NO. I INSECT THORAX — SNODGRASS 5 1 



of flight. If flight devolves principally upon the hind wings, as it does 

 in the Orthoptera, Euplexoptera, and Coleoptera, the second phragma, 

 is attached to the anterior part of the metatergiim, and the third 

 phragma, though it may not be separated from the first abdominal 

 tergum, becomes functionally a part of the metatergum through the 

 forward extension of its precosta as a postnotal plate of the meta- 

 thorax. Where the fore wings are the most highly developed for 

 flight, as they are in the majority of insects, the second phragma 

 goes to the mesotergum. In other words, the segment containing the 

 largest wing muscles has a phragma at each end, an arrangement 

 which concentrates the force of the muscles upon the tergum of this 

 one segment. 



The postnotal plates are probably contemporaneous in their origin 

 with the acquisition of motion by the wings. They are well developed 

 in the oldest known fossil insects ; in the Palaeodictyoptera there is 

 a large postnotum liehind each of the wing-bearing terga (fig. 19 B, 

 PN2, PN3). The statement by Handlirsch (1908) that there are 

 eleven abdominal segments in this group appears to be the result of 

 counting the postnotum of the metathorax as the first abdominal ter- 

 gum. In the Isoptera postnotal plates are lacking, but this condition 

 is here correlated with the degeneration of the dorsal thoracic muscles 

 and the weak nature of the termite flight. Postnotal plates are present 

 in the Ephemerida and Odonata, though in the latter also the dorsal 

 muscles have been lost. 



It is not difficult to see a reason for the development of postnotal 

 plates in wing-bearing segments. Where the primitive tergal plates 

 are separated by membranous areas of the posterior parts of the seg- 

 ments (fig. 2 B, 23 B, Mb), a contraction of the dorsal muscles causes 

 an overlapping of the terga (fig. 2 C) , a result which must be counter- 

 acted if the muscles are to produce motion in the wings. The inter- 

 segmental membranes, therefore, must be reduced or obliterated, and 

 their suppression has been accomplished in most cases by a forward 

 extension of the precostal margins (fig. 23 C, D) until the resulting 

 postnotal plate and the tergum in each segment form an uninterrupted 

 arch between consecutive phragmata. The force of the muscle con- 

 traction is now expended against the tergal arch, with the result that 

 its upward flexion gives a down-stroke to the wings. Thus, also, the 

 longitudinal dorsal muscles become antagonists to the tergo-sternal 

 muscles, if the latter muscles previously existed, or they call for the 

 development of such muscles, which then become elevators of the 

 wings. 



