PREFACE. 



The present work constitutes the first part of the studies on parasitic 

 Acari in which the author, Mr. S. Hirst, has been engaged for some 

 time. The representatives of the genus Bemodex are skin-parasites of 

 microscopic size. Although certain species give rise to troublesome 

 affections of the skin, others are not definitely known to be harmful. 

 I), folliculorum, a parasite which is so common that it has been stated 

 to occur in " practically every human being," is regarded as harmless by 

 the majority of the authorities who have studied it, although it has been 

 accused by other students of the group of being instrumental in carrying 

 certain grave diseases. Since these animals live embedded in the skin 

 of their host, their detection is not easy without practice ; but the 

 success of the author in finding them in Mammals in which they had 

 not previously been recorded, or had been supposed to be rare, makes it 

 appear probable that many species remain to be discovered. An accurate 

 knowledge of this group is highly desirable, as there are many instances 

 of parasites at first believed to be harmless which have been found, with 

 increased knowledge, to be extremely injurious to the health of their 

 hosts, either by their direct effects or by acting as the carriers of disease. 



The thanks of the Trustees are due to Messrs. Taylor and Francis for 

 permission to reproduce figs. 39, 41, 42 a, 46, 47 ; to the Belgian 

 Government, Lt.-Col. A. W. Alcock, F.R.S., Lieut. Leslie Linzell, and 

 M. II. van Saceghem for specimens ; and to Mr. M. A. C. Hintou for 

 assistance in connexion with the Mammalian hosts. 



SIDNEY F. HARMER, 



BKiTisn Museum (Natural History), Keener of Zoology. 



London, S.W.7. 

 Aj)ril 1, 1919. 



