Natural History 631 



If the supply of food gives the insect a material inducement 

 to frequent flowers, colour and fragrance guide the visitor in 

 choosing its particular source of supply. The golden standard 

 of the broom, the glowing purple of the sage, heather, or thyme, 

 the massed yellow flower-heads of sunflower or marigold, the 

 white expanses of hedge-parsley and hawthorn, are illuminated 

 signs advertising the goods within. That insects can distinguish 

 between different colours is still a little doubtful, for it is not 

 easy to discriminate between colour as such and the intensity of 

 light reflection. To our senses colour is the most prominent 

 feature of the flower, but to the insect scent may be more im- 

 portant. We know that insects can perceive odours to us quite 

 imperceptible; it may be that they can discriminate between 

 scents which to our coarser senses are the same. 



While colour and scent are the guides, floral structure deter- 

 mines what insects can profitably visit a particular flower, and 

 often decides the manner of the visit. Thus a flower like the 

 hedge-parsley has its nectar freely exposed so that flies and such 

 short-tongued insects can easily reach it; the nectar cup being 

 shallow all may drink, and flies wander about over the inflor- 

 escences distributing pollen indiscriminately. But in a flower 

 like the sage the nectar is concealed at the bottom of the deep 

 corolla tube ; it can be reached only by long-tongued bees, and the 

 bee must enter the flower in one particular way. The lower lip 

 forms a landing-stage to which it clings; it thrusts head and 

 thorax into the corolla throat, pushing against the lower ends of 

 the two pivoted, lever-like stamens, the upper ends of which swing 

 down and dust the bee's back with pollen at a definite point. On 

 subsequently visiting a somewhat older sage flower the bee first 

 pushes against the forked stigma, which at this stage projects 

 from the hooded upper lip, and so the pollen from one blossom is 

 deposited on the stigma of another of the same kind. In such a 

 flower as the sage, pollination can take place only in a perfectly 

 definite fashion. Thus, small flies cannot work the lever of the 



