HYMENOPTERID FLOWERS 115 



and vMets on the other hand, where the insect visitors are practically the same as for 

 flowers with concealed nectar, which also are almost always red, blue, or violet. 



The agreement between floral colours and insect visitors in these groups can 

 only be explained by assuming that the more highly organized insects prefer red, 

 blue, and violet colours, from which it also follows that these colours are to be 

 regarded as a higher stage of floral coloration. 



The connection be- 

 tween the groups of 

 flowers and insects above 

 referred to, which at first 

 appears so remarkable, 

 thus becomes intelligible. 

 Hermann Muller had 

 already noticed this 

 colour-preference on the 

 part of insects. 



In respect of fre- 

 quency of insect visits, 

 the two groups of social 

 flowers which are made 

 conspicuous by aggre- 

 gation surpass consider- 

 ably the two other floral 

 classes that correspond 

 to them (Knuth, 'Bluten- 

 besucher,' II, p. 3). 



As already remarked 

 on p. 95, the little bee 

 Andrena Hattorfiana F. 

 confines its visits entirely 

 to Knautia. 



Transitional forms 

 between S and L also 

 occur. The following, 

 for instance, belong to 

 SL, since their visitors are almost entirely Lepidoptera : Eupatorium cannabinum ; 

 Adenostyles alpina, albifrons, and hybrida. 



Fig. 25. Adenostyles alpina, Bl. el Fing., a social flower of the 

 division SL. 



6. Hymenopterid Flowers (H). 



These are regularly plundered and pollinated only by Hymenoptera (membrane- 

 winged insects). The shapes and colours of the flowers are exceedingly varied. 

 Bilaterally symmetrical (zygomorphous) forms largely predominate, and red, blue, or 

 violet colours. Although hymenopterid flowers are visited by insects belonging to 

 very different groups, it is only the less -specialized members of the class that can be 

 poUinated by insects other than Hymenoptera. In the most highly specialized types 



I 2 



