This illustration shows the growth stages of the “American dog tick”, Dermacentor variabilis, from eggs to adult insects. Ticks belong to the class Arachnida, as do spiders and mites. The wood tick D. variabilis is a known carrier of Rocky Mountain spotted fever caused by the bacterium Rickettsia rickettsii, which it can transmit to humans through its bite.

Picture & text from CDC/PHIL. For more information on source & for information on color coding used above for different types of ticks, see Tick Pictures from CDC