LOCOMOTION 



231 



lowering and raising the tergum, to which the wings are attached, 

 lever the wings up and down upon the side plates (pleura) upon 

 which they rest (Fig. 167), and a direct set attached to the base 

 of each wing, which they can both rotate upon its axis and 

 extend from the body or retract. The hind-wings are beaten 

 down and up, and at each downstroke the strong front edge 

 (costa) is by muscular action rotated downwards and forwards 

 so that the somewhat concave lower surface faces obliquely 

 downwards and backwards. This process is assisted by the 



Fig. 166. — The ventral aspect of a male American cockroach with the wings 

 extended. An imaginary median line has been inserted. — From Thomson. 



A, Antennae ; C, cercus ; Co, coxa, the breadth of which makes it look, in its present position, like a 

 ventral plate on the body ; E, eye ; F, femur ; P.T, prothorax ; St, style ; Ta, tarsus ; Ti, tibia ; Tr, 

 trochanter; W^, first pair of wings ; W^, second pair of wings. 



resistance of the air below bending the thin hinder part of the 

 wing upward. As a result, during the beat the wing exerts pressure 

 both downwards and backwards while a region of decreased 

 pressure is created above and in front of it. Thus the insect is 

 pressed and drawn upwards and forwards. The fore wings are held 

 at right angles to the body, but do not beat. 



ALIMENTARY SYSTEM 



The alimentary canal (Fig. 168) has long, ectodermal fore- and 

 hind-guts, lined with cuticle as in the crayfish. The fore-gut 

 comprises (i) the mouth, with a tongue-like ridge (hypopharynx) 

 which bears on its under surface the duct of the salivary glands 



