Comparative Morphology of the Orthopteran Thorax with a Discussion of Phylogenetically Relevant Characters

Abstract

Orthoptera, or Saltatoria, represents the most diverse insect group among the lower neopterans or Polyneoptera. The present study provides a detailed comparative investigation of the skeletal and muscular thoracic morphology of 23 orthopteran species. For the first time, we investigate unstudied ensiferan key taxa including Gryllacrididae (raspy crickets), Stenopelmatidae (Jerusalem crickets), and Prophangalopsidae (hump-winged crickets). We identify novel thoracic characters that might represent apomorphies of Orthoptera: The connection of propleura and prosternum as pleurosternal bridge, the pterothoracic furcae that enclose the respective pleural arm from the ventral side, the mesofurca and mesospina that are situated in a single line along the sternacosta, and a stalked mesospina with a delimited dorsal plate. In particular, the morphology of the sternal apophyses turned out to show major differences between the two major subgroups of Orthoptera: Caelifera (short-horned grasshoppers) and Ensifera (long-horned grasshoppers). For example, the profurca bears a single arm in Caelifera and is branched in the majority of Ensifera. A number of thoracic muscles, like the Musculus mesofurca-propleuralis (IIspm9), could be identified and defined for Orthoptera, muscles that have never been described before to be present in other neopteran insects. Additionally, the obtained data set is used to reconstruct the orthopteran ground pattern of the thoracic skeletomuscular complex. Moreover, all characters potentially containing phylogenetic information are discussed and compiled in a morphological character matrix in order to be accessible for future phylogenetic studies.