14 ENTOMOLOGY FOR MEDICAL OFFICERS 



may always be found in company without, so far as can be 

 seen, influencing each other either for good or ill. Thus 

 certain small shrimp-like Crustacea of the genus Spongicola 

 are always found caged within living Basket-sponges. Or 

 two animals of different kinds may always be found associ- 

 ated, one of them getting obvious benefit, without, so far as 

 can be seen, either benefiting or incommoding the other. 

 Thus the small soft defenceless crabs of the genus Pinnoteres 

 always use the mantle-chamber of living Oysters and Mussels 

 as a fortress and refuge. Or two animals of different kinds 

 may live together for obvious mutual advantage. Thus a 

 Hermit-crab, Paguropsis typica, always carries — and carries in 

 a particular manner — a particular species of sea-anemone on 

 its back, using the sea-anemone as a blanket, which it can 

 pull on and off as it likes, and in return giving the sea- 

 anemone carriage to fresh pastures. Thus the mere fact of 

 association does not by itself imply parasitism, and this is 

 recognised by applying the term commensalism to such cases 

 as these. 



On the other hand one kind of animal may associate with 

 another, with the obvious intention (so far as the apparently 

 purposive actions of Arthropoda imply deliberation) of 

 getting what it can, regardless of any annoyance or injury 

 that it may incidentally inflict, but not to gratify any 

 imperative predatory instinct. Thus the house-fly and 

 blow-fly all the world over live in and around human 

 dwellings, and may in some circumstances be harmless or 

 even actually useful to their human hosts, though in most 

 circumstances they are objectionable and detrimental. 



Such a doubtful messmate as a blow-fly or a bluebottle 

 may perhaps treat a live man as if he were dead : it may 

 casually lay its eggs in the nostrils of a helpless or incom- 

 petent human being, and the maggots may live on the 

 tissues of the human being, quite like parasites. Here we 

 see how an almost innocent commensalism may pave the 

 way for parasitism. 



Next as regards the factor of aggression : An animal that 

 can support life quite well on the sweet juices of plants may 

 take to sucking the blood of other animals. This is the case 



