842 ECOLOGY 



Flies and beetles. As a class the flies (Diptera) are not very important polli- 

 nating insects, largely because of their absence of precision in making floral visits. 

 Some, however, notably the drone flies (Syrphidae), have extended probosces and 

 depend largely upon flowers for food, and thus are important pollinating agents. 

 Most flies pollinate only flowers with exposed nectar and pollen (as in Euonymus}. 

 Color seems to have but little attractive significance, but odors (especially those 

 offensive to human nostrils) attract numerous flies, particularly carrion flies and 

 dung flies, which may thus be important pollinating agents in ill-smelling flowers 

 like Rafflesia. The pollination of Arum and Aristolochia is thought to be effected 

 largely by small flies, which are able to crawl through the narrow apertures. Beetles 

 are still less important than flies, though some species with narrow elongated heads 

 are of some significance; as a class their floral visits result in more harm than benefit. 

 Pollinating animals other than insects. Apart from insects the most important 

 pollinating animals are birds, especially those with long slender bills and protrusile 

 tongues, such as the humming birds, which visit honeysuckles, trumpet flowers, and 

 other long-tubed blossoms containing nectar ; in some cases birds visit flowers in 

 search of nectar-feeding insects. Bird pollination is much commoner in the tropics 

 and in the southern hemisphere than in northern latitudes; in parts of South Amer- 

 ica, humming birds almost equal insects in importance as pollinating agents, and 

 in South Africa the sunbirds and their relatives are even more important, pollinating 

 insects being much less conspicuous than in the northern hemisphere. The struc- 

 ture of bird-pollinated flowers does not differ from that of flowers which are polli- 

 nated by insects with elongated mouth parts. A few instances of pollination by 

 bats have been reported, but they are not regarded as important. Pollination by 

 slugs or snails is of possible importance in a few cases, as in Calla and in other aroids 

 with numerous blossoms close together near the ground. 



The food of pollinating insects. Pollen. Pollinating insects visit 

 flowers to obtain pollen, nectar, or sap, and sometimes for shelter, and 

 it is while they are engaged in one or more of these activities that pollina- 

 tion takes place incidentally. Bees obtain nectar, which they store for 

 future use, and pollen, which is in large part utilized more immediately 

 by the larvae, while butterflies and moths obtain only nectar and that for 

 immediate use; it is largely because of this that the bees are more useful 

 pollinators than are the more highly specialized butterflies. In some 

 flowers there is little or no nectar (as in Papaver, Hypericum, and Sola- 

 num) and insect visits are made mainly for pollen, which usually is pro- 

 duced in considerable abundance. The insects presumably get most 

 of the pollen, but some of it is pretty certain to be rubbed off on the 

 stigmas. Nectarless insect-pollinated flowers commonly are regular 

 (actinomorphic) and wide open, with the anthers prominently exposed. 

 Sometimes there are two kinds of stamens (as in Cassia), one which the 

 insects visit for pollen and another which sprinkles pollen over the insects 



