HOW FLOWERS ATTRACT INSECTS 211 



guests are influenced in the choice of flowers, even more by food-value than by- 

 external appearance.' 



4. The strong-smelling flowers of Ruta, Anethum, and the like, attract flies 

 more particularly. The species of Prosopis, which themselves have a peculiar odour, 

 display marked preference for strong-smelling flowers (Reseda, Lepidium, Ruta, 

 Anethum, Achillea, Matricaria). Sweet aromatic odours (Thymus, Lavandula, Rosa, 

 and others) strongly attract bees without excluding other insects. The odour not 

 strongly exhaled till towards evening of many white flowers with long corolla- 

 tubes (Lonicera Periclymenum and Caprifolium, Melandryum album, and the like), 

 attracts hawk-moths (Sphingidae) and other crepuscular and nocturnal Lepidoptera. 

 Flowers with the odour of carrion specially attract carrion-flies, and the urinous 

 odour of Arum attracts moth-flies (Psychodidae). 



Numerous facts in Flower Pollination support the view that the corolla plays 

 a prominent part in attracting insects. First let us ask ourselves why else does it 

 exist, why are flowers provided with bright and varied colours if not for the 

 purpose of alluring insects ? Why, let us ask further, are the male flowers of 

 diclinous plants larger than the female flowers if not to attract by their greater 

 conspicuousness the earlier visits of insects, so that the female flowers may be 

 pollinated ? What can be the object of the remarkable change of colour in flowers 

 that have been fertilized (e.g. Ribes sanguineum, R. aureum, Weigelia rosea, 

 IMelampyrum pratense, Aesculus Hippocastanum, and so forth) unless to render 

 more conspicuous the entire inflorescence ? And many more such questions might 

 be propounded. 



Plateau's experiments only show that the sense of smell perhaps guides insects 

 to a greater extent than has hitherto been supposed. Apparently there is need of 

 further experiment to decide questions as to the attraction of insects to flowers by 

 means of the senses of smell and sight. Meanwhile the following law may be 

 provisionally accepted. Attraction from considerable distances is certairily effected for 

 the most part by the odour of the flowers, which fills the air as with invisible clouds, and 

 indicates the direction for flight : when the insects approach nearer (1-2 m.), the colours 

 of flowers undertake the task of attracting them further, and when they finally settle, the 

 lines and points long since described by Sprengel under the name of'Saftmal' (i.e. sap- 

 mark) serve to point out the way to the nectar. 



p 2 



