PARASITIC ADAPTATION. 97 



infest the intestines of man and other animals. The degree 

 of decadence will depend upon the degree of dependence 

 upon the host. In this latter respect the parasitism may be 

 optional, as in the case of the mosquito, which may live upon 

 the juices of plants, but which prefers a meal of warm blood, 

 or it may be obligate, depending upon the body of another for 

 its means of subsistence, though such obligate parasites as 

 the biting flies, fleas, and certain household bugs, may also 

 live free and only occasionally visit their hosts, this form 

 being accompanied by little modification. In the event of 

 the parasite becoming progressively degraded into one which 

 not only seeks its host for food, but has become dependent 

 upon it for both its nutrition and place of abode, all of the 

 above mentioned phenomena of adaptation become more 

 conspicuous. We are furnished an interesting example of 

 such a transformation in the so-called sheep tick {Melophagus 

 ovinus), not a true tick, however, but a fly, which from an 

 occasional visitor has, like the louse, taken permanent abode 

 upon its host. No longer taking the aerial flight of its dis- 

 carded free life, this fly has become wingless, and, further- 

 more, is enabled to pass its entire life cycle upon the body of 

 the host animal by a remarkable method of reproduction 

 involving the retention of the eggs in the oviducts until 

 development has passed through the larval stage. It is not 

 until ready to pass into the stage of the pupa that these 

 are extruded, when the pupal case is attached to the indivi- 

 dual wool fibres. From this case the young insect, on becom- 

 ing sufficiently developed, makes its escape and proceeds to 

 feed and grow, thus rounding out a complete parasitic cycle. 

 While the easy life of the parasite tends to degeneration, 

 the perpetuation of the species becomes more precarious, and 

 the organs of reproduction undergo a strong development. 

 If a host animal dies, most of its parasites, especially those 

 existing in the interior of its body, die with it and, were it 

 not that the eggs find lodgment in a new host, the parasitic 

 species would in a short time become extinct. The transmis- 



