108 HELIOTROPIC MECHANISM 



and twist of the head showed that when the body tilted 30° from the 

 vertical the sagittal plane of the head incUned 50°, with a body tilt 

 of 45° the head was inclined 75°, while in extreme cases, with Very 

 bright illumination and over reactive flies, the body frequently tilted 

 as much as 55° or 60°, sometimes even more, and the sagittal plane 

 of the head was horizontal. 



The same postural asymmetry may be produced by simply focusing 

 onto the left eye of a normal robber fly a beam of light from the 

 objective of the optical system of a string galvanometer; thus the 

 right eye is relatively dark, there is an unequal photochemical reac- 

 tion in the two eyes, and asymmetry in the tonus of the muscles of 

 the two sides of the body is produced just as it is by producing the 

 unequal photochemical change by blackening one eye. 



In general it may be stated that all butterflies, including yellows, 

 whites, meadow-browns, coppers, skippers, and fritillaries, . react in 

 the same way. They show abnormal postures w^hen quiet, and 

 circle toward the unblackened eye both when walking or flying. 

 Circionis alope, Vanessa huntera, and Argynnis aphrodite are good 

 types for illustration. When resting, the wings are held together 

 above the back and show the tilt of the body in a striking way when 

 one eye is blackened. The average body tilt in the bright light of the 

 laboratory was 40° as shown in Fig. 4. This figure also shows the 

 rotation of the head by the fact that the antennae are displaced toward 

 the seeing eye, often both being carried to this side beyond the sagit- 

 tal plane of the body; in this feature there may also be some con- 

 traction of the muscles of the antennae, which in extreme cases 

 carries the lower antenna parallel with, or even touching the hori- 

 zontal surface on which the butterfly is resting. Vanessa often holds 

 its wings spread horizontally; this is the normal position of the wings 

 of moths; and in these cases the wing on the side of the blackened eye 

 is elevated while the other wing is depressed and touches the surface 

 on which the animal rests. This asymmetrical condition of tonus of 

 the wing muscles is maintained both when resting and when walking 

 and it is probably the cause of the circus motions when flying. The 

 tonus of the muscles of the anterior pair of walking legs of butterflies 

 is so altered as to produce the asymmetrical postural conditions de- 

 scribed for Proctacanthus , and to an exaggerated degree. The changes 

 in the posterior legs are not so clear. 



