CHAPTER VII 



THEIR RELATION TO OTHER ANIMALS 



In the course of their development insects have 

 estabUshed the closest kind of relations to the rest of 

 the animal kingdom, and there is scarcely a vertebrate 

 terrestrial animal that is not more or less affected by 

 parasites — man not excluded. Some of this parasitism 

 is of the most highly specialized character. We have 

 somehow come to think of parasites as being simple, 

 lowly organized creatures, of very inferior rank, and 

 yet a moment's thought will show that there could be 

 no parasites of vertebrates until the vertebrates them- 

 selves existed, and as the insects long antedated verte- 

 bi-ates, parasitism must have come as a specialization 

 from an already well-developed organism. 



Nor is such parasitism confined to what we call the 

 lower orders, for we find none of it in the Thysanura 

 or primitive forms; but its most elaborate development 

 occurs in the Diptera, or flies, which are the highest in 

 the scale so far as physiological specialization goes. 



As might be supposed, the Heniiptera, gaining their 

 food by piercing and sucking, rank well among the 

 orders containing animal parasites; indeed, broadly 

 speaking, a large percentage of the order is strictly 

 parasitic on either plants or animals. The scale insects, 

 for instance, are absolutely dependent upon the host 

 plants to which they attach themselves, and many of 

 them if once removed from their attachment, are help- 

 less and die. The plant lice are less strictly parasitic 

 and yet the term "lice" is a good one when we compare 

 it with the same term used for those suckers that feed 

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