SearchinQ

http://www.ucmp.berkeley.edu/arthropoda/arthropoda.html
Features an arthropoda essay from the University of Califonia - Berkley Museum of Paleontology.

http://animaldiversity.ummz.umich.edu/site/accounts/information/Arthropoda.html
Details on crustaceans, insects, spiders, and relatives from the Animal Diversity pages at the University of Michigan.