394 THE BIOLOGY OF INSECTS 



believed that certain plant-bugs infect potato-leaves with 

 *' mosaic disease," and that the woolly aphids {Schizoneura 

 lanigera) piercing apple-bark, facilitate entrance of the 

 spores of Nectria, the canker-fungus. Among biting insects 

 it may often be noticed that a strong-jawed creature such as 

 a wireworm, by biting through the tough epidermis of a root 

 or tuber, opens the way for hosts of frail springtails and 

 mites to invade the soft internal tissues. 



In the survey of the activities of social insects given in 

 a previous chapter the specialisation in form and habits 

 of bees (p. 225) in correspondence with the floral struc- 

 tures and products (nectar and pollen) which serve as the 

 source of their food-supply was emphasised. Later, in 

 discussing the evolutionary history of the insect orders 

 (p. 343) it was pointed out that the rise and develop- 

 ment of the flowering plants had been brought about con- 

 currently with the specialisation of those groups of insects 

 among the Hymenoptera, Diptera, and Lepidoptera that 

 seek in blossoms for their food. The relations between 

 such insects and flowers are among the most remarkable of 

 associative features shown by living beings, and suggest 

 irresistibly that the insects and the plants have grown, as it 

 were, to fit each other in the course of their long histories. 

 The adaptation of many flowers of diverse families to the 

 visits of insects, so that the insects obtain nectar or pollen, 

 and incidentally effect the cross-pollination of the flowers, 

 as they pass from plant to plant in their search for food, has 

 been a fascinating subject of biological study since the 

 fundamental observations on the subject made by C. Darwin 

 on orchids (1862) and other plants (1877), by J. Lubbock 

 (1875) and by H. Miiller (1878). It is unnecessary to 

 repeat here details as to the pollination of orchids, arum, 

 primrose, labiates, and many other plants set forth by those 

 and subsequent writers, especially as the subject has been 

 already discussed in detail in another volume of this series 

 (M. Skene, 1925). It may suffice to recall summer hours 

 spent in watching the visits of insects to blossoms : 

 a humming-bird hawk-moth poised in air, with its wings 



