INSECT BEHAVIOR 315 



are performed as a rule in a definite, predicable sequence, and even a 

 slight interference with the normal sequence disconcerts the insect. 

 Just as the performance of one reflex act may serve as the stimulus for 

 the next reflex in order, so the completion of one instinctive action may 

 be in part the stimulus for the next one. 



Modification of Instincts. An action can be regarded as purely 

 instinctive in its initial performance only, because every subsequent 

 performance may have been modified by experience; in other words, 

 habits may have been forming and fixing, so that the results of instinct 

 become blended with those of experience. Thus the first flight of a 

 dragon fly is instinctive and erratic, but later efforts, aided by experience, 

 are well under control. 



When once shaped by experience, reflex or instinctive actions tend 

 to become intense habits. Thus, certain caterpillars, having eaten all 

 the available leaves of a special kind, will almost invariably die rather 

 than adopt a new food plant, whereas larvae of the same species will eat 

 a strange plant if it is offered to them at birth. An act is strengthened 

 in each repetition by the influence of habit, to the increasing exclusion 

 of other possible modes of action. Many a caterpillar, having eaten its 

 way out of the egg-shell, does not stop eating, but consumes the remain- 

 der of the shell a reflex act, started by a stimulus of contact against the 

 jaws and continued until the cessation of the stimulus, unless some 

 stronger stimulus should intervene. It has been said that the larva eats 

 the remains of the shell because they might betray its presence to its 

 enemies. Whether this is true or not, to assume conscious foresight of 

 such a result on the part of an inexperienced caterpillar is worse than 

 unnecessary. 



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

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

 instincts. Thus the gregarious habit of larvae 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 

 operations, moreover, are performed in a sequence that is characteristic 



