FOOD. 13 



into more eomplox ovoj-anic eomponnds and in turn supply food for otliors. 

 The scavengers feed on the decaying plant life and animal Me, clearino- 

 the earth of deeayino' refuse, making- it clean and sweet ; eventually they 

 die and the Avhole mass of organic material breaks up into the compounds 

 available for plant life. The parasites and predators feed upon the living 

 insects, checking the increase of both herbivore and scavenger, so that 

 the destruction of living plant life by herbivores can then never go 

 beyond a certain point, the balance thus being maintained. We are in 

 this volume mainly concerned with the herbivorous insects, those which 

 feed upon the living plant. They are largely injurious to man, though 

 also beneficial. We cannot neglect the parasites and predators, for in 

 actual work we meet them at every turn ; the part they play is not very 

 evident, but the practical study of pests requires that every student of 

 agriculture should be familiar with them and recognise them almost at a 

 glance. The scavengers are not of direct importance and we see them 

 but little ; grain aud household pests are of direct importance to man as 

 also the insects parasitic upon cattle and warm-blooded animals. 



Food Plants. 



Caterpillars and other herbivorous insects may have one, a few, or 

 many plants on which they can feed and thrive. Evidently an insect 

 that can live on a variety of plants has an advantage over one that lives 

 only on one or a few, and injurious insects are largely those which have 

 a great range of food-plants, enabling them to spread widely, to increase 

 abundantly, and to find food when crops are not available for them. The 

 list of injurious insects is nearly synonymous with the list of insects 

 having many food-plants. 



In general, insects feed upon one or more closely allied plants ; thus 

 cotton pests are found also on hUnda and other species of Hibiscus, cane 

 pests on maize and sorghum, and so forth. In other cases they feed on 

 plants which bear similar fruits ; an insect that eats tlie oily seeds of 

 cotton will perhaps feed on the oily seeds of other plants not closely allied 

 to cotton. 



The food-plants of some species, e.g., the gram caterpillar, are to be 

 numbered in scores. The food-plants of others are few, and there are 

 insects, for instance, which can feed on certain varieties of cotton aud 

 not on others. The composition of the tissues of the plant probably 

 determines its suitability to insects, and some plants appear to have no 

 pests. Plants protect themselves in various ways, but insects in their 

 turn seem equally to accustom themselves to the oils, alkaloids, hairs. 



