The mesosomal skeletomuscular system of workers of Myrmecia nigrocincta was examined. A broad spectrum of methods was used, including micro-computed tomography combined with computer-based 3D reconstruction. An optimized combination of advanced techniques not only accelerates the acquisition of high quality anatomical data, but also facilitates a very detailed documentation and visualization. This includes fine surface details, complex configurations of sclerites, and also internal soft parts, for instance muscles with their precise insertion sites. Myrmeciinae have arguably retained a number of plesiomorphic mesosomal features, even though recent molecular phylogenies do not place them close to the root of ants. Our mapping analyses based on previous morphological studies and recent phylogenies revealed few mesosomal apomorphies linking formicid subgroups. Only five apomorphies were retrieved for the family, and interestingly three of them are missing in Myrmeciinae. Nevertheless, it is apparent that profound mesosomal transformations took place in the early evolution of ants, especially in the flightless workers. The modified mesosoma is characterized by four character complexes: a) an enlarged prothorax with elongate procoxae, a large plate-like pronotum, strongly developed muscles of the forelegs and especially of the neck region; b) highly differentiated legs with complex cleaning and attachment devices; c) a reduced flight apparatus with greatly simplified pterothoracic musculature and mechanically reinforced exoskeleton and d) strongly developed specialized muscles inserted on the base of the metasoma. Structural modifications of the prothorax and neck region allow ant workers to transport items efficiently with a highly movable head with strongly developed cervical muscles. Their differentiated legs enable them to move efficiently on various surfaces and to maintain their complex apparatus of sensilla. The mechanically reinforced mesosoma provides protection against predators and likely against detrimental environmental agents. The enhanced movability of the metasoma increases the defensive capacity with a sting or other mechanisms.

Myrmecia, Formicidae, ants, mesosoma, phylogeny, evolution.