456 



PHYSIOLOGY. 



266. 



Flight, the last of the voluntary motions to be considered by us, is 

 the most difficult to explain of all, as not only the muscles which are 

 attached to the organs of flight, but all those found within the thorax, 

 participate in producing it, and therefore it is not merely the wings 

 during flight but the entire thorax, by means of the motion according 

 to its several plates, which are detected as contributing to effect it. 

 We can consequently distinguish two chief motions which are visible 

 during the flight of an insect, namely, the individual motion of the 

 wings themselves, and the contemporaneous motion of the thorax. The 

 above described respiratory motion of the thorax during flight is iden- 

 tical with the latter, so that the same motion which effects the in- and 

 expiration of the air from the thorax produces also the flapping of the 

 wings. 



The motion peculiar to the wings consists in their expansion and 

 bending backwards ; the expansion is produced by the extensors and 

 the bending back by the flexors. From 178 we know that the 

 extensors are by far the largest of these muscles, and that they vary in 

 compass according to the varying size of the anterior and posterior 

 wings. By the contraction of an extensor, therefore, the wing is 

 expanded, and by the continuance of this contraction it is retained in 

 this position. In those orders with four membranous wings or coria- 

 ceous anterior wings we find no difference in the position of the wings 

 during expansion. The anterior ones lie in front of the posterior onesj 

 and in one plane with them, sometimes separate and sometimes con- 

 nected with them by means of a peculiar apparatus. In the beetles, 

 however, in which the anterior wings are transformed to hard elytra, 

 their position is quite different. Sometimes these elytra are not at all 

 expanded during flight, and this is the case in the genus Cetonia and 

 in the earwig. In other instances the elytra are expanded it is true, 

 but in a very different direction, namely, perpendicularly upwards ; 

 whereas the wings are extended horizontally, as we observe in Necro- 

 phorus, in the genus Ulster, and in many Staphylini. In many other 

 beetles, lastlv, they lie in the same direction with the wings, yet not in 

 general upon the same plane, but a little higher. In all these cases, 

 therefore, the elytra do not participate in the blow of the wing, but 

 they retain the same position and situation during the whole flight. 



The remaining muscles of the thorax, but particularly those of the 

 two segments upon which the wings are placed, and which above 



