INSECT BEHAVIOR 35 
UL 
Phototropism and thermotropism, either together or singly, 
as Wheeler suggests, may explain the up and down migration 
of insects in vegetation. ‘On cold, cloudy days few insects 
are taken because they lurk quietly near the surface of the soil 
and about the roots of the vegetation, but with an increase in 
warmth and light they move upwards along the stems and 
leaves of the plants, and, 1f the day be warm and sunny, escape 
into the air.” 
Thermotropism.—Ants are strongly thermotropic; they 
carry their eggs, larvee and pupz from a cooler to a warmer 
place or vice versa, and thus secure optimum conditions of 
temperature. Caterpillars and cockroaches migrate to regions 
of optimum temperature. 
In thermotropism it appears that the direction of heat rays 
has little or no effect as compared with differences of intensity. 
Tropisms in General.—Other kinds of tropisms are known, 
for example, tonotropism, or the control of the direction of 
locomotion by density, and electrotropism; not to mention any 
more. 
All these phenomena are responses of protoplasm to definite 
stimuli and are almost as inevitable as the response of a needle 
to a magnet. 
The tropisms of the lower organisms have been experi- 
mented upon by many skilled investigators, whose results fur- 
nish a broad basis for the study of the subject in the higher 
animals—a study which has scarcely begun. Even in the 
simplest organisms, behavior is the resultant effect of several 
or many stimuli acting at once, and the precise effect of each 
stimulus can be ascertained only by the most guarded kind of 
experimentation; while in the higher animals, with their com- 
plex organization, including specialized sense organs, the study 
of behavior becomes intricate and cannot be carried on intelli- 
gently without an extensive knowledge of the behavior of 
unicellular organisms. The properties of protoplasm are the 
key to the behavior of organisms, though comparatively little 
is known as yet in regard to these properties. Furthermore, 
