34 The Structure and Special Physiology of Insects 
elaborateness of many insect instincts, such as those of the ants, wasps, and 
bees, to choose examples at once familiar and extreme in their complexity, 
makes it very difficult to analyze the trains of reactions into individual ones, 
and to determine, if it is indeed at all determinable, the particular stimuli 
which act as the springs for these various reactions. The attitude of the 
modern biologist in this matter would be to keep first in mind the theory 
of reflexes, to look keenly for physico-chemical explanations of the reac¬ 
tions, and only when forced from this position by the impossibility of find¬ 
ing mechanical explanations for the phenomena to recognize those com¬ 
plex reflexes which we call instincts, and finally those acts which we call 
intelligent, or reasonable, and which are possible only to the possessors of 
associative memory. The investigations, mostly recent, which have been 
directed toward a determination of the immediate springs or stimuli of 
insect reactions indicate clearly that many of these responses, even some 
which were formerly looked on as surely indicative of considerable intelli¬ 
gence on the part of their performers, are explicable as rigid reflex (mechan¬ 
ical) reactions to light, gravity, the proximity of substances of certain 
chemical composition, contact with solid bodies, etc. On the other hand 
the position of the extreme upholders (Bethe, Uexkull, and others) of the 
purely reflex explanation of all insect behavior will certainly prove untenable. 
As one of the phases of insect biology to which this book is particularly 
devoted is that which includes the study of habits, activities, or behavior, 
we may dispense with any special discussion of instinct in this introductory 
chapter. It is sufficient to say that no other class of invertebrate animals 
presents such an interesting and instructive psychology as the insects. 
