. 122 ' NATURAL HISTORY OF WASPS. 
the turn which a rook gives her wings to lift her up 
before she steps on the ground. Under these cir- 
cumstances the end of each separated wing describes 
an elliptical figure, standing vertically, or nearly so, 
just as in the Syrphus. The same may be seen in a 
humble-bee, as she scrambles, rustling and humming, 
up an inverted tumbler held obliquely towards the 
light. 
When the wings are locked together and vibrating 
in unison, in rapid flight, we cannot of course follow 
the outlines of the figures which they describe. But 
we know that they must trace a series of cycloidal 
curves along the line which the insect follows, which 
are wider according to the rapidity of the flight. 
Fig. 6.—Diagram shewing the figure described by the wings of the 
wasp in flight: the 
thin line is an 
imaginary section 
of the wing in the 
up-stroke, the broad 
band represents the 
space covered du- 
ring the stroke given downwards and forwards. 
And the little creature may be compared to a 
steamer urged on by two paddle-wheels which are 
of a size wholly disproportioned to her tonnage, and‘ 
in which all the difficulties of feathering floats have 
been happily solved. It might seem, at first sight, 
that to drive the insect along forwards, and to 
support her in the air, the blow of the wings should 
be given downwards and backwards. But Dr. Petti- 
grew, whose assistance and personal courtesy in 
investigatmg this subject I have great pleasure in 
acknowledging, from careful examination of the whole 
question, concludes that the stroke is given down- 
