INTRODUCTION 5 



able importance if we wish to understand the parasitic relationship 

 itself. 



It is, of course, a truism that all living organisms are inter-dependent, 

 but the origins and development of that acute dependence displayed by 

 parasites and commensals has a special fascination. At one end of the 

 scale there is a rove beetle which can apparently only breed in birds' 

 nests where the temperature is raised to about 40 degrees centigrade 

 by the presence of nestlings and, at the other end, the cuckoo, which 

 has also to seek out the nests of small birds in which to lay her eggs. 

 Once on the track of this sort of relationship, the naturalist becomes 

 more and more curious. He just has to go on. 



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Bird-bottle fly, Protocalliphora azurea, resting upon 

 a flower (x 4.7) 



