The character-state distribution of various external and internal pro- and mesothoracic structures of Hymenoptera is described. Structures discussed are the transscutal articulation, axillae, notauli, median mesoscutal sulcus, parapsidal lines, pronotal lobe, postspiracular sclerite, prepectus, netrion, epicnemium, basalare, pronotal-mesothor-acic attachment mechanism, and 2 mesothoracic muscles,viz.mesotrochanteral depressor and second-phragmal flexor. Ancestral state and transformation of the characters are hypothesized, and shared derived states of higher taxa are determined. Hypotheses of synapomorphy are based on the shared derived states, and these hypotheses used to test previous hypotheses of relationship and classification. Relationships of Symphyta to Apocrita and of Aculeata to Terebrantes are discussed, and various aspects of classification of Symphyta and Terebrantes are examined. Finally, prior use of terms is reviewed for the thoracic structures studied. A single set of terms is selected for use throughout the order based on original use of each term and on hypotheses of homology.