Vol. 1.] ^Voodwort]l. — ^¥ing Veins of Insects. 25 



action, so that the ratio shown in the diagram between the 

 vertical and horizontal forces is entirely out of proportion, the 

 former being much exaggerated. 



Besides the to-and-fro motion just discussed, there is also a 

 motion along the plane of the wing resulting in the figure 8 

 path already referred to. The width of the loops varies 

 greatly in different insects, and in the same insect the exact 

 course is dependent in part on the speed, but more on the 

 character of the stroke, which the insect seems to have the 

 power of varying more or less. This power is seen very strik- 

 ingly in those insects that poise themselves in the air, as do 

 most of the bees and such Hies as Bombilida' and many of the 

 hawkmoths. 



Now, Marey has shown that a body constructed like an 

 insect wing moving to and fro will be forced from its direct 

 course and caused by the resistance of the air to travel over 

 just such a path as an insect's wing is known to follow. The 

 front part of the wing being the more rigid, the effect of the 

 air resistance will be such that the whole wing will become 

 twisted near its base, and if the wing attachment permit, this 

 resistance acting obliquely on the wing disk will also force the 

 tip of the wing forward during each stroke. At the end of the 

 stroke the elasticity of the articulation and of the wing mem- 

 brane will bring the wing back to its normal position. This 

 occurring at each stroke, both up and down, the result will be 

 the characteristic curve. In such an insect as the bvitterfly, air 

 pressure, acting as here described, appears to be sufficient to 

 account for the curve of motion. Marey, indeed, believes the 

 air pressure alone is sufficient to explain the curved path in 

 the case of all insects. 



There is, however, the possibility that air pressure will pro- 

 duce just the opposite effect from that described al)ove, provided 

 the hinge at the base is sufficiently rigid to prevent the forward 

 motion of the tip of the wing, and provided the membrane is 

 sufficiently lax to bag and take on the helix shape. This is 

 the condition which has l)een insisted on by Pettigrew ('71), 

 and is illustrated in the beetles. The result of the " l)agging," 

 in this case, is the pulling backward of the tip of the wing and 

 its release at the end of each stroke, allowing the wing to 

 straighten out and the tip to move forward by its own elasticity. 

 The curve thus produced would reseml^le that previously 



