S Spiny Ants (Polyrhachis) Simon Robson School of Life & Environmental Sciences, The University of Sydney, Sydney, NSW, Australia Diversity, Distribution, and Evolutionary History With over 700 described species divided into 13 subgenera and numerous species groups, the formicine ant genus Polyrhachis represents one of the most morphologically and behaviorally diverse genus of ants. These medium-sized ants are often recognizable by the presence of spines, their colorful integument, the lack of a meta pleu- ral gland, and the enlarged first segment of the gaster, which can be as long as ½ to ¾ of the length of the gaster as a whole. Restricted to Old-World biogeographic regions, the genus achieves its greatest diversity in the Indo- Malayan, Oriental and Australasian regions [2]. Molecular evidence suggests the group orig- inated in SE Asia approx. 58 million years ago, dispersing once to Africa and several times to Australia [8]. With the exception of the polyphyletic subge- nus Myrmhopla (a recognized “waste basket group” [2]), each of the remaining 12 subgenera are morphologically distinct and easily recogniz- able, a consistency at the subgeneric level that is closely matched by their related ecology and nesting habits. Morphological Diversity Petiolar spines are present in the workers of most species, accounting for the genus name (Greek: poly ¼ many, rhachis ¼ ridge or spine). The position and size of the spines, however, varies widely within the genus. Most Polyrhachis pos- sess spines on either part or all of the mesosoma (on either the first or last segments: the pronotum and propodeum) as well as the gaster. Within the Cyrtomyrma subgenus, spines are predominantly found on the last segment of the mesosoma (the propodeum) and the gaster, while Chariomyrma always possess spines on the first and last seg- ments of the mesosoma (pronotum and pro- podeum) as well as the gaster. The subgenus Polyrhachis is unique in also possessing meso- notal spines, as well as petiolar spines of such extraordinary length that can be as long as the entire gaster of the ant itself (Fig. 1). The color of the Polyrhachis integument is highly variable. Many are covered with a metallic gold or silver pubescence but there are spectacular examples of gold, silver, black, red, green, and even blue workers (Fig. 2). The diversity of worker coloration is expanded even further with variation in the actual region of the worker body that is colored. Even for species with similar col- oration, the actual region of the body color can © Springer Nature Switzerland AG 2020 C. Starr (ed.), Encyclopedia of Social Insects, https://doi.org/10.1007/978-3-319-90306-4_115-1