336 '^^ ANIMAL KINGDOM 



Forward flight, 

 showing air f lov\7 



^7 



Hoverind Ba.cKin^ Turning 



Figure 17.11. Flight maneuvers in the honeybee. The figure eight in the first three 

 diagrams traces the path of the left wingtip. The lines on the last diagram show the posi- 

 tions of the two wings during a full beat. (After Stellwaag.) 



up or down and greatest when the wings are horizontal. Hence, as the 

 wings begin to move they do so agaijist the torce of these small muscles 

 until they reach the half-way point, when they move with the force and 

 are suddenly accelerated. This guarantees full amplitude to the wing 

 beat and a sudden, rapid stretching of the relaxing set of muscles just 

 before their next contraction. Such stretching is known to improve the 

 strength of contraction in many kinds of muscles, including those of the 

 vertebrates. 



In all winged insects, smaller muscles in the thorax attached to the 

 sides and wing bases are used to alter the posture of the wings as they 

 move up and down. Suitable contraction of these muscles enables the 

 insect to turn, hover or back up (Fig. 17.11). While the details of these 

 processes are too intricate to present here, the general pattern of ordi- 

 nary flight is such that the wings act as propeller blades, drawing air 

 from above, in front, and to the sides, and propelling it posteriorly as 

 a sharply driven column of air. The details are modified endlessly in 

 the various groups of insects. 



153. Vision 



The functioning of the compound eyes is a most intriguing physi- 

 ologic problem. It was recognized early that images formed by such 

 eyes must be very different from those formed in our eyes. Their struc- 

 ture (Figs. 16.7, 17.12) suggests that each ommatidium records the 

 amount of light received from a particular direction, and that all of 

 them together provide a mosaic impression of the world. This theory 

 received considerable support when Exner, in 1891, sliced off the com- 

 pound eye of a firefly and used it as a lens for making a photograph. 

 The film image was a single large one and was erect rather than in- 

 verted as in our eye. 



