NO. 2 THORACIC MECHANISM OF A GRASSHOPPER — SNODGRASS <)J 



In the fully exended wing of Dissosteira the axillary sclerites lie 

 approximately flat and in the same plane as the general wing surface 

 (fig. 47 A, B). When the wing is turned posteriorly, however, the 

 axillaries take different positions. In the living grasshopper it is 

 probable that the first movement of flexion is produced by the elas- 

 ticity of the wing base when the extensor muscles are relaxed, for the 

 wing of a dead specimen automatically assumes a partly flexed position. 

 The fully flexed and folded condition, however, undoubtedly depends 

 on the pull of the flexor muscle (fig. 49, D) on the third axillary. 



On the relaxation of the wing, the initial flexing causes the outer 

 end of the third axillary to turn upward, and the pull of the flexor 

 muscle brings this sclerite to a vertical position. The movement of 

 the third axillary turns the attached median plate (m) likewise to a 

 vertical position on its hinge with the second axillary (2Ax). In the 

 forewing (A), the revolution of the first median plate (w) draws the 

 second median plate (m') inward. The second median plate, however, 

 is firmly attached to the united bases of the median, radial, and sub- 

 costal veins, and the head of the radius (i^) is flexibly attached to the 

 anterior end of the second axillary. As a consequence, the movement 

 of the first median plate turns the entire anterior part of the wing 

 posteriorly on the hinge between the radius and the second axillary. 

 But, since the basal connection of this part of the wing forms an 

 oblique line from the head of the first axillary to the articulation 

 between the two median plates, the entire preanal area of the wing 

 is deflected as it turns posteriorly. At the same time, the anal area is 

 lifted but maintains its horizontal plane as the third axillary assumes 

 a vertical position. When the wing finally comes to a longitudinal 

 position over the back, therefore, the anal area is uppermost and the 

 preanal area slants downward on the side (fig. 50 A). During the 

 final revolution of the wing the first axillary turns upward on its 

 hinge with the tergum, the second axillary rotates slightly on the 

 pleural wing process, and the third axillary revolves posteriorly in its 

 vertical position on the fourth axillary. 



In the hind wing the mechanism of flexion is in general the same 

 as that of the forewing, but, in addition to the posterior turning of 

 the wing, the great anal area is folded fan-like into many plaits. The 

 third axillary of the hind wing (fig. 47 B, sAx) is relatively much 

 larger than that of the forewing (A), its muscle process stands out 

 prominently from the shaft, and the flexor muscle inserted on it con- 

 sists of two bundles of fibers. A distal median plate is lacking in 

 the hind wing, but the single plate (A, m) attached to the third ax- 

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