BY INSECTS IN FLYING. 379 



is also exhibited, the front pair between CD and BB, tiie hinder pair 

 between EF and CC. Besides these lateral muscles, there is a pair si- 

 tuated behind EF, and another pair in the intermediate space between 

 CD and EF, which has a slanting position, and the whole course of 

 which may easilj' be perceived in fig. 8. at DD. These muscles, namely 

 the single great dorsal muscle and the four pair of lateral muscles, are 

 capable of contracting the cavity of the thorax in different directions ; 

 the first only from the fore to the hind part, so as to increase the con- 

 vexity of the back, the other four pair from the upper to the inferior 

 part, so that the sides become more arched, and the back flatter. The 

 other elevations are less interesting to us, and contain the muscles for 

 the movement of individual organs. Thus, for example, in the elevations 

 G at fig. 7. lie the muscles which serve to expand and elevate the wings: 

 they are represented in fig. 8. by E E and F F : in the elevation behind A 

 lies the muscle which serves to close the air-hole fig. 7*; in the elevations 

 «, b, c, which are the hips of the three pair of thighs, are the muscles 

 for the motion of the legs, &c. The depressions between the elevations 

 are immaterial: there are however three of them, of which we must take 

 a more particular notice ; namely, the one wherein the further air- or re- 

 spiration-hole (stigma) is situated (fig. 7 *•), the second in which the 

 hinder air-hole is perceived (fig. 7**), and the third in which the wing 

 is placed, and which we perceive (fig. 8) in the section at the point of 

 insertion of each wing (c?rf). Having thus far considered the structure 

 of the thorax, we have now only to observe that the spaces between the 

 muscles are filled up by the tubes which carry air to the tracheae, and 

 that the chain of nerves and the aorta, which are supported by the forked 

 prolongation of the covering represented at fig. 8, traverse the larger 

 space beneath the dorsal muscle of the intestinal canal. 



We find as the external organs of the thorax, the wings, the poisers 

 (halteres), the scales, and the legs. The wings, d, are placed in a depres- 

 sion on the side of the thorax, which is formed by a very thin prolon- 

 gation of the exterior envelope in a slanting direction, as may be seen 

 in fig. 7. They consist of a pocket-like folding of the epidermis, sup- 

 ported and extended by the horny veins. The muscles by which the 

 wings are expanded and elevated commence at the base of those veins. 

 Behind the wing is the scale, a circular pellicle (fig.l.g), which is placed 

 perpendicularly, connected with the basis of the wings, and moved by 

 muscles of its own. Behind this, quite at the end of the thorax, are si- 

 tuated the 2)oisers (fig. 8./.), which are the rudiments of the undeveloped 

 hinder wings, and are likewise capable of independent movement by 

 their proper muscles. We have already mentioned that the legs are at- 

 tached to the hips (fig. 8. a, b, e,); tiicir structure is unimportant to our 

 investigations. 



The movtonent of the wing, on whicii the sound in the next ])lace de- 

 VoL. 1.— Part III. 2d 



