CHAP. \ HI. 



SYMBIOSIS AND PARAS 



69 



oviposit in insects alread) parasitized, so that superpai 

 occurs, with the result that verj few oi the parasites survive in the 

 nexl generation to attack the possible hosts which have escaped 

 being parasitized. The numbers both of host and parasite there- 

 fore undergo a sudden drop (see fig. 34), the host becausi 

 large proporl ion destroyed bj the vast numbers of parasites present. 

 and these latter on account of internecine struggles caused by 

 superparasitism. It is probable that in such cases the mo 

 amongst the parasites is pro] much higher than in the 



the insect parasitized and this latter is therefore able to 

 in. rease its numbers very rapidly, the parasites again increasing as 

 their hosts provide more abundant food until the increase of the 

 parasites once more overtakes that of their host, with the result 

 that superparasitism again occurs and the wave of increase is again 

 checked. 



1 



1 inal.) 



In economic work the study of the parasites of the various 

 pests is oi the verj firsl important e and at the same time it 

 is often a matter of greal difficult} and complexity, not onlj to find 

 out which parasites attack any particular crop-pest, what are their 

 lifehistories and what are their names, but to work out a] 

 various relations between them and to decide therefrom which are 

 beneficial and which are injurious. For it is not to be supposed 

 (though too often taken for granted) that a parasite must be bene- 

 ficial merely because it is a parasite. Main parasites are injurious, 

 since the) attack insects which are beneficial. If we take I 



iterpillar, which is a crop-pest, and of a parasite which 

 checks it, the I. ii' be looked on as beneficial; but this 



parasite maj itsell be attacked by a hyperparasite, which in its 

 turn may be looked on as injurious in this particular case. Bui 1 



