NO. 2 THORACIC MECHANISM OF A GRASSHOPPER SNODGRASS 7 



they are correlated with the part the terga of these segments play in 

 the mechanism of flight. In the Apterygota the corresponding terga 

 are simple plates showing none of the special characters of the wing- 

 bearing plates of pterygote insects. 



The first important tergal modifications connected with the develop- 

 ment of the paranotal lobes into movable organs of flight pertain to 

 the ridges upon which the dorsal muscles of the mesothorax and 

 metathorax have their attachments. These ridges, which are the ante- 

 costa of the mesotergum, the antecosta of the metatergum, and the 

 antecosta of the first abdominal tergum, bear each a pair of apodemal 

 plates, var}^ing in size, that project into the body cavity to give in- 

 creased surfaces of attachment for the greatly enlarged dorsal muscles 

 (fig. 2 B, DMcl) which have become depressors of the wings. The 

 antecostal apodemes, primarily intersegmental, are the thoracic phrag- 

 mata {iPh, 2Ph, sPh). 



The lengthwise pull of the dorsal muscles on the phragmata de- 

 mands sclerotic continuity in the dorsum, since the function of these 

 muscles as depressors of the wings depends on their ability to produce 

 a dorsal curvature in the terga on the relaxation of the antagonistic 

 tergo-sternal muscles. To insure action by the dorsal muscles the 

 intersegmental membranes between the mesotergum and metatergum 

 and between the latter and the first abdominal tergum must be prac- 

 tically eliminated, and their suppression has been accomplished either 

 by a fusion of the succeeding terga, or by a forward extension of the 

 precostal lips of the terga into the territory of the membranes. In the 

 second case, the precostae become postnotal plates (fig. 2 B, PN2, 

 PN3), often of large size, lying behind the true tergal plates of the 

 mesothorax and metathorax {T2, T3), where they appear to be parts 

 of these segments, to which, in fact, they do belong since they lie 

 anterior to the antecostal sutures {ac^ ac) which are the primary 

 intersegmental lines. 



In those insects in which the fore wings are the principal organs of 

 flight, the second thoracic phragma becomes partially or wholly de- 

 tached from the metatergum, and both the phragma and the postnotal 

 plate establish a close association with the mesotergum, while the ex- 

 tremities of the postnotum commonly unite for security with the pos- 

 terior dorsal angles of the mesothoracic epimera. In those insects in 

 which the hind wings have taken on the chief function of flight, the 

 middle phragma always remains attached to the metatergum, and the 

 precosta is not enlarged. The third phragma may preserve its connec- 

 tion with the first abdominal tergum, as it does in the Orthoptera 

 (fig. 25, PN3), but in most cases it becomes more or less separated 



