PHYSIOLOGY: W. E. CARREY 
603 
by Loeb and Garrey^ that Amhly stoma larvae reacted to the constant 
current in such a way that the muscle tonus was different depending 
upon the direction of the current. With the anode at the anterior end, 
the body curved ventrally and the legs were directed backward, while 
with the cathode at the head end the body was in the opisthotonic con- 
dition, concave dorsally, and the legs were directed forward. These 
differences in muscle tension made motion difficult or impossible in 
any direction except to the anode, toward which the animals were 
forced to move. 
In the hehotropic insects the characteristic reactions are mediated 
through the eyes, the central nervous system, and thus refiexly, the mus- 
culature. Loeb proposed the following theory of the direction of the 
motions of such heliotropic animals.^ The ''photosensitive elements 
are arranged symmetrically in the body and through nerves are con- 
nected with symmetrical groups of muscles. Light causes chemical 
changes in the eyes If the rate of photochemical reaction 
is equal in both eyes the effect upon symmetrical muscles is equal, and 
the muscles of both sides of the body work with equal energy. If a 
positively hehotropic animal is struck by Hght from one side, the effect 
on the tension or energy production of muscles connected with this eye 
will be such that an autoro.atic turning of the head and the whole animal 
toward the source of light takes place," until the eyes are equally illu- 
minated, when the symmetrical muscles work equally so that the animal 
will continue to move in this direction, or any deviation from this line 
of orientation will result in a repetition of the manouver. 
It is our purpose to show that inequality in the illumination of the 
two eyes of m.any insects does in reality produce the differences in mus- 
cular tension demanded by this theory, that their movements are de- 
tei mined by this condition, that the phenomena are quite general in 
this group of animals and identical with those described for galvano- 
tropism where the tension theory is proven. 
Holmes^ showed that the tonus of the muscles of the water scorpion, 
Ranatra, was markedly affected by illuminating the animal from dif- 
ferent directions and by blackening different parts of the eyes, thus 
changing the equality and symmetry of illumination of the two sides. 
His results were in absolute harmony with Loeb's theory, the ani- 
mals reacted according to its demands with 'machine-like' precision. 
The striking similarity to the behavior of PalcEtnonetes or Amhly stoma 
when under the influence of the galvanic current would have sufficed 
to establish the identity of heliotropism with galvanotropism, had it 
not been for the diverting effect of Holmes's psychological speculation, 
