NO. 2 ENCYCLOPEDIA OF INSECT ANATOMY — SNODGRASS 35 



with the wing base, contraction of the basalar muscles turns the wing 

 somewhat forward and deflects its margin during the downstroke. 

 With the upstroke, the subalar muscles turn the wing posteriorly and 

 deflect its posterior margin. The wing thus acts as a propeller, and 

 with the downstroke it exerts a backward pressure on the air that 

 drives the insect forward. 



This account of the flight mechanism must be understood to give 

 only its fundamentals and the action of the muscles that is the basis 

 of the wing action with most insects. Other factors, however, may 

 complicate the picture, and variations in the thoracic structure in the 

 different insect orders involve modifications in the wing and its mecha- 

 nism. A notable example is the conversion of the hind wings of 

 Diptera into small knobbed oscillating stalks called halteres, so called 

 because they were first regarded as balancers. It has now been shown 

 that they have a gyroscopic action in some way stabilizing the in- 

 sect's flight. The development of the halteres leaves no doubt that 

 they are the reduced metathoracic wings. Another example is the 

 modification of the forewings of beetles into elytra which seem to 

 be good protective shields but are of no help in flying. 



For a fuller discussion of the principles of flight mechanics and 

 aerodynamics than can be given here the student is referred to the 

 book "Insect Flight" by Pringle (1957). 



Returning to the anatomy of the wings, the primitive wing lobes 

 have been lengthened and properly shaped during their evolution 

 into organs of flight. Necessarily, the wings must be as thin and light 

 as possible, and at the same time stiff enough to withstand air pres- 

 sure. This involved the development of lines of rigid, branching thick- 

 enings k-nown as the wing veins. An old idea is that the veins were 

 formed around tracheae, but this is not supported by recent critical 

 studies (see Whitten, 1962). Another old idea is that the wings 

 were first gills, but this idea has been superseded by the glider theory 

 of wing origin. The venational pattern must have been established 

 early in the evolution of wings since the wing veins seem to con- 

 form to a fundamental pattern which permits a generally uniform 

 nomenclature. 



In the holometabolous insects the wing buds of the embryo are 

 sunken into pockets of the epidermis. These pockets become closed 

 off externally, and remain thus concealed throughout larval life. The 

 larva is thus not encumbered with externally growing wings, and 

 none of the thoracic modifications related to the wings are developed 

 until the pupal stage. Throughout the whole span of its life, then, 



