204 LIFE : OUTLINES OF GENERAL BIOLOGY 



squirrek, vegetarians though they be, are unable to resist the 

 gustatory appeal of the young pigeons they see in the nests on the 

 trees. So the fewer squirrels, the more wood-pigeons, and the worse 

 for tlie farmer. 



Of all these natural checks, the one that means most to man's 

 interests is between insectivorous birds and injurious insects. When 

 we think of the legions of plant-bugs (Rhynchota), the hosts of 

 larv.T. such as caterpillars, leather-jackets, and wireworms, the 

 minute Diptera like the Frit-fly, the phytophagous beetles like 

 cockchafers and weevils, besides sawflies and scale-insects, and the 

 frankly destructive tribe of locusts, we realise that the increase of 

 injurious insects is a continual menace to the kingdom of man, 

 which, after all, depends as yet on the green plants of the field. If 

 the cloud of insects should tliickenand spread for a few years, then 

 is all the labour of creation undone. Local plagues, now of locusts 

 and again of caterpillars, here of cotton boll weevils and there of 

 Phylloxera in the vineyard, hint to us loudly that our whole eco- 

 nomic system may be readily imperilled if the natural checks to the 

 multiplication of injurious insects should cease. Changeable weather 

 puts an end to many insect-pests; a few commit race-suicide by 

 devouring all the food, but this is rarely possible with field-crops; 

 fortunately for man, insects are often against insects, ladybird 

 beetles against green-flies, and ichneumons against caterpillars, and 

 so on; spiders, frogs, toads, lizards, and other animals do their bit 

 in decimation; but on the whole, what matters most is that there 

 should be an abundance of insectivorous birds, for they form the 

 most important of all checks to the multiplication of injurious 

 insects. We do not ourselves believe that there are data for pro- 

 I)hecy. but some naturalists of distinction have said that if our 

 insectivorous birds were wiped out — and they are being continually 

 menaced — our whole bionomic system would come to an end within 

 six to ten years. But whether this is or is not a sound prediction, 

 it is absolutely certain that every reduction in the numbers of those 

 sjH^cies of birds that feed on injurious insects means a loss to 

 agricuhure. 



Fi-owKKs AM) THEIR Insect VISITORS.— No naturalist can 

 have any antipathy to insects, even when they puncture the 

 farmer's and gardener's and colonist's inflated hopes. They are so 

 intriguing, so subtle, so masterful with as much right to live, if 

 the phrase has any meaning, as any other kind of creature. They 

 are fascinating, even when they are sinister. Many of them are 

 directly iK-neficial to man, as silk and honey so well illustrate; 

 othfTs, like ichneumon-flies, are invaluable in checking pests; but 

 the insects that mean most to man are those which secure the 

 cross-pollination of flowers. In search of nectar and pollen, so often 

 advertised by brilliance and colour, by fragrance and form, many 



