PARASITISM 7 



animal. Possibly there is some justification for those biologists who 

 consider these two habits of life — which are found everywhere and 

 which have been evolved as part of the general struggle for food and 

 shelter — as fundamentally distinct. 



In nature we find extremely varied and diverse types of parasitism. 

 It is an easy matter to point to a louse and say with confidence, "There 

 is a parasite." It is equally obvious that a golden eagle is a bird of prey. 

 On the other hand it is difficult to decide if certain larval water beetles 

 and leeches, which sometimes kill and eat their prey outright and at 

 other times merely abstract a little fluid from living victims, are 

 carnivores or parasites. It is also obvious that there is a wide gap 

 dividing the type of parasitism displayed by a worm which lives 

 permanently in the veins of a sea-gull, immersed in a perpetual food 

 bath of blood, and a female gnat which occasionally visits the gull, 

 punctures its skin and withdraws a few drops. 



Certain of these animals are described as obligate and permanent 

 parasites. The Protozoa which cause avian malaria, the tapeworms and 

 the feather lice, for instance, are compulsorily parasitic throughout 

 their lives. Tapeworms and Plasmodium live inside the bird (endo- 

 parasites), feather lice on the outside (ecto-parasites), but they are 

 always dependent upon their hosts and cannot live apart from them. 

 For these parasites, prey and environment merge and become one. 



Although so-called obligate parasites are at some period of their 

 lives dependent upon a host, many of them are able to spend long 

 spells in the free state and it is normal for them to do so. Ticks gorge 

 themselves with blood and then drop off the birds into their nests or on 

 to the ground and remain there until they have digested the meal. 

 Bugs hide in cracks and crevices or in the deeper layers of the nest 

 during the day, but steal out at night and feed upon their roosting host. 

 Leeches drop back into the water after engorging around the mouths of 

 cattle and horses drinking in ponds or streams. These animals are gener- 

 ally referred to as periodical parasites, whereas those which are dependent 

 on a host during one stage of the life-cycle only, are designated as tempor- 

 ary parasites. A good example of the latter type is a beautiful metallic fly 

 {Protocalliphora azurea) which as an adult insect hovers over flowers in the 

 sunshine and sips nectar and dew, but as a larva lives by sucking the 

 blood of nestling birds. Another fly, Camus hemapterus, is parasitic as an 

 adult but the larva is a scavenger and dung-eater in the nest. Many 

 Diptera (flies) can be included in both categories. Gnats, midges and 



