AbstractA unique set of morphological characters based on the hemipteran (sensu lato) thorax are used to define the pleuron of Psylloidea. New external and internal topographical descriptions of pleurites of all three thoracic segments are provided based on observations of specimens from various genera representing the current taxonomic arrangement of Psylloidea. Variations in propleurite morphology and the anteroventral angle of the episternum among taxa are clarified. The mesothoracic pleural sulcus is found to be a distinct groove formed by the deep fossa of the pleural apophysis and is not a secondary structure, as assigned by previous authors. A newly discovered internal apodeme of the metathoracic trochantin and a serially homologous structure in the mesothorax isolate the trochantinal territory of the second segment. The metathoracic pleural sulcus appears to be pressed against the ventral edge of the metepimeron, as previously described for the mesothorax of certain species of other insects having a strongly developed meron. Use of morphological interpretations and newly discovered apodemes to assess primary homology of structures for phylogenetic and taxonomic studies is discussed. A glossary of standardized morphological terms for thoracic structures of non‐heteropteran Hemiptera based on this study and other recent and former dissertations on the insect thorax is provided. J. Morphol. 252:276–290, 2002. © 2002 Wiley‐Liss, Inc.