: Introduction . Plants . Animals . Mammals . Birds . Reptiles & Amphibians . Arthropods . Spiders . Insects
© 2001-2004 The Cofrin Center
for Biodiversity and the University of Wisconsin Green Bay,
All Rights Reserved
Last
updated on
October 27, 2009