1320 A TEXTBOOK OF THEORETICAL BOTANY 



5. Flowers visited by Special Groups of Insects 



i. Hymenopterous Flowers 



Flowers belonging to these groups are pollinated only by nriembers of the 

 Hymenoptera. This group includes the Hive Bees, Humble Bees, Wasps, 

 Ichneumon Flies and Ants. The flowers which are pollinated by these 

 insects are varied in form but the majority are zygomorphic, and red, blue 

 or violet colours predominate. Many of the flowers are so specialized that 

 they can be pollinated by no other type of insect. Some flowers, indeed, are 

 suitable only for the heavier and more massive bees, or those with extra long 

 probosces. Finally we have a number of cases where the flower is so con- 

 structed that it is only possible for one particular species of bee to effect 

 pollination, e.g., Aconitiim, Delphinium and several species of Corydalis. 



Hymenopterous flowers fall naturally into six groups according to the 

 type of insect which pollinates them: 



(«) Hive Bee Flowers (Such flowers can be pollinated by 



bees with a proboscis of 7 mm. or 

 less), e.g., Trifolium repens. 



{h) Humble Bee Flowers (Such flowers can be pollinated by 



bees with proboscis of more than 

 7 mm.), e.g., Trifolium pratense. 



(r) Bee and Humble Bee Flowers (Plants with two types of flowers, 



with corolla tubes of different 

 lengths), e.g., Calamintha alpina. 



(ii) Bee and Butterfly Flowers e.g., Rhinanthus hirsutus. 



{e) Wasp Flowers e.g., Scrophularia nodosa. 



(/") Ichneumon Fly Flowers e.g., Listera ovata. 



[a) Hive Bee Flowers 



Under this heading must be included not only the true Hive Bee, but 

 also a number of fairly long-tongued bees with a proboscis up to 7 mm. in 

 length. Most of the flowers which are normally pollinated by these insects 

 are rarely visited by any others, except occasionally by butterflies. The long 

 probosces of the latter make it possible for them to reach the nectar, but 

 their bodies remain so far out of the flower that they rarely effect pollination 

 and they must therefore be regarded merely as robbers of the flowers. 



The Hive Bee, as already pointed out, does not visit a wide variety of 

 flow^ers and as far as Britain is concerned it shows a marked preference for a 

 relatively small number of species. Early in the season, when pollen and 

 nectar are scarce, it may be obliged to visit a wider range of plants than it 

 does later on when it is freer to choose. The greater part of the honey 

 stored in a hive is obtained successively from the cultivated fruit trees, the 

 White Clover, the Blackberry, the Lime and later, in appropriate localities, 

 from Heather. These flowers provide the bulk of the honey, but many other 

 flowers are visited, depending very largely on local circumstances. A colony 



