One often sees medium to large sized ants moving in trails on a plant or small tree in the forest or even in one’s own garden. Take a closer look, and one may find an odd one out in this busy colony - a spider that appears to be mingling undercover with the ants.
These spiders, taxonomically belonging to the genus Myrmarachne (literally meaning “ant spider”), under the large jumping spider family Salticidae, are found throughout the world’s tropics, and have evolved to perform a near-perfect imitation of the ants whose habitat they share. It is commonly thought that the spider does this to get closer to and prey on the ants, but though this is true for very few species, most of them do it for quite the opposite purpose - theoretically called “Batesian mimicry” - in which a harmless or weaker species (the spider in this case) imitates the more dangerous one (the ants). They do this to escape predation, spiders being on the menu for birds and other larger animals, and even for a few spider-eating spiders like Portia fimbriata. These predators relish spiders, but wouldn’t dare take on an army of ants armed with jaws, formic acid and of course favoured by their sheer numbers.
The ant-mimicking spiders have a few structural features unique among spiders, which aid their mimicry:
- The cephalothorax (the front half, round or oval in most spiders) has a distinct constriction to resemble the separate head and thorax of ants.
- The pedicel (stalk) connecting the cephalothorax to the abdomen is long and conspicuous (it is externally invisible in most other spiders)
- The chelicerae (claws) are extraordinarily long and straight, often with bulbous ends to mimic the ant’s head.
- Unlike in most other spiders, the male here is marginally larger than the female.
- Even the gait and other habits of these spiders closely resemble those of the ants they mimic. They often use the first of their four pairs of legs in a manner similar to the antennae of insects (though functionally these are still walking legs)
There are a few ways to tell the two apart:
- The spiders make short walks with intermittent stops. An ant would typically (though not always) walk continuously without pausing too often.
- The spider’s 4 pairs of walking legs, all of equal size, is one of the easiest ways to tell them apart from ants, which have 3 pairs of walking legs; one front pair is modified into much smaller antennae.
- A closer look reveals the typical eight spider eyes, including a group of four large ones in the front.
These spiders generally range 6-10mm in length (the male reaching the upper limit of the range). The females are more or less similar to the males in external appearance, but have regular sized chelicerae instead of the elongated ones. They occur on shrubs, small trees and sometimes on walls. The web is a dense oval silken retreat built under leaves of the plant or in the angles of walls. 24 species have been scientifically described from India; the two most common ones have been pictured in the following posts.
[Taxonomic and anatomical info from Spiders of India by PA Sebastian & KV Peter]
Myrmarachne plataleoides. This one commonly mimics the Asian Weaver Ant Oecophylla smaragdina. M. plataleoides- Karwar, Karnataka; August '11; Host plant- Mango Mangifera indica
