90 



THE CAUSES WHICH PROPAGATE 



promotes reproduction, and favours migration. It finds its function 

 of variation in the dynamics of flight. Thus, as Dr. Pettigrew 

 has shown, the vertical projection of insect atmospheric paths, 

 arising from alternate rising and forward wing-pulses, are in- 

 variably curvilinear by the second law of motion, although in 

 flies it will be found (Fig. 1) the wing blur indicates a recipro- 

 cating twist, converse to that observed in butterflies (Fig. 2). 

 So likewise it is immaterial whether the wings hook and move 

 in unison, like those of bees, moths, and other insects, or act 

 similarly and simultaneously as those of Dragon-flies ; whether 



Fig. 1. a,h, c, forward impulse ; c, V, a', rising impulse. 

 Fig. 2. a, forward impulse ; i, rising impulse. 



they feather as those of flies, or shut like those of butterflies, as 

 they move upwards. 



The wings of an insect may be considered as levers, and 

 during flight they take two strains, a longitudinal and trans- 

 verse. The first is provided for in the large and often prominent 

 longitudinal air-veins, which in thin-membraned wings increase 

 conformably in diameter, strength, and prominence, as we 

 approach the wing-base, where this strain attains a maximum. 

 The latter is received and sustained by the wing membrane and 

 cross veins, that unite with the longitudinal and transmit them 

 the strain. The position of the centre of percussion is ap2:)roxi- 

 mately indicated by the intersection of the two chief longitudinal 

 veins. The wing is fvirther an elastic jJi'opeller, and the imjjetus 

 of the stroke arises in great measure from its spring. In the 

 exotic genus of butterflies Euplea the pliancy, and in Idea the 

 flexibility, of the pinions are marvellous. The costal or front 



