132 



ZOOLOGY 



worms. The fleas are transient parasites, as are many other 

 insects which are free in the adult stage but lay their eggs in 

 or on the body of the host where they undergo partial develop- 

 ment as parasites. In other instances the parasite must spend 

 its whole life in the body of one or more hosts. These are 

 called permanent parasites. 



Fig. 63. 



Fig. 63. — Caterpillar of Samia cecropia parasitized. From Lugger. 



Questions on the figure. — Seek in your reference literature all figures and refer- 

 ences to caterpillars attacked by parasites. Why would caterpillars be rather 

 favorable hosts for parasites? What are a few of the parasitic enemies of cater- 

 pillars? What economic importance has this phenomenon? 



In addition to the drain on the resources of the host, the 

 presence of the parasite may so irritate the tissues of the host 

 as to produce abnormal growth and disease therein. In many 

 of the transient parasites the life of the individual host is of 

 no consequence after the end of the period of parasitism and 

 hence the entire destruction of the host's body may occur just 

 as truly as in the ordinary preying species. Very profound 

 modifications occur in the structure of the parasite, which are 

 the outcome of, and in part an adaptation to, the special mode 

 of life. There is usually a degeneration of the organs of di- 

 gestion, of motion, and of sensation, since the parasite de- 



