638 Reflexes, Instincts, and Intelligence 



of insect behavior been undertaken. The truth is that insects are all highly 

 specialized animals, and by that fact, offer themselves as thoroughly inter- 

 esting but extremely difficult subjects for the student of animal behavior. 

 It seems at first sight impossible in the face of the well-developed nervous 

 system and highly specialized and complex character of the behavior of 

 insects to expect to analyze successfully this behavior into its separate com- 

 ponent reactions with their stimuli; to recognize among insects simple rigidly 

 mechanical reflexes of the nature of direct inevitable response to external 

 physico-chemical stimuli as light, temperature, humidity, gravitation, con- 

 tact, etc. Yet recent studies have shown that this can in certain cases be 

 done. It has been done, in fact. And a few examples of this work and 

 its results are given in the following paragraphs. 



Reflexes and Tropisms. The first considerable piece of careful experi- 

 mental study of insect reflexes was probably that of Loeb on the heliotropism 

 of caterpillars, moths, fly larvae, flies, plant-lice, and ants ; the results of which 

 were published in 1899. This original and able experimenter obtained 

 positive results from this work and from them was led to conclude that light 

 -controls or causes certain movements or behavior of these insects in a very 

 definite mechanical way; that it is the direction of the light rays that is the 

 chief controlling factor in the influence exerted by the light on the insects, 

 and finally that "the conditions which control the movements of the animals 

 toward light are identical, point for point, with those which have been shown 

 to be of paramount influence in plants." Therefore Loeb maintains that 

 these phenomena of insect orientation and movement in the presence of light 

 cannot depend upon specific characteristics of the central nervous system, 

 that is, are not manifestations of intelligence or instinct (in the common 

 understanding of the term), but are phenomena much more like those rigidly 

 mechanical ones of iron filings in a field of magnetic force, or of sunflowers 

 turning toward the sun. 



Since Loeb's epoch-making work numerous observations and experi- 

 ments have been made which have revealed other similar reflex phenomena 

 among insects. These phenomena are commonly called "tropisms" and 

 include whatever movements or special behavior can be explained as the 

 immediate rigorously controlled reaction in response to localized external 

 physico-chemical stimuli, such as heliotropism or phototropism, reactions 

 to light; geotropism, responses to gravitation; chemotropism, behavior 

 dependent on the influence of chemical stimuli affecting the organs of taste, 

 smell, etc. ; stereotropism or thigomotropism, movement or cessation of move- 

 ment produced by the stimulus of contact with solid substance; hydrotropism, 

 reactions to the stimulus of moisture or water; anemotropism, response to 

 the stimulus of air-currents; thermotropism, behavior controlled by tem- 

 perature, etc., etc. Thus Davenport has ascertained the movements of 



