HYMENOPTERID FLOWERS Its 
and vzole/s 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- As 
tween the groups of | 
flowers and insects above 
referred to, which at first 
appears so remarkable, 
thus becomes intelligible. 
Hermann Miller » 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, ‘ Bliiten- 
besucher,’ IT, p. 3). 
As already remarked 
on p. 95, the little bee 
Andrena Hattorfiana 7. 
confines its visits entirely 
to Knautia. 
Transitional forms 
between S and L also 
occur. The followin g; FIG. 25. <Adenostyles aahrne re “ pie a social flower of the 
for instance, belong to 
SL, since their visitors are almost entirely Lepidoptera: Eupatorium cannabinum ; 
Adenostyles alpina, albifrons, and hybrida. ; 
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 
pollinated by insects other than Hymenoptera. In the most highly specialized types 
I 2 
