DIFFERENTIATION OF INDIVIDUALS AND ADAPTATION 133 



titions of the stimulus are all the more likely to be answered 

 by the same kinds of response as in the first instance. This in- 

 dividual acquirement of a special mode of responding to stimuli 

 is known as habit. Since responses in higher organisms occur 

 by means of the nervous system we rightly associate habits 

 with the nervous activities. In reality, however, mere pro- 

 toplasm may acquire these habitual modes of action, and one 



FIG. 65. 



.m 

 ma. 

 1 



FIG. 65. The head of female Mosquito (Culex). After Dimmock. a. antennae; c, clypeus; h, 

 hypopharynx; m, mandibles; ma., maxillae; m.p., maxillary palpus; I, 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 antennas and the mouth-parts of a male and female mosquito. See 

 also Fig. 42. 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? 



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 environment, 

 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 in the waters 

 of hot springs. Such must have been the way in which other 

 animals were changed from the marine to the fresh- water habit, 

 since all fresh-water animals are believed to have been derived 

 from marine forms. 



