Spiders are NOT insects.
More Info: Spiders belong to a special class of organisms known as Arachnida. Like insects, Arachnidas belong to the phylum Arthropoda, which also include ticks, scorpions, centipedes, and other similar organisms. (“Introduction to the Arthropoda”, ucmp.berkeley.edu )
Similarities between Insects and Spiders
Belonging to the phylum Arthropoda, insects and spiders share similarities, which include:
- Segmented body.
- Paired segmented appendages.
- Bilateral symmetry.
- Chitnous exoskeleton.
- Tubular alimentary canal with mouth and anus.
- Open circulatory system, a tubular dorsal blood vessel.
- Body cavity or coelom.
- Nervous system of anterior ganglia and paired nerve cords.
- Striated muscles in skeletal system.
- Respiration by gills, tracheae, or spiracle.
“Phylum Arthropoda.” Department of Entymology . University of Minnesota, n.d. Web. 5 Mar. 2012. <www.entomology.umn.edu/cues/4015/handouts/ Orders.htm>
In addition to the characteristics that place spiders in the phylum Arthropoda, Arachnidas possess book lungs–so named because they look like the pages of a book, a cephalothorax (head attached to thorax) and abdomen, and fangs with venom glands or chelicerae–the first pair of fang-like appendages near the mouth. (“Systematics of the Arachnida.”, ucmp.berkeley.edu)
 “Introduction to the Arthropoda.” UCMP – University of California Museum of Paleontology. N.p., n.d. Web. 5 Mar. 2012. http://www.ucmp.berkeley.edu/arthropoda/arthropoda.htm
 “Systematics of the Arachnida.” UCMP – University of California Museum of Paleontology. N.p., n.d. Web. 5 Mar. 2012. <http://www.ucmp.berkeley.edu/arthropoda/ arachnidasy.html>
 “Phylum Arthropoda.” Department of Entymology . University of Minnesota, n.d. Web. 5 Mar. 2012. <www.entomology.umn.edu/cues/4015/handouts/ Orders.htm>