Mimic Spotlight: The AntySpider (hurr hurr, puns)

Much like the Orchid Mantis I talked about in March, Myrmarachne melanotarsa uses mimicry both defensively and aggressively. This jumping spider (See Figure 1) is one of many jumping spider species which mimic ants, specifically the cocktail ant (Crematogaster sp.) in M. melanotarsa's case. 

Figure 1: Myrmarachne melanotarsa

Many jumping spiders (Salticidae) are afraid of ants, as they are often preyed on by the 6-legged fiends. Additionally ants are well-protected against salticids which might view ants as potential prey. For this reason it is common in ant-adverse jumping spiders, such as Menemerus sp. for the females to abandon their broods when in the presence of ants more often than when in the the presence of other salticids. Ants have trouble navigating the web-built nests of jumping spiders, and hence it is usually safe for mothers to leave their brood unattended for a short time as the risk of predation by ants is low.

Our ant-mimic uses this behaviour to its advantage, as it raids the abandoned nests of the aforementioned Menemerus sp. for eggs and newly-born young. Unlike the ants it mimics, M. melanotarsa has no difficulty entering the nest (it is a spider, after all). The figure below shows that M. melanotarsa is often mistaken for an ant by the females of  Menemerus sp - indicating a successful mimetic display by M. melanotarsa. 

Figure 2: Frequency of test spider (Menemerus sp.) females remaining inside nest with hatchlings or vacating nest when surrounded by groups of 20 individual ants (Crematogaster sp.), ant-like salticids (M. melanotarsa), non-ant-like salticids (Menemerus sp. and Pseudicius sp.), midges (Chaoborus sp.) and empty stimulus chambers (control). Black = outside nest; grey = inside nest.

In addition to the aggressive uses of its mimicry, M. melanotarsa also gains some protection from its ant disguise. As mentioned earlier, ants pose a serious threat to salticids - so jumping spiders tend to avoid ant encounters whenever possible. As a result, salticids which prey on other jumping spiders often mistake M. melanotarsa as an ant, and hence, our mimic avoids being eaten. Success. 

While Batesian mimicry relies on being seen by a potential predator, aggressive mimicry relies on blending in; not being seen. Other salticids, such as Cosmophasis bitaeniata, use this ant disguise to infiltrate ant nests and prey on the eggs. It is due to this that ant-mimicing spiders are generally regarded as being aggressive mimics, because the defensive aspect of their mimicry is a considered a secondary benefit.

- Nelson, X. J. and Jackson, R. R. 2009, ' Aggressive use of Batesian mimicry by an ant-like jumping spider', Biology Letters, vol. 5, pp. 755-757

- Figure 1: http://img3.rnkr-static.com/user_node_img/50007/1000134056/full/myrmarachne-melanotarsa-photo-u3.jpg, unknown photographer, retrieved 02/05/14
- Figure 2: Nelson, X. J. and Jackson, R. R. 2009, ' Aggressive use of Batesian mimicry by an ant-like jumping spider', Biology Letters, vol. 5, pp. 756