INDIVIDUAL DIFFERENTIATION AND ADAPTATION. 127 



stimulus they respond to it in some way. This. response may 

 be either advantageous or disadvantageous to the organism. 

 If unfavorable, the result may be disastrous. If favorable 

 later repetitions of the stimulus are all the more likely to be 

 answered by the same kinds of response as in the first instance. 



FIG. 63, 



m. p. 



FIG. 63. The head of female Mosquito (Cule.r). After Dimmock. a, antennae; c, 

 clypeus; h, hypopharynx; m, mandibles; ma., maxillas; tn.p., maxillary palpus; /, 

 labium; la., labrum (epi-pharynx). 



Questions on the figure. In what way and for what purpose are the 

 mouth-parts of the mosquito used? What are the probable functions of 

 the antennae? Compare the antennae and the mouth-parts of a male and 

 female mosquito. See also Fig. 40. Mention some respects in which the 

 mosquito is adapted to its mode of life? What extent of horopter do 

 its eyes command? To what degree is the mosquito parasitic? 



This individual acquirement of a special mode of responding 

 to stimuli is known as habit. Since responses in higher organ- 

 isms occur by means of the nervous system we rightly asso- 

 ciate habits with the nervous activities. In reality, however, 

 mere protoplasm may acquire these habitual modes of action 

 and one might say that all such adaptations are dependent on 

 the power of protoplasm to respond to external stimuli. By 

 reason of this power of adaptive responses, organisms may 

 become habituated or acclimatized to changes in their environ- 

 ment, their habits or responses changing according to the 

 necessities of the case. It is a matter of common observation 

 that animals can thus gradually be brought to the endurance 

 of conditions which would originally have killed them. Such 

 must have been true of the animals which have come to live 



