INSECTS AND OTHER ORGANISMS 395 



in rapid vibration and its long suctorial maxillae thrust 

 accurately into the heart of some bell-flower ; a bee alighting 

 on and depressing the lip of a snapdragon, crawling into the 

 corolla which closes behind her, and then emerging, her 

 back dusted with pollen some of which must adhere to the 

 stigma of the next flower visited ; a miscellaneous assembly 

 of Diptera, mostly drone-flies, on the flower-heads of some 

 richly blooming composite, where the drone-fly constantly 

 thrusts her proboscis, extended and flexed in rapid alterna- 

 tion, in and out of the yellow florets, pausing at intervals 

 to clean the sucker from superfluous pollen by using her 

 fore-legs as combs ; a large variety of much smaller Diptera 

 on the white, flattened flower-heads of a roadside umbelli- 

 ferous weed, attracted by the rank scent as well as by the 

 large, conspicuous area presented by the expanse of white 

 petals individually small. One cannot watch familiar sights 

 like these without the thought that the insects and the 

 flowers are fitted to each other as the eye to the light or the 

 lock to the key. 



Though all insects are dependent on plants directly or 

 indirectly for their supplies of food, it is well known that 

 there are some plants which are definitely destructive to 

 certain insects. While the forms and appearance of flowers 

 are adapted to attract such insect visitors as bees and 

 Lepidoptera which eftect pollination, there are often 

 specially arranged barriers of hairs which guard the nectaries 

 from the access of small Diptera and ants which could 

 render no service in return for the nectar that they might 

 obtain if admitted. Some plants — the '' Catch-flies " of 

 the Pink family, for example, species of Lychnis and Silene 

 — have on the calyx glands that secrete a sticky fluid which 

 entangle the feet of small insects as they seek to crawl 

 upwards to reach the corolla ; unable to move they perish 

 outside the unfriendly flower. Other plants possess more 

 elaborate modifications by which they not only catch and 

 kill insects but digest the nutritious parts of their bodies 

 thus supplementing the food-supply as elaborated by green 

 plants generally from inorganic substances. Of these 



