356 ENTOMOLOGY 
the study of tropic reactions is complicated by the fact that 
they are due not only to external stimuli, but also to little- 
understood internal stimuli, arising from unknown conditions 
of the alimentary canal, reproductive organs, ete. 
A newly recognized property of protoplasm is that of adap- 
tation, as manifested in the acclimatization of protoplasm to 
untoward conditions of temperature, light, contact and other 
stimuli; and this adaptation to unusual conditions may take 
place without the aid of natural selection. 
A tropic reaction occurs, whether it 1s to prove useful to the 
organism or not. Thus a lady-bird beetle walks upward, on 
a branch, on a fence, on one’s finger. It walks upward as far 
as possible and then flies into the air. If it happens to reach 
the tip of a twig and finds aphids there, the beetle stops and 
feeds upon them. This adaptive result is in a sense incidental. 
Yet, upon the whole, tropic reactions are wonderfully adaptive 
in their results. Here natural selection is of: special value as 
affording an explanation of the phenomena. 
As Loeb and Davenport have insisted, the mechanical reac- 
tions to gravity, light, heat and other influences determine the 
behavior of the organism. 
2. INSTINCT 
Insects are eminently instinctive; though their automatic 
behavior is often so remarkably successful as to appear ra- 
tional, instead of purely instinctive. 
Instinct, as distinguished from reason, attains adaptive ends 
without prevision and without experience. for example, a 
butterfly selects a particular species of plant upon which to lay 
her eggs. Caterpillars of the same species construct the same 
kind of nest, though so isolated from one another as to exclude 
the possibility of imitation. Every caterpillar that pupates 
accomplishes the intricate process after the manner of its kind, 
without the aid of experience. 
Instinctive actions belong to the reflex type—they consist 
of co-ordinated reflex acts. A complex instinctive action is a 
