220 A BOOK OF INSECTS 



stage, however, the bee alights of necessity upon the 

 stigma, which projects in the place previously occupied by 

 the anthers, the latter being now bent aside. Thus pollen 

 brought from the younger flower is transferred to the 

 stigma of the older. 



In the common primrose {Primula vulgaris) cross- 

 pollination is secured by a different plan. The five petals 

 are united at their bases to form a tubular corolla, and the 

 flowers are dimorphic. In one form, known popularly as 

 " thrum-eyed," the pistil has a short style, while the 

 anthers of the stamens are situated at the mouth of the 

 corolla tube ; in the other form, called " pin-eyed," the 

 arrangement is reversed. The style is long, the globular 

 stigma is in the mouth of the tube, while the anthers are 

 low down — being on the same level as the stigma in the 

 "thrum-eyed' flowers. The tw r o forms of flower are 

 not found on the same plant. Long-tongued flies, and 

 occasionally humble-bees, visit primroses in search of the 

 nectar which is secreted at the base of the ovary — i.e. at 

 the far end of the corolla tube. When an insect comes 

 to a long-styled flower, its proboscis is dusted with pollen 

 at a part which — if it subsequently visits one with a short 

 style — is brought into contact with the stigma. Con- 

 versely, pollen collected from the anthers of a short-styled 

 flower is transferred to the stigma of a long-styled form. 

 The flowers of the primrose may sometimes be self- 

 fertilised, as when pollen falls upon the stigma of the 

 short-styled form ; but cross-pollination can only be 

 effected through the agency of insects. 



While some kinds of flowers are accessible to insects 

 of many kinds, the majority are specially reserved, so to 

 speak, for the delectation of a few species. Thus, the red 

 clover relies upon the aid of humble-bees with mouth- 

 parts sufficiently long to penetrate its floral tube ; and 



