be parasitic only in tlie immature stages — the hair- 

 worm larvae, parasitic in aquatic insects ; only the 

 adults parasitic — fleas, on birds and mammals ; or 

 both larvae and adults may be parasitic — the blood- 

 sucking lice and flies, biting lice, mites, and ticks 

 that occur on birds, mammals, and sometimes reptiles, 

 and the monogenetic trematodes on fish. Similar re- 

 lations obtain among endoparasites, although it is 

 more common to have all stages parasitic : entozoic 

 amoebae, trichomonad flagellates, opalinid ciliates, 

 si)oro2oans. ])entastoniids, nematodes, digenetic trem- 

 atodes, acanthocephalons, cestodes, and some cope- 

 pods. 



Animals may also be parasitic on plants. Nema- 

 todes infest the roots of plants. Galls are formed by 

 wasps or gnats especially on oaks, hickories, willows, 

 roses, goldenrods, and asters. Mites stimulate forma- 

 tion of witches' brooms in hackberry. A variety of 

 insects the larvae of which are leaf miners, wood 

 borers, cambium feeders, and fruit eaters, should be 

 included here. Plants themselves may be parasites 

 either on other plants or on animals. Bacteria and 

 fungi are among the most important disease-produc- 

 ing organisms in animals. 



Social parasitism describes the exploitation of one 

 species by another, for various advantages. Old 

 World cuckoos and the brown-headed cowbird of 

 North America do not build nests of their own ; 

 rather, they deposit their eggs in the nests of other 

 species, abandoning eggs and young to the care of 

 foster parents (Weller 1959). The bald eagle some- 

 times robs the osprey of fish that it has just caught. 

 One species of ant waylays foraging workers of an- 

 other species and snatches away the food they are 

 transporting : the robber species may deliberately rob 

 another nest of food. Some species of ants make 

 slaves of the workers of other species. Various other 

 types of dependency of one species on another have 

 evolved, not only between ants, but also in other 

 social insects, such as termites, wasps, and bees. So- 

 cial insects are apparently the only animals other 

 than man to have succeeded in domesticating other 

 species, and of cultivating plants, particularly fungi, 

 for food (Wheeler 1923). 



Evolution and adaptations 



The ancestors of ectoparasites were clearly free- 

 living forms. It is not difficult to imagine how a 

 small organism living freely in water or vegetation 

 could accidentally have settled on the outside of a 

 larger species and found conditions favorable for sur- 

 vival. There would even be selective advantage in 

 such a niche if the organism found a rich source of 

 food. The biting lice probably evolved from psocid 

 insects that live beneath the bark of trees. They may 



have transferred from this niche to bird nests and 

 then to the birds themselves. Most ectoparasitic in- 

 sects ])robabIy are derivatives of carnivores, sapro- 

 vores, or suckers of plant juices. 



Kndoparasites may in some cases have evolved 

 from ectoparasites ; more likely, they came directly 

 from free-living ancestors or from commensals. For 

 e.xam]3le, free-living nematodes and scavenger beetles 

 both feed upon decaying organic material, and it is 

 easy to visualize how the beetles could have acci- 

 dentally consumed one or more nematodes. Many 

 kinds which have since become parasites, such as 

 protozoans and flatworms, could have had their first 

 entrance into the alimentary tracts of prospective 

 hosts via drinking water, and subsequently invaded 

 other organs in the body. The invaders would have 

 found their hosts abundant food sources, but would 

 have needed some preadaptation to live at the low 

 oxygen concentrations characteristic of digestive 

 tract, to resist being consumed by the digestive juices 

 of the host, and to keep from being carried out with 

 the feces. As succeeding generations of parasites be- 

 came increasingly adapted to live either on or in their 

 hosts, many kinds lost the capacity for a free-living 

 existence. Specialization to internal parasitism has 

 cost the loss of locomotor, sense, and digestive or- 

 gans, none of which are needed, and led to the 

 development of organs of attachment, increased re- 

 productive capacity, and, in several forms, to polyem- 

 bryony, intermediate hosts, and a complicated life 

 cycle (Lapage 1951). 



Some parasitic species are more highly evolved 

 than others. Many parasites, for instance, pass their 

 entire existence in a single host ; others require one, 

 two, even three intermediate hosts. It is of ecological 

 significance that both primary and intermediate hosts 

 of a parasite occur iti the same habitat or community. 

 Even then the hazards to successful passage from one 

 host to another are so great and mortality so high 

 that prodigious quantities of offspring are produced 

 to insure that at least a few individuals will complete 

 the cycle. 



Parasites are transferred from one host to another 

 by active locomotion of the parasite itself; by in- 

 gestion, as one animal sucks the blood of or eats an- 

 other ; by ingestion, as an animal takes in eggs, 

 spores, or encysted stages of the parasite along with 

 its food or drinking water ; as a result of bodily con- 

 tact between hosts ; or by transportation from host 

 to host by way of vectors. As an illustration of vec- 

 tors, the bacteria that cause tularemia in man are 

 carried from rabbit to rabbit by ticks. Man contracts 

 the disease when he handles infected rabbits, but the 

 incidence of infection is greatly reduced in the autumn 

 when cold weather forces the ticks to leave the rabbits 

 and go into hibernation (Yeatter and Thompson 

 1952). 



Cooperation and disoperation 



179 



