240 ZOOLOGY: S. 0. MAST Proc. N. A. S- 
MathematicSy 1919, as a consequence of a misprint in my "second report 
on recent progress in the theory of groups of finite order," Bull. Amer. 
Math. Sec, 9, 1903, p. 118. 
PHOTIC ORIENTATION IN INSECTS 
By S. O. Mast 
ZooivOGiCAi, Laboratory, Johns Hopkins University 
Communicated June 6, 1922 
The idea that orientation in organisms is dependent upon the relation 
in the physical and chemical states produced by the stimulating agent in 
symmetrically located receptors and the relation in the activity of the 
locomotor apparatus on opposite sides is widely accepted. This idea 
extends back to the botanist, Ray (1693), it was applied to lower animals 
by Verworn in 1894 and to higher animals including insects by Bohn in 
1904 and Loeb in 1906. It has been referred to as the difference of in- 
tensity theory, the continuous action theory, the tropism theory, the 
Verworn theory, the Ray- Verworn or DeCandolle theory, the muscle- 
tension theory and Loeb's muscle-tension theory. We shall call it the 
Ray- Verworn theory. 
Referring to insects, Bohn, Loeb, Garrey and others maintain that light 
affects the tonus of the muscles in proportion to the intensity of the illumi- 
nation of the receptors with which they are connected and in such a way 
that when the eyes on opposite sides are unequally illuminated, the legs 
on one side move faster than those on the other resulting in turning until 
the eyes are equally illuminated, when the tonus of the legs on opposite 
sides becomes equal and the turning ceases. These authors maintain 
that whenever insects are oriented, the receptors on opposite sides receive 
the same amount of stimulating energy. They hold that orientation is 
the result of balanced action in the two halves of the body. 
There are numerous reactions involved in the process of orientation 
of insects and other animals which are not in accord with these views. 
Some of these follow. 
If one eye of the drone-fly, Eristalis tenax, is covered it deflects strongly 
toward the functional eye making circus movements, but after a certain 
time it orients fairly accurately, especially in a well defined beam of light. 
Similar results have been obtained in observations on various other in- 
sects by Radl, (1903), Holmes (1905), Carpenter, (1908), Dolley (1916), 
and Minnich (1919). The eyes on opposite sides are not equally stimu- 
