50 FLEAS, FLUKES AND CUCKOOS 



association, the more easily is the balance upset. One partner can 

 then suddenly take a mean advantage of the other. Thus we have 

 already seen that certain debris feeding fly-larvae which find shelter in 

 birds' nests will sometimes return their hospitality by surreptitiously 

 eating the fledgelings alive. Some of the predatory mites, which live 

 permanently on birds and hunt other small arthropods in the forests of 

 feather and down, have abandoned the chase and turned parasitic 

 themselves. They have lost their powerful jaws and now chew the pith 

 of feathers or the various layers of the bird's integument. Although 

 commensals obviously expose themselves to treachery of this sort, it 

 would be entirely wrong to imagine that commensalism is an inevitable 

 step in the development of the parasitic habit. It merely represents one 

 of a number of ways in which parasitism can arise. 



In the cases we have cited the prey is a small animal, which has 

 occasionally been able to turn the tables on the predator by becoming a 

 parasite. The more usual course of events is for the predator itself to 

 find the prey too large to kill but nevertheless it can feed upon it and be- 

 come permanently attached to it. This is undoubtedly one of the 

 commonest ways in which the parasitic habit has originated. Some 

 leeches, which have not developed a specialised taste for one particular 

 food, will kill any small animal they come across in their wanderings 

 in ponds and streams. They attach themselves to their unfortunate vic- 

 tims and suck them dry. If, however, a leech finds an elephant taking 

 a casual bathe in the river and can creep into its anus, the days of pre- 

 carious wanderings are over. However successfully and however long a 

 leech maintains this position it will certainly never suck the elephant 

 dry. Large size is fundamentally a bar to the parasitic habit. An 

 elephant's trunk, like the leech's sucker, may pre-adapt it to ecto- 

 parasitism, but it is clearly impossible for any large mammal to secure 

 a life of ease and plenty in such a manner. 



The rove beetles (Staphylinidae) have developed a wide range of 

 habits. The majority are saprophagous and swarm where there is dead 

 and decaying organic matter such as dung and corpses, but many are 

 predacious, both as adults and larvae. A large number of these beetles 

 are found exclusively in the homes of other animals. We can guess that 

 they were first attracted to this habitat by the concentration of animal 

 life or animal excrement and later became adapted to, and possibly 

 largely dependent on, the higher temperature characteristic of nests. 

 In the case of the species which favour the homes of birds and mammals 



