INTRODUCTION 19 



Parasites that are not wholly and manifestly constrained to 

 that mode of life ; and (c) Unconditioned Parasites that are 

 entirely and obviously dependent on parasitism for their 

 existence. 



The species that, whatever other direct effect they may 

 produce, affect the human organism indirectly, by introducing 

 other germs or parasites, may be broadly classified as (a) 

 Accidental and Casual Carriers, or Miastors ; (d) Qualified or 

 Adapted or Specific Carriers, that are essential to the exist- 

 ence of some particular species of endoparasite ; and (c) 

 Porters^ that help in distributing a particular species of endo- 

 parasite, but are not essential to its existence. 



A. ARTHROPOD PARASITES OF MAN. 



(a) Accidental and Casual Parasites. 



Under this head we may include (i) certain trespassers in 

 the alimentary canal ; (ii) certain trespassers in the tissues ; 

 and (iii) certain casual blood-suckers. 



(i) The larvae of many kinds of Arthropoda may get 

 into the human intestine, having been swallowed (either as 

 eggs or as larvae) in food, or having crawled in at some 

 natural orifice. The maggots of Eristalis (p. 144), of 

 Apiochceta (p. 145), of Sepsidce (p. 147), of several species of 

 MuscidcB (p. 153) and AnthomyidcB (p. 180), of Sarcophaga 

 (p. 179), the larvae of moths and beetles and mites, and even 

 young centipedes, have been recognised among such intruders ; 

 and as for most of them the human intestine is far from being 

 the normal and natural station, they cannot, even though 

 they may do harm, be classed as parasites. It is better to 

 call them Pseudoparasites. It must be remembered, however, 

 in the case of Apiochceta (p. 145) that the family to which this 

 species belongs {J^horidd) has in all stages a definite charnel- 

 house taint. 



(ii) The larvae of Sarcophaga (p. 179) and of several 

 species oi MuscidcB (p. 149) may get into neglected wounds, or, 

 gaining access by nostril or ear, may undermine the face and 

 burrow into the skull of living persons ; and the larvae of 

 several families of mites (e.g. Tronibidiidce, Tarsonemidce, 

 Tyroglyphidce^ may burrow into the human epidermis. 



