Parasitism 387 



contained dissolved minerals. Mistletoe plants may be definitely in- 

 jurious to the tree on which they grow, but, since they are abundantly 

 supplied with chlorophyll, they photosynthesize their own carbo- 

 hydrates. Blood-sucking bugs, flies, and leeches and vampire bats 

 may be in contact with their hosts for only short periods, and, al- 

 though such animals are commonly regarded as parasites, their mode 

 of life is clearly on the borderline between parasitism and predation. 

 Little basis exists for considering mosquitoes, which live on the blood 

 of animals, to be parasites and not so considering aphids, which suck 

 the juices of leaves. But if aphids are regarded as parasites, their use 

 of the material of a living plant is no difterent in principle from that 

 of caterpillars which eat the leaf substance from the outside as herba- 

 ceous predators— or from that of deer which browse on the same leaves 

 from the ground. Many partial parasites can derive nourishment 

 from a wide variety of host species. These considerations bring us 

 to the conclusion that every gradation exists between obligate para- 

 sitism at one extreme, in which the parasite is completely dependent 

 upon a living host with which it is permanently associated, and preda- 

 tion at the other extreme, in which a free-living predator catches, 

 kills and devours its prey. 



Many kinds of plants and animals have taken up a completely para- 

 sitic mode of existence. Although the largest representation comes 

 from the lower organisms, certain members of more advanced groups 

 have also resorted to parasitism. Plant parasites, mostly fungi and 

 bacteria, may attack animals or other plants. Animals that parasitize 

 other animals are found in the Protozoa, in various other invertebrate 

 groups, and rarely in vertebrates. Animals that parasitize plants are 

 represented by gall wasps and gnats. The eggs of these insects are 

 commonly laid on stems and leaves, and the activities of the young 

 cause the formation of galls on the plant host. No organism is known 

 that is not susceptible to attack by parasites of some sort. 



Some parasites are restricted to one host species, or to one type 

 of host, whereas other parasites can attack widely different plants and 

 animals. Parasitism may even occur within a species. For example, 

 in the deep-sea angler fish, PJwtocoryniis spiniceps the male lives as 

 a tiny permanent parasite upon the head or side of the female and ob- 

 tains his entire nourishment from her blood supply. Thus does this 

 species solve the problem of the location of one sex by the other in 

 the inky blackness of the deep sea. This type of intraspecific relation 

 occurs frequently in the plant kingdom, as seen, for example, in the 

 growth of the pollen tube— representing the male plant— as a "parasite" 

 on the tissues of the stigma and style of the flower of an angiosperm, or 



