472 LESLIE B. AREY AND W. J. CROZIER 
gression and light intensity, but a limit is quickly reached beyond 
which no sensible increase is possible in the rate at which pedal 
waves succeed one another upon the foot. Moreover, it is very 
probable that the pedal nervous mechanism, as is to some degree 
indicated by other responses, is 'set off' as a whole, and not bilat- 
erally, by impulses originating in the mantle and passing through 
the central ganglia. So long as the photoreceptive mechanism 
is in bilaterally balanced excitation, the rate of operation of the 
pedal musculature might then proceed upon the 'all-or-none' 
principle, uninterfered with by the mechanism concerned in 
turning movements. This conception is already w^ell founded 
for vertebrates (cf. Brown, '14). The rhythm of the 'scratch 
reflex' of mammals is independent of the frequency of the excit- 
ing stimuli (Sherrington, '06). More generally stated, the rate 
of contraction of symmetrical locomotor muscles is very nearly 
constant so long as the rate of stimulation of the two sides of 
the animal is the same; the bilateral halves of the central nerv- 
ous mechanism of control together behave as an independent 
unit so long as they are not stimulated differentially. Con- 
siderations of this sort make it clear why careful measurements of 
the locomotor rate of insects oriented in a field of Ught show no 
correspondence between rate of creeping and the light intensitj^ 
(Dolley, '17; Minnich, '19). One must insist that before an 
animal reaction can properly be made the basis of quantitative 
experiments, the nature of the reaction must be carefully reviewed 
as to its suitability for the purpose in mind; measurements of a 
phenomenon not itself sufficiently understood are likely to 
prove a waste of time. Minnich ('19, p. 406) has also pointed 
out that light intensity affects the posture of the legs of the 
bee, but not their rate of rhythmic activity. 
The speed and precision of the orientation vary in a char- 
acteristic manner with the wave length of the hght used. The 
experiments upon this topic were qualitative in nature, but were 
repeated a sufficient number of times to make sure of the essen- 
tial features of the results, which were as follows: When sunhght 
or light from a tungsten filament is made to pass through dif- 
ferent ray-filters, blue and green lights are very powerful in indue- 
