RELATION i<> "i IN r \ M \i 235 



through the nostril- and pupate in I In- -oil. The hor-e hot lly ( (> 

 pliilns cijui] Allies its eggs to tin- 1 bOFSCS, e-pe. i.dly OB it;' 



ml shoulder-, whence the larva- are lit ked oil ,md IWallowed ; once 

 in the stnmarh, the hot- I'aMen them-elve- to it- liiii ns of 



spei ial hooks, and withstand almost all efforts to dModu'e t hem; though 

 when the hots have attained their growth tip e their hold and 



pass \\ith tin- excrement to the soil. Hots of the Lremis II \f)ndcrma form 

 tumors on cattle and other mammals, domesticated or wild. The OX- 

 warhle (//. lineata, Fig. 213, 7) reaches the oesophagus of its ho-t in the 

 same manner as the horse hot, according to Curtice, but then makes 

 it- way into the subcutaneous tissue and causes the well-known tumors 

 on the hack of the animal; when full grown the bots squirm out of these 

 tumors and drop to the ground, leaving permanent holes in the hide. 



Parasitism in General. Parasitic insects evidently do not consti- 

 tute a phylogenetic unit, but the parasitic habit has arisen independently 

 in many different orders. These insects do, however, agree superficially, 

 in certain respects, as the result of what may be termed convergence of 

 adaptation. Thus a dipterous larva, living as an internal parasite, in 

 the presence of an abundant supply of food, has no legs, no eyes or anten- 

 nae, and the head is reduced to a mere rudiment, sufficient simply to 

 support a pair of feeble jaws; the skin, moreover, is no longer armor-like 

 but is thin and delicate, the body is compact and fleshy, and the diges- 

 -ystem is of a simplified type. The same modifications are found in 

 hymenopterous larvae, under similar food-conditions, except that the 

 head undergoes less reduction. The various external parasites lack 

 wings, almost invariably, and the eyes, instead of being compound, 

 ither simple or else absent.' In some special cases, as in a few 

 dipterous parasites of birds and bats, the wings are present, either 

 permanently or only temporarily, enabling the insects^o reach their 

 hosts. 



This so-called parasitic degeneration, widespread among animals in 



general and consisting chiefly in the reduction or loss of locomotor and 



sensory functions in correlation with an immediate and plentiful supply 



of food, results in a simplicity of organization which is to be regarded 



not as a primitive condition but as an expression of what is, in one 



, a high degree of specialization to peculiar conditions of life. 



This exquisite degree of adaptation to a special environment, however, 



tices the general adaptability of the animal, makes it impossible 



for a parasite to adapt itself to new conditions; and while parasitism 



may he an immediate advantage to a species, there are few parasites 



