INTRODUCTION AND METHODS. 7 



by the same change, and for this reason the various experimental methods 

 are organized with respect to the change concerned. There is almost no 

 limit to the number of changes and combinations that ingenuity can devise, 

 but the following discussion is restricted essentially to changes that have 

 been used or are now in process of being used. 



Organization of experiments. — All experiments have been made in 

 the field under natural conditions. Controls have been regularly employed 

 and the results checked by repetition. Comparative values have been 

 secured by using a definite number of flowers, an equal area, or an equal 

 number of them. The latter method alone seems to be entirely without 

 error and has completely replaced the others, since it yields exact and 

 directly comparable expressions of choice. For the same reasons experi- 

 ments should be observed at similar times and for periods of the same 

 length in so far as possible. For the best results it is often necessary to 

 have two or three observers, so that the time difference can be eliminated 

 and the same group of competing insects followed under the same condi- 

 tions. The grouping of the plants and the relative position of the flowers 

 are matters of much importance and must be considered with reference to 

 the habits of the insects and the type of behavior to be tested. All experi- 

 ments carried on with a plant or group that insects have been in the habit 

 of visiting must reckon with the effect of the habit itself. While such 

 results are dependable as to behavior, they do not permit an exact analysis 

 of the factors entering into it. On the other hand, advantage may be taken 

 of habit to insure certainty in regard to a response, as when artificial or 

 mutilated flowers are alternated in the cluster with normal ones. It must 

 also be recognized that the reactions of mature insects contain a large 

 element of habit and that the real response to certain stimuli can be obtained 

 only by using young ones that have just emerged. Moreover, it has become 

 more and more desirable to deal adequately with individual behavior, 

 and this can be done only by working with marked bees. 



Plateau has more than once pointed out that experiments with flowers 

 and insects demand the greatest patience and almost unlimited time, and 

 that one must expect to have many of them rendered incomplete by changes 

 of weather and other hazards. The problem is the same as in all experi- 

 mentation in nature and must be met in so far as possible in the choice of 

 the region, the detailed organization of plans, and the introduction of the 

 maximum degree of control. 



The modifications brought about for experimental purposes may be 

 grouped as follows: (1) changes of place, grouping, or time; (2) concealing 

 or disguising flowers or clusters; (3) removal or mutilation of flowers, 

 parts, markings, etc.; (4) artificial or painted flowers or parts; (5) addition 

 of flowers, parts, substances, etc. ; (6) combinations of two or more changes. 

 The distinctions between the different types of modification are far from 

 absolute and certain changes might well be placed in another group. 



Change of position or place. — These may concern the plants or flowers 

 of one species, or of two or more species. In the latter case they have to 

 do primarily with competition and are considered later under that heading. 

 Changes that have to do with the time of blooming or the grouping may be 



