. 190 - NATURAL HISTORY OF WASPS. 
The motions of which a wasp’s wings are capable, 
according to the form of the articulating surfaces 
and the movements of the thorax, are up and down, 
and backwards and forwards, with a slight rotatory 
motion. And during all these movements there is a 
special arrangement for keeping the wings parallel to 
each other. The action of the great muscles of flight 
is directed to the alternate raismg and depression of 
the arch of the meso-thorax, to which the fore-wings 
are attached, and the motions of which they follow. 
The hind-wings, while they are locked, must of course 
vibrate with the fore-wings, but they make the 
stroke oblique as they hang behind it. And the same 
thing occurs on the return stroke, which is effected, 
not by a spring, but. by another set of muscles, the 
fore-wing still leadmg. The result of this is to pro- 
duce a figure-of-eight movement; so that the one 
half of the vibration is not the exact counterpart of 
the other, but a blow is given with more effect in 
one direction, that is on the down- than on the 
return-stroke; the anatomical arrangement deter-— 
mining in which direction the more effective stroke 
shall be delivered. » 
The wings, in their combined action, have a much 
greater forward-propellng power than when sepa- 
rate. The flight of many moths which lock their 
wings, as compared with that of butterflies, which do 
not lock them, is an instance of this. Now as it is 
length and not width which propels, the two separate 
narrow wings should give a greater speed than the 
broad wing which results from their union. And it 
must be by modifying the direction of the blow that 
the locking of the wings tells on the speed, as on 
