Thorax

An outstanding feature of the Diptera is that the mesothorax is greatly enlarged to accommodate the muscles for the single pair of wings, whereas both the prothorax and metathorax are very reduced. The prefixes pro-, meso-, and meta- are usually attached to the different sclerites of the corresponding thoracic segments, e.g. prosternum, mesonotum, and metepimeron, but because of the predominance of the mesothorax, the prefix meso- is frequently omitted when referring to the sclerites of that segment. These prefixes are not to be confused with ante-, pre-, and post-, which are used to define particular parts of each sclerite, e.g. antepronotum, prescutellum, and postpronotum.
Basically each thoracic segment of an insect has a dorsal tergal plate, the notum; a lateral plate, the pleuron; and a ventral plate, the sternum. Generally, the notum can be more or less divided into the prescutum, scutum, and scutellum; in the Diptera, however, this division of the notum is usually made only for the mesonotum. The lateral pleural sclerites are considered to have been derived from subcoxal elements of ancestral legs, which became incorporated into the body wall. They consist of an anterior episternum and a posterior epimeron separated by a pleural suture which runs between the leg base and the wing base and serves to strengthen the pleuron. The episternum may be divided into an upper anepisternum and a lower katepisternum; the epimeron may similarly be separated into the anepimeron and the katepimeron. These general terms can all be modified by the suffixes pro-, meso-, and meta- to designate the particular segment of the thorax with which they are associated. However, as indicated above, when referring to the sclerites of the mesothorax, all these terms are usually used unmodified except for the term mesonotum. The principal thoracic landmarks in the Diptera are the attachments of the head, abdomen, legs, wings, and halteres, and the anterior (mesothoracic) and posterior (metathoracic) thoracic spiracles. In the Diptera, as in all insects, true prothoracic spiracles are absent, but the mesothoracic spiracles frequently migrate forward and hence are often incorrectly called the prothoracic spiracles (ex McAlpine 1981).