ORIGINS OF parasitism: evolution of parasites 49 



can eventually result in a change to an endo-parasitic mode of life. It 

 is not, of course, suggested that all internal parasites have passed 

 through an ecto-parasitic stage. 



The females of many animals are predisposed to take this route, for 

 they frequently seek sheltered nooks and crannies in which to breed. 

 We find that the females of the roe-deer flea {Dorcadia dorcadia), for 

 example, are fixed permanently inside the nostrils of their host, but the 

 males are free and wander about over the whole body. 



One species of black-fly (Simuliidae), of which the males are not 

 parasitic at all, mates in the ears of its host — for where the female 

 leads the male follows and in due course both sexes may take to an 

 endo-parasitic life. 



Numbers of temporary insect parasites are only parasitic in the 

 female sex — the males feeding romantically on dew and nectar. In 

 many cases the development of fertile eggs has become dependent on a 

 blood meal and so tied the species to a vertebrate host. 



There are multiple ways in which parasitism can arise and in fact 

 there is only one vital element in the genesis of a parasitic relationship 

 and that is opportunity. Flies are continually, although unwillingly, 

 brought into contact with spiders and it comes as no surprise to find 

 a group of flies (Cyrtidae), which in the larval stage parasitises spiders. 

 Ducks eat leeches with extreme relish and there is one case on record 

 when some of these birds arrived overnight at a leech farm and by 

 breakfast time had devoured the entire stock of 20,000 leeches. Never- 

 theless we find that at least one species of leech parasitises ducks. There 

 is an even more curious situation existing between certain birds and 

 mosquitoes. Swallows, for example, devour them by day and by night 

 are devoured by the insects. 



The most favourable condition, therefore, for the dawn and develop- 

 ment of dependence is a social environment and it is in crowded 

 communities, whether of birds or ants or men, that one finds parasitic 

 relationships developed most consistently. Here the scene is set, the 

 dangerous opportunity is ever present, and it is merely a question of 

 time before one of the organisms concerned exploits the situation. It 

 may thereby obtain some advantage, however transitory, which starts 

 it upon a course of irreversible specialisation — the risky road to 

 dependence. 



A commensal relationship is potentially even more dangerous than 

 a merely social tie, for by nature it is more intimate. The closer the 



