21 



Parasitic Insects. 



The role of parasitic insects in the control of injurious forms is better 

 understood today than ever before, but here again the web is very complex 

 on account of the presence of secondary and tertiary forms. Man is begin- 

 ning to utilise parasitic insects in controlling some of the serious pests 

 (See my article on " Parasitic Insects" in the Fifth Report). It is probable, 

 however, that man will never have full control of parasitic forms. Their 

 influence will appear without any effort on his part. Arm^^ worm and tent 

 caterpillar scourges come and go, but we are to a large extent ignorant of 

 the real causes of their fluctuations. To what extent climatic conditions 

 and parasites are responsible for the oscillations in their numbers we are 

 not certain. We cannot yet predict the time of recurrence of army worm 

 visitations. Possibly other and unknown factors are partly responsible 

 for their appearance. The time is still far distant when we will be able to 

 suppress insect outbreaks by having on hand a supply of parasitic forms 

 ready to be liberated at the proper time. 



The Web of Life. 



Only a few of the threads of the intricate web of life have been touched 

 in this short discussion. Examples may be found everywhere about us in 

 nature. 



Sir Ray Lankester gives an illuminating classification of the economic 

 relations between animals and man as follows: — 



A. Those animals captured or slaughtered for food and other products. 



B. Those bred or cultivated for food and other products. 



C. Those promoting operations of civilised man, but not by being 



killed, captured or trained — scavengers, burying beetles, earth- 

 worms, pollinating insects. 



D. Those causing bodily injury, death, disease, e.g., lions, wolves, 



snakes, stinging insects, germ-carriers, parasites, worms, etc. 



E. Those injuring man's domestic animals, cultivated plants, or wild 



forms important to him — e.g., as in D, but also injurious in- 

 sects, voles, etc. 



F. Those injurious to man's worked-up products of art and industry, 



such as buildings, furniture, books, clothes, foods and stores. 



G. Those beneficial in checking the increase of D,E,F., e.g., certain 



carnivorous and insectivorous birds, reptiles and amphibians; 

 some parasitic and predaceous insects. 



