294 ENTOMOLOGY 



With insects, as with other animals, many instincts are transitory; 

 even when partially fixed by habit, they are replaceable by stronger in- 

 stincts. Thus the gregarious habit of larva? is finally overpowered by 

 a propensity to wander, which does not mature, however, until the 

 approach of the transformation period. The reproductive instinct is 

 another of those impulses that do not ripen until a certain age in the 

 individual. 



Inflexibility of Instincts. Broadly speaking, instinctive actions 

 lack individuality are performed in the same way by every individual 

 of the species. The solitary wasps of the same species are remarkably 

 consistent in architecture, in the selection of a special kind of prey, in the 

 way they sting it, carry it to the nest and dispose of it; all these opera- 

 tions, moreover, are performed in a sequence that is characteristic of the 

 species. Examples of this so-called inflexibility of instinct are so omni- 

 present, indeed, that insect behavior as a whole is admitted to be in- 

 stinctive, or automatic. Insects are capable of an immense number of 

 reflex impulses, ready to act singly or in intricate correlation, upon the 

 requisite stimuli from the environment. 



To normal conditions of the environment, the behavior of an insect 

 is accurately 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 insufficient and the larva is unable to adopt an- 

 other 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 sur- 

 prising stupidity, capable as they are amid ordinary circumstances. 



Flexibility of Instincts. Notwithstanding such examples, the 

 common assertion that instincts are absolutely "blind," or inflexible, is 

 incorrect. Instinctive acts are not mechanically invariable, though 

 their variations are so inconspicuous as frequently to escape casual ob- 

 servation. A precise observer can detect individual variations in the 

 performance of any instinctive act variations analogous to those of 

 structure. 



To take extreme examples, the Peckhams found that an occasional 

 queen of Polistes fusca would occupy a comb of the previous year, instead 

 of building a new one; and that an individual of Pompilus marginatus, 

 instead of hiding her captured spider in a hole or under a lump of earth 

 as usual, hung it up in the fork of a purslane plant. They observed also 



