ANIMAL INTER-RELATIONS 27 



up these fascinating side-tracks. Although the first 

 half-hour of field work wiU suggest hundreds of 

 intricate problems connected with the details of food- 

 habits and other phenomena, it is necessary for the 

 moment to concentrate on the very general principles 

 that can be seen at work among animal communities 

 in nature. 



Animals may be classified according to their food- 

 habits into herbivores and carnivores and scavengers. 

 Carnivores can be roughly divided again into pre- 

 dators and parasites, but the distinction is not a 

 sharp one, although it is of fundamental importance 

 ecologically, in connexion with problems of numbers 

 in an animal population. It has been pointed out 

 (Elton, 1927) that this distinction between parasites 

 and predators depends on the relative sizes of the 

 carnivore and its prey. If the latter is relatively 

 large it affords, besides a source of food, a possible 

 home and also a means of transport. But there are 

 numerous intermediate types of carnivore which 

 cannot be logically classified either as predators or 

 parasites. Examples are the lamprey, blood-sucking 

 insects, queen ants that become temporary ' social 

 parasites ', and many more. An extremely important 

 borderline class is that of insects that parasitize 

 other insects. These are sometimes caXLed parasitoids. 

 They Hve in their host as larval parasites, but when 

 they grow beyond a certain size emerge as free-living 

 insects, having destroyed their host. Scavengers Hve 

 upon dead or dying animals and plants. Examples 

 are the jackal, the burying beetles, and many Collem- 

 bola. 



Herbivorous animals can oe further classified into 

 a huge number of different types. Every species «tf 

 plant is probably attacked by some kind of animal. 

 The first thing to do in analysing the food habits 

 of animals from an ecological survey fist is to deter- 

 mine the forms which are strictly dependent upon 

 one species of plant. Others wiU be found to occur 



