130 



WASPS 



[CH. V 



almost horizontal into a vertical position. This is possible 

 in virtue of the "wing-plate" lying in a recess or fold. 

 It must be borne in mind that when the " wing-plate " is 

 horizontal the wing itself, being set at right angles to the 

 plate, is vertical and vice versa. Hence when the longi- 

 tudinal muscles contract the wing is depressed the 

 down-stroke is made. The up-stroke is brought about 

 by the contraction of the transverse dorso- ventral muscles 

 which pull the scutum and scutellum down again, sepa- 

 rating the sides of the hinge and at the same time 

 lowering the "wing-plate" and restoring it to the hori- 

 zontal, but the wing to the vertical position. - 



The action may be roughly imitated as follows: 

 crease a strip of fairly stiff paper as in Fig. I. and 

 push a pin through BG: then 

 the pin P represents the wing, 

 BG = "wing-plate," D = dorsal and 

 A ventral surface of meso-thorax. 

 BG is nearly horizontal, P nearly 

 vertical. Now hold BA firmly 

 and pull D sharply upwards, imi- 

 tating the effect produced by the 

 longitudinal muscles; the crease 

 opens out, BG becomes vertical, 

 the point of P (wing) travels over an arc downwards into 

 the position shown in the second figure; this completes 

 the down-stroke. The up-stroke is made by pressing 

 D down, when the crease becomes folded once more and 

 the point of P travels up, BG resuming the nearly hori- 

 zontal attitude. 



A 

 Fig. I. 



A 



Fig. II. 



