396 



GENERAL BIOLOGY 



2. Parasitism. 



Parasitism is a relation between two species that costs the 

 one its substance and the other its independence; the one 

 species is called host, the other parasite. 



The cost to the host species may be light or severe, 

 according to the extent of the parasitism. It is compara- 

 tively light in such case as that of the song sparrow that 



hatches the cow-bird's ^'g'g. The 

 latter is parasitic only to the ex- 

 tent of the rearing of her brood. 

 She deposits her ^gg in the nest of 

 the sparrow (as shown in figure 

 230), supplanting a sparrow ^gg 

 for the purpose, and leaves it 

 there for the sparrow to hatch, and 

 to feed through the nesting period. 

 The cost to the host species may 

 amount to personal discomfort 



Fig. 231. Downy flower gall 

 of the goldenrod. h, a gall 

 ami a flower head: i, a double 

 gall split open, showing the 



^S,?1hi^;;pf='o"T'ofrlc"oni3) merely, as in the case of many 



parasite in the other (right 

 hand) chamber. 



small external and internal para- 

 sites of the larger mammals — lice, 

 fleas, ticks, worms, etc. — or it may amount to loss of strength 

 or even oi life of many individuals. The host may be eaten 

 by degrees by a single large parasite, as is the midge that 

 makes the downy flower gall of goldenrod when parasitized 

 by the braconid shown in fig. 231 ; or it may be eaten by a 

 large number of smaller parasites, as is the caterpillar shown 

 in fig. 232. In any case parasitism is the burden of the host 

 species; but the manner of life of the host is little altered 

 thereby. 



vSuch is not the case, however, with the parasite, which, 

 according to the nature and extent of its dependence upon 

 the host species, becomes always more or less degenerate. 



