Several groups of spiders have evolved to look and behave like various species of ants. This is known as mimicry. There are also spiders that mimic other insects such as flies and wasps. Most spider mimics belong to a few families of Hunting Spiders, notably the Salticidae (Jumping Spiders), Corinnidae (Sun Spiders), Thomisidae (Flower Spiders) and Zodariidae (Spotted or Ant Spiders).
Ants are particularly good animals to mimic, firstly because they are very numerous and secondly because many animals find them distasteful or dangerous to eat. By looking like an ant, the spider mimic is less likely to be noticed or eaten.
A few spiders not only look like ants but smell like them as well. These spiders are such good mimics that they trick not only animals that eat ants but they trick the ants as well. By smelling like the ants, the spider is able to enter the ant nest unchallenged and steal their young.
Info taken from www.amonline.net.au
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