Abstract The thorax of Mengenilla was examined using traditional morphological techniques and its features were documented in detail using scanning electron microscopy and computer-based 3D reconstructions. The results were compared to conditions found in other holometabolan insects. The implications for the systematic placement of Strepsiptera are discussed. The observations are interpreted in the light of the recently confirmed sistergroup relationship between Strepsiptera and Coleoptera (Coleopterida). The synapomorphies of the thorax of Strepsiptera and Coleoptera are partly related with posteromotorism (e.g., increased size of the metathorax), partly with a decreased intrathoracic flexibility (e.g., a fused pronotum and propleurum), and partly independent from these two character complexes (e.g., not connected profurca and propleuron). Strepsiptera are more derived than Coleoptera in some thoracic features (e.g., extremely enlarged metathorax) but have also preserved some plesiomorphic conditions (e.g., tegulae in both pterothoracic segments). All potential apomorphies of Mecopterida are missing in Strepsiptera. The last common ancestor of Coleopterida had already acquired posteromotorism but the wings were still largely unmodified. Several reductions in the mesothorax likely occurred independently.