48 INSECTS AND HUMAN WELFARE 



known as secondary parasites, and in at least a few cases the 

 secondary parasites are known to be infested with others 

 known as tertiary parasites. 



A parasite stands in marked contrast to a predacious 

 species since it does not feed indiscriminately upon whatever 

 suitable prey it may discover, but has a certain host species 

 or series of hosts which it always selects to the exclusion of 

 others. From generation to generation these same hosts are 

 always chosen with most extraordinary persistence, for rea- 

 sons which are at present in most cases obscure. Nevertheless 

 such appears to be almost invariably the case, although some 

 species of parasites have been found to attack a much larger 

 series of hosts than others, and a very few appear to be some- 

 what indiscriminate in their tastes. 



Parasitic species belonging to a number of the different 

 orders of insects are known, and from our knowledge of the 

 relationships of the different groups it is evident that the 

 phenomenon of parasitism has arisen independently in each 

 group, but by far the most important from an economic 

 standpoint are those included in the Hymenoptera' and Dip- 

 tera. The parasitic Hymenoptera have the greater signifi- 

 cance, and include a vast complex of species of the most va- 

 ried habits, included in a long series of families. Of these the 

 Ichneumonidse and Braconidse attack the larval or more 

 rarely the pupal stages, the female ordinarily depositing her 

 eggs within the body of the insect and the larva feeding upon 

 the blood and tissues of its host, which it finally leaves for 

 pupation, or destroys entirely by consuming the vital organs. 

 In the former case, an affected larva may perhaps survive 

 under exceptional circumstances, but ordinarily succumbs to 

 the attack. According to the comparative size of the host 

 and parasite, one or a series of "parasites may be nourished by 

 a single host. The members of a series of families belonging 

 to the superfamily Chalcidoidea, which includes a vast 

 number of small or minute species, are parasitic in habits, 

 some of them attacking their hosts like the previously men- 

 tioned families (Fig. 23) . Others present a most remarkable 



