THE ART OF PARASITISM 



221 



another may not always be sharply drawn. However, for the sake of 

 convenience an attempt will be made to outline briefly a few examples 

 of such relationships. 



External Parasites 



External parasites are found throughout the plant and animal king- 

 doms. Even among the minute protozoa, ectoparasitic organisms 

 occur, such as Cyclochaeta, a parasite on fishes, which may cause an 

 appreciable economic 

 loss under epidemic 

 conditions. The lam- 

 prey eel among the 

 chordates is a large 

 external parasite on 

 certain fishes. 



For the sake of con- 

 venience, external par- 

 asites may be classified 

 as to whether they are 

 temporary, periodic, or 

 permanent. Some 

 forms, like the house 

 fly, do not really belong 

 in any of these cate- 

 gories. Yet the house 

 fly certainly deserves 

 mention, since it serves as a mechanical carrier from one host to 

 another for the transfer of numerous bacteria and their spores, as 

 well as the cysts and eggs of various other parasites. 



Temporary Parasites. As an example of temporary parasitism 

 may be mentioned the parasitic Hymenoptera that lay their eggs 

 on the eggs, larvae, or even the adults of other insects. During the 

 developmental interlude they remain as true parasites within the 

 body of the host until they eventually destroy it, at which time they 

 cease their parasitic existence and become free living. The ichneu- 

 mon flies, that belong in this group of parasitic Hymenoptera, each 

 year attack and destroy great numbers of injurious as well as some 

 beneficial insects. Another example of a temporary parasite is the 

 ox botfly, the free living adult of which attaches its eggs to hairs on 

 the legs of cattle. Upon hatching, the larvae penetrate the hide and 



American Museum of Natural History 



These brook lampreys are close relatives of a 

 larger form which frequently attacks fish and remains 

 as a temporary external parasite until the host is 

 destroyed. What type of mouth is characteristic 

 of this group ? 



