1903] FOLSOM :~ INSECT PSYCHOLOGY 1 7 



operations, moreover, are performed in a sequence that is characteristic of the spe- 

 cies. Examples of this so-called inflexibility of instinct are so omnipresent that 

 insect behavior as a whole is admitted to be instinctive, or automatic. Insects are 

 capable of an immense number of reflex impulses, ready to act singly or in intri- 

 cate correlation, upon the requisite stimuli from the environment. 



To normal conditions of the environment, the behavior of an insect is accu- 

 rately adjusted ; in the face of abnormal circumstances, however, demanding the 

 exercise of judgment, most insects are helpless, The specialization to one kind of 

 food, though usually advantageous, is fatal if the supply becomes insufBcient and 

 the larva is unable to adopt another food. A species of Sphex habitually drags 

 its grasshopper victim by one antenna. Fabre cut off both antennae and then 

 found that the Sphex, after vain efforts to secure its customary hold, abandoned the 

 prey. Under such unaccustomed conditions, insects often show a surprising stu- 

 pidity, capable as they are amid ordinary circumstances. 



Notwithstanding such examples, the common assertion that instincts are 

 "blind," or inflexible, is incorrect. Instinctive acts are not mechanically invari- 

 able, though their variations are so inconspicuous as frequently to escape casual 

 observation. A precise observer may detect individual variations in the perform- 

 ance of any instinctive act, — variations which are analogous to those of structure. 



To take extreme examples, the Peckhams found that an occasional queen of 

 PoUstes fusca would occupy a comb of the previous year, in place of building a new 

 one. They observed also that one Ammophila, in order to pound down the earth 

 over her nest, actually used a stone, held between the mandibles. 



While most of the variations that one encounters are small and, in a sense, 

 accidental, or purposeless, such novel departures as those of the Polistes or the 

 Ammophila denote adaptability. 



Even the despotic power of habit may be overborne by individual adaptability. 

 Among caterpillars that have exhausted their customary food, there are often a few 

 that will adopt a new food-plant and survive, leaving their more conservative fel- 

 lows to starve. 



As Darwin himself held, the doctrine of natural selection is applicable to 

 instincts as well as to structures. All reflex acts are to some extent variable. 

 Disadvantageous reflexes or combinations of reflexes eliminate themselves, 

 while advantageous ones persist and accumulate. 



Indeed, structures and instincts must frequently have evolved hand in hand. 

 The remarkable protective resemblance of the Kallima butterfly would be useless, 

 did not the insect instinctively rest among dead leaves of the appropriate kind. 



Though manifestly dominant, instinct alone fails to account for all insect 

 behavior. The ability of an insect to profit by experience indicates some degree 

 of intelligence. 



