Leg allometry in ants: Extreme long-leggedness in thermophilic species

The thermophilic ant genera Cataglyphis and Ocymyrmex share a variety of specialisations that enable them to engage in high-speed foraging at considerably higher temperatures than less heat-tolerant species. In the present account we test the hypothesis that thermophilic ants have longer legs than closely related species from more mesic habitats. By comparing large-sized, medium-sized, and small-sized species of Cataglyphis and Ocymyrmex with size-matched species of the closely related non-thermophilic genera Formica (Formicinae) and Messor (Myrmicinae), respectively, we show that the thermophilic species are equipped with considerably longer legs than their less heat-tolerant relatives. Hence phylogenetically, extreme long-leggedness has evolved at least twice in desert ants: in the Formicinae and the Myrmicinae. Functionally, this morphological trait is adaptive for a number of reasons. The long legs raise the body into cooler layers of air and enable higher running speeds, which increase convective cooling and reduce foraging time. These are important adaptations all the more as due to the low food density prevailing in desert habitats foraging Cataglyphis and Ocymyrmex ants have to cover large distances within their physically demanding foraging grounds.

► Thermophilic ants have significantly longer legs than non-thermophilic ants.
► Leg length increases with decreasing geographical latitude.
► Extreme long-leggedness is a highly adaptive trait in thermophilic desert ants.
► In thermophilic ants extreme long-leggedness has evolved at least twice.