604 
PHYSIOLOGY: W. E. CARREY 
centering as it did upon other features in the behavior of Ranatra. 
This case stands as an isolated example whereas, the features it pre- 
sents are in reality generally appKcable to hehotropic animals, since 
our experiments have demonstrated their existence in practically every 
group of the insecta. They are well shown by many genera of butter- 
flies and moths, by the bees, by many of the commoner flies including 
Musca, Caliphora, Tab anus, and Eris talis. While future reference will 
be made to these forms, the present communication will deal mainly 
with the reactions of the robber flies, in some forms of which the effects 
of light on the muscle tonus are most striking. The best of these which 
has come to our notice is one of the large brown forms, Proctacanthus, 
although Promachus and Deromyia show the reactions well. 
Methods were used to produce unequal illumination, usually by black- 
ening some part of the eyes with asphalt black, which is practically 
opaque and hardens into a brittle shell-like film upon drying. 
1. Blackening all of one eye. When one eye of positively heliotropic 
insects is blackened, circus motions are made toward the opposite 
(normal) side and in all the forms studied they occur both when flying 
and creeping, as noted by Parker for Vanessa antiopa."^ They may be 
noted in practically all of the commoner butterflies. Differences in the 
tonus of the muscles of the two sides are in evidence when the animals 
are at rest under constant illumination. The bodies are tilted well 
toward the side of the good eye and the legs of that side are flexed with 
the body resting against their upper segments, while the terminal seg- 
ments of that side are well under the body, and those of the opposite 
side are extended away from the body. The head is usually rotated 
on the body so that both antennae may actually be below the line of 
the wings. 
The robber fly, Proctacanthus (Sp. ?) shows the tonus changes of 
the legs much more strikingly than any other insect examined. On the 
side of the good eye they show a continued state of flexion involving all 
three legs. The anterior leg may be so far adducted as to cross under 
the body farther to the side of the blackened eye than the corresponding 
leg of that side. The legs on the side of the blackened eye are more 
extended than normally and spread farther apart. The body may tilt 
so far toward the side of the normal eye as to press the legs to the table. 
The head not only rotates on the long axis of the body toward the good 
eye but is also flexed toward that side, a considerable angle appearing 
between the head and thorax. In some cases the abdomen also shows 
flexion concavely toward the side of the good eye even when the animal 
is at rest. The posture assumed is the characteristic one assumed by 
