386 THE INVERTEBRATA 



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 be- 

 coming deflected the wing encounters a certain amount of pressure 

 from behind 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 contract immensely faster than those of any other 

 animals. It is interesting 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 em- 

 ployed. 



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. 289 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 w^ing base and raises the wing. 



The abdomen consists of a series of segments less differentiated 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 bear the 

 cerci anales (as for instance in the cockroach). In specialized insects 

 the apparent number of abdominal segments may be greatly reduced. 



In insect embryos rudiments of appendages are borne on each of 

 the abdominal segments, but these rudiments disappear in the adult 

 except in the Apterygota. Only those which become the cerci anales 

 in the nth segment are frequently retained. In the 8th and 9th 



