438 THE POPULAR SCIENCE MONTHLY 



ever, the parasite quickly destroys it by consuming its vital tissues. 

 It then completes its own growth, pupates and eventually emerges as 

 a very active, highly organized and beautifully colored fly, provided 

 with a splendid nervous system, exquisite sense-organs and powerful 

 locomotor organs in the shape of legs and wings. It is either a male 

 or a female and, if of the latter sex, soon proceeds to place its offspring 

 in immediate contact with the host. Although the larval Ichneumon 

 exhibits modifications of structure almost as extreme as those of the 

 adult Sacculina, these produce no effect on the organization of the 

 adult insect. The association of the larva with its host is the work of 

 the mother insect, a creature gifted with complex instincts that enable 

 her to ferret out the host even in the most intricate concealment. The 

 large size and small number of her eggs and her highly specialized 

 method of oviposition indicate very clearly that chance, which plays 

 such a role in the life-cycle of the tape-worm and Sacculina, has given 

 way to an almost inevitable association of the parasite with its host. 



Of course, the Ichneumon represents only one of many forms of 

 parasitism among insects. I have chosen it because it is the most 

 characteristic and most highly specialized. There are insects like the 

 Strepsiptera and the Ehipiphorid and Meloid beetles which seem to 

 combine the Sacculina with the Ichneumon type in that they produce 

 many small eggs that hatch as very active triungulin larva? and only 

 later develop into legless, bag-like larvae of the Ichneumon type. It is 

 interesting to note that in the Strepsiptera the adult female prolongs the 

 parasitic habit of the larva, while the adult Meloidae or oil-beetles are 

 rather sluggish and seem to show other after-effects of their larval life. 

 There are also many insects, like the true lice and bird lice which are, 

 to all intents and purposes, permanent parasites comparable with the 

 ectoparasitic flukes, though they never exhibit such extreme modifica- 

 tions. And, finally, there are other animals besides insects that have 

 parasitic larval and free adult stages, e. g., the fresh-water mussels. 7 



Zoologists have naturally been deeply impressed by such wonderful 

 parasites as the tape-worms, flukes and Sacculina and have regarded 

 these as fine examples of degeneration or degradation. Many, indeed, 

 have dwelt on these words in a manner which leaves no doubt that they 

 are used in a purely anthropomorphic sense as implying deterioration 

 or " an impairment of natural or proper qualities " in the parasites, 

 notwithstanding Eay Lankester's assertion that " degeneration may be 

 defined as a gradual change of the structure in which the organism 

 becomes adapted to less varied and less complex conditions of life." 8 

 It is easy to trace the source of this anthropomorphism to the atrophy 

 of the parasite's neuro-muscular system, a system by which we as intel- 



7 Cf. Lefevre and Curtis, "Reproduction and Parasitism in the Unionidse, " 

 Journ. Exper. Zool., IX., No. 1, 1910, pp. 79-115, 5 pis. 



8 " Degeneration, " p. 32. 



