“In the United States there are a few rare spiders that mimic ants.  Here at Taperinha we find a good score of species of these spiders aping the various kinds of ants very closely; even the odd, spiny wood-ant, cryptocerus, furnishes a pattern, and there are spiders that mimic the wingless ichneumons.  We find, after a while, that the spiders prey upon ants just as our spiders catch flies; indeed, this fact has already been noted by other observers.  But we go a step beyond the books when we discover not only that the spiders eat the ants, but that they eat the particular ants which they mimic.  At all events, we verify this fact in a great number of cases, and we never find the spiders eating any but the mimicked species....  I do not like to hazard a theory on this case of mimicry.  It is difficult to suppose that the quick-witted ants would be deceived even by so close a resemblance; and, in any case, it would seem that the spiders do not require such a disguise in order to capture slow-moving ants.  Most birds will not eat ants; it seems likely, therefore, that this is simply another example of protection; the spider deceives its enemies, not its prey; it mimics the particular species that it feeds on, because it is seen in that company when it is hunting, and among a host of similar forms is likely to pass unnoticed.”

At first sight, and especially in view of the fact that such cases are not uncommon among insects, it would be naturally supposed that the object of the mimicry was to enable the spider to approach its victim without exciting suspicion; and it is difficult to account, on any other supposition, for the very close resemblance between certain species of spiders and the particular species of ants which they prey upon.  It seems as though the highest point of protective benefit would have been reached long before the resemblance of the spider to the ant had become so close as it really is.  On the other hand, it is difficult to believe that ants are deceived, even by those spiders which mimic them most closely, when we remember that their perceptions are so keen that they discriminate not only between ants of their own and different species, but even between ants of their own species living in two different communities.

The mimicry of ichneumon flies by spiders was noted some years ago by Mr. Herbert Smith.  This case comes under Class 3, in which one species mimics another which preys upon it.  Great destruction is caused by ichneumons which lay their eggs on the bodies of the live spiders, and the disguise probably protects the spider by leading the fly to mistake it for one of its own species.

We have no proof that spiders ever mimic ants as a method of escaping from them, but it is possible that this sometimes happens.  We know that some ants prey upon them.  The foraging ants of South America destroy spiders as well as many kinds of insects, and Wallace mentions a small, wood-boring ant which fills its nest with small spiders.