NO. 2 THORACIC MECHANISM OF A GRASSHOPPER SNODGRASS 99 



A careful study of the forms of the folded wings of the grass- 

 hopper, as seen in transverse section (fig. 50), will suggest that many 

 details of structure, both in the tegmina and in the hind wings, are 

 adaptations to the passive state of flexion rather than to the active 

 phases of flight. 



The extension of the wings is effected probably by the action of 

 both the basalar and the subalar muscles (fig. 49 E, M' and M"). 

 The basalar sclerites (fig. 48, iBa, 280) are connected by a tough 

 membranous fold (a) with the base of the wing anterior to the wing 

 process. A depression of these sclerites on their episternal articula- 

 tions, caused by the contraction of their muscles (fig. 49, E, M'), must 

 therefore release the flexed wing from its position over the body and 

 turn it outward. The principal extensor of the wing, however, ap- 

 pears to be the muscle of the basalar sclerite (fig. 49, M"). In the 

 flexed wing, the second axillary sclerite is elevated between the first 

 axillary on the one hand, which now stands in a vertical plane on its 

 tergal hinge, and the median plate {m) on the other, which rises ver- 

 tically from its hinge on the second axillery. The ventral plate of the 

 second axillary is connected with the subalar sclerite by a thickening 

 of the intervening membrane (figs. 48, 49, h). The downward pull of 

 the basalar muscle (fig. 49, M") is therefore exerted on the second 

 axillary. It is easy to demonstrate that a downward pressure on the 

 second axillary flattens the entire wing base by restoring the first 

 axillary, the median plate, and the third axillary to the horizontal 

 plane, and thereby extends the wing. 



When the wings are extended, the mechanism of flight becomes 

 operative. This includes the direct and indirect muscles, which ac- 

 complish the movements of levation, depression, and rotation, and 

 which have already been described. 



VI. THE SPIRACLES 



The generalized ancestors of modern insects possibly had a pair 

 of tracheal invaginations on each of the 17 body segments between 

 the primitive head, or procephalon, and the periproct. Evidence of 

 the existence of such invaginations has been found, however, on only 

 14 segments, namely, the second maxillary segment, the three tho- 

 racic segments, and the first ten abdominal segments. 



Tracheal invaginations of the second maxillary segment have been 

 reported by Nelson (1915) to be present in the embryo of the honey- 

 bee. They arise. Nelson says, on the lateral surfaces of the anterior 

 half of the segment above the bases of the rudiments of the second 



