334 



THE ANIMAL KINGDOM 



provide us with explanations to offer the puzzled student who says as he 

 looks in dismay at the crayfish: "It will never work, it has too many 

 moving parts." 



1 52. The Flight Mechanism in Insects 



Unlike birds and bats, most insects do not have large flight muscles 

 attached to the wings. Instead, the wings are articulated with the thorax 

 in such a way that very slight changes in the shape of the thorax cause 

 the wings to beat up and down. The flight muscles are located entirely 

 within the thorax and are not attached to the object moved. 



The thorax can be compared to a box (Fig. 17.9) having an under- 

 sized cover. The inner end of each wing is attached by a movable joint 

 to the upper edge of the sides of the box. When the vertical muscles 

 of the thorax contract, the notum (box cover) is depressed and the wings 

 flip upward. When longitudinal muscles contract, the notum arches 

 upward and the wings flip down. The flight muscles are very stout and 

 change little in length during contraction. The two sets of muscles are 

 opposed and pull alternately against each other. 



In many insects (all the Exopterygota and Lepidoptera, and most 

 Coleoptera), the frequency of the wing beats correlates closely with the 

 nerve impulses to the muscles. These impulses are evenly spaced in 

 time and staggered in the nerves to the two sets of muscles, so that 

 rhythmic up and down movements of the wings result. The rate varies 

 with the rate of the nerve impulses, from 8 wing beats per second in 

 large moths to 75 or more in the smaller insects. 



In many of the Diptera, Hymenoptera, and possibly a few Coleop- 

 tera, however, the wing beats are not correlated with the frequency 

 of the nerve impulses. Low frequency nerve impulses have little or no 

 effect, but when the frequency rises above 100 or so per second the flight 

 muscles begin to contract rapidly but at a higher frequency. Pringle 

 has studied this phenomenon and finds that not only are the nerve 

 impulses not correlated with the wing beats, but their frequency is ir- 

 regular and not staggered in nerves to opposing muscles. Hence, al- 

 though the frequency of the nerve impulses must exceed a certain 

 threshold, once this is exceeded the rhythm of muscular contraction 



■Notum curves down fore and aft 

 NotuTn of thorax: 



rWin^ 



LoDoitudinal Tnusclz 

 Vertical xnusclc 



-Stamum 



Figure 1 7.9. Diagram of the primary flight muscles. Vertical muscles extend between 

 the notum and the sternum. Longitudinal muscles extend between the downturned ends 

 of the notum. 



