This large Class of arthropods includes over 60,000 described species (and most likely a very large number of so-far undescribed ones). Spiders make up the majority of these (over 50,000 described species); with mites and ticks next largest (around 48,200 species). The Arachnida also includes a diverse array of smaller groups, including scorpions (1200 species), whip scorpions (100 species), palpigrades (60 species), pseudoscorpions (2000 species), solpugids (900 species), and harvestmen (5000 species). Nearly all species are terrestrial.

 

Arachnids have a pair of tagmata called a prosoma and opisthosoma. The prosoma is partially or completely covered with a carapace-like shield. The opisthosoma may be segmented or unsegmented. The appendages on the opithosoma are absent or modified, being used as spinnerets (spiders) or pectines (probably sensory in function, found in scorpions). Respiration is via tracheae or book lungs; it is cutaneous in many small arachnids.

 

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Sources:

 

 
• Hickman, C.P. and L. S. Roberts. 1994. Animal Diversity. Wm. C. Brown, Dubuque, IA.
 
• Brusca, R. C., and G. J. Brusca. Invertebrates. 1990. Sinauer Associates, Sunderland, MA.
 
• Pearse, V., J. Pearse, M. Buchsbaum, and R. Buchsbaum. 1987. Living Invertebrates. Blackwell Scientific Publications, Palo Alto, Ca.