14 POLLEN. [CHAP. 



roaming over the plant and constitute a most efficient 

 bodyguard, not only driving off the leaf-cutting ants, 

 but, in Belt's opinion, rendering the leaves less liable 

 to be eaten by herbivorous mammalia. Delpino 

 mentions that on one occasion he was gathering 

 a flower of Clerodendrons fragrans, when he was 

 suddenly attacked by a whole army of small ants. 

 M. Boissier also makes the interesting observation that 

 many plants produce honey in some countries and 

 not in others. Thus Potentilla tormentilla and Geum 

 urbanum give honey in Norway, and none, or hardly 

 any, near Paris. Indeed a careful comparison showed 

 that most plants gave more honey in Norway than at 

 Paris. No doubt, in consequence of this, some plants 

 which are visited by insects in the north are neglected 

 in the south. Thus he observed five species of Hier- 

 acum to be frequented both by bees and humble- 

 bees in Denmark, while near Paris they are never 

 visited by those insects. 



M. Boissier found that by watering a plant copiously 

 he could increase the supply of honey ; nay, more, 

 that he could even cause some species to give honey 

 which do not generally do so. 



The pollen of course, though very useful to insects, 

 is also essential to the flower itself; but the scent 

 and the honey, at least in their present development, 

 are mainly useful in securing the visits of insects, 

 though the honey is also sometimes of service in 

 causing the pollen to adhere to the proboscis of the 

 insect. 



Bees rarely visit flowers unless for some real advant- 

 age. The Diptera (flies) appear to be less intelligent, 



