430 THE INVERTEBRATA 



and in the male Coccid bugs and all Diptera, the posterior wings 

 are minute structures, flight being performed by the remaining 

 pair, which are normally developed. Thus, either by linking two 

 pairs of wings together, or by dispensing with one pair, flight is 

 commonly brought about by one functional unit on each side of the 

 body. The variations in form, consistency, and size of the wings are 

 briefly dealt with under the different orders. 



Simple up-and-down movements of the wings are sufficient to 

 account for the elementary phenomena of insect flight. In moving 

 through the air the anterior margin remains rigid but the rest of the 

 membrane yields to the air pressure; so that when the wing moves 

 downward it is bent upwards (cambered) ; as the wing moves upward 

 the membranous part is bent downwards, therefore, by becoming 

 deflected the wing encounters a certain amount of pressure from be- 

 hind which is sufficient to propel it. The faster the wings vibrate the 

 more they are cambered, the greater the lateral pressure and the 

 faster the flight. Smaller insects have as a rule a greater rate of wing 

 beat. Thus a butterfly may make only 9 strokes a second while a bee 

 makes 190 and a housefly 330. The wing muscles of insects thus con- 

 tract immensely faster than those of any other animals. It is inter- 

 esting to note that the intracellular respiratory pigment, cytochrome, 

 occurs in high concentration in them. 



To bring about wing movement direct muscles attached to the wing 

 base and others called indirect inserted on the body wall are employed. 



The extent to which direct and indirect muscles are present varies. 

 In the Odonata a direct musculature is strongly developed, the 

 muscles being attached to the intucked wing base. In the specialized 

 orders Lepidoptera, Diptera and Hymenoptera, indirect muscle action 

 is responsible for most of the movement and those muscles attached 

 directly to the wing base serve for folding the wing to a position of 

 rest as well as for flight purposes. 



Fig. 303 represents diagrammatically the condition in the winged 

 aphides. The thorax is a box whose roof is capable of being arched 

 and flattened by longitudinal and dorsoventral muscles respectively. 

 Since the wing base has two points of attachment, (i) to the pleural 

 plate, and (ii) to the edge of the tergum, the wing operates as a lever 

 of the second order. The arching of the tergum raises the wing base 

 and depresses the wing, while a flattening of the tergum depresses the 

 wing base and raises the wing. 



The abdomen consists of a series of segments less dift'erentiated 

 than those of the head and thorax. The number is eleven, as seen to 

 be present in the embryo insect (with the addition of a transient 

 telson) and in primitive groups (Thysanura and Odonata). In other 

 groups, the nth segment is represented by the podical plates which 



