FLOWERS AND THEIR USES 271 



the features in which angiosperm flowers differ from one 

 another including the shape and color of sepals and petals, 

 and the position of stamens, pistils, and nectaries have 

 developed with reference to insect-pollination. 



The relation of their flowers to insects, therefore, has been 

 a very important factor in the evolution of angiosperms ; and 

 one reason why the angiosperms as a group have been so 

 successful in the struggle for existence is the fact that so 

 many of them have secured the help of insects in pollination. 

 The relations of insects to flowers have likewise been an im- 

 portant factor in the evolution of insects. Thus, butter- 

 flies and moths live entirely or chiefly upon nectar; they 

 have a long, flexible, sucking proboscis, which can be pushed 

 deep into the parts of flowers where nectar is to be found. 

 The proboscis is of different length in different butterflies 

 and moths, and each species seeks the flowers to which the 

 length of its proboscis best adapts it. Bees not only feed 

 upon nectar themselves, but they gather and store it up in 

 the form of honey for the use of their young a habit which 

 makes the honey-bee and the nectar that it gathers of great 

 practical interest to us. The mouth-parts of bees are 

 especially adapted to the work of biting and sucking, which 

 enables them to obtain nectar ; and some of them, includ- 

 ing the honey-bee, have a special apparatus on their hind 

 legs, consisting of a mass of rough, stiff hairs to which 

 pollen grains cling in great numbers. 



281. Pollination of the Lady's-Slipper. The lady's- 

 slipper offers a very good illustration of the way in which 

 a flower may be adapted to pollination by particular in- 

 sects. The flower of the lady's-slipper has the form of a 

 hollow sac with an opening at the top ; this opening is partly 

 closed by a flap. The edges of the opening in front of the 

 flap are curved inward, so that insects of various sizes can 

 get into the sac but cannot well get out at the same place. 

 Inside, at the bottom of the sac, are juicy hairs that are 



