PUBLIC 

 HEALTH 

 LIBRARY 



PREFACE. 



The appearance of a volume of this form needs, I feel, 

 some explanation. When first I expressed the intention of 

 publishing the results of my study of the House-fly in parts 

 as they were completed, it was suggested to me that, as the 

 separate parts of the monograph would appear at different 

 times and therefore in different volumes of the Quarterly 

 Journal of Microscopical Science, a useful purpose would be 

 served if, on the completion of the work, I were to have the 

 separate parts bound and published in volume form. Such 

 is my apology for the appearance of what, obviously, is 

 by no means a perfect production from a publisher's stand- 

 point. Sir Ray Lankester, K.C.B., the Editor of the 

 Q.J.M.S., kindly allowed me to obtain two hundred copies 

 of the letterpress and plates of each part, the three parts 

 appearing in 1907, 1908 and 1909 respectively. The number 

 of copies of this edition of the monograph is, therefore, very 

 limited. The Manchester University Press has been good 

 enough to undertake the re-publication of these reprints in 

 a volume, and has given me an opportunity of adding some 

 fresh introductory matter as well as appendices giving 

 additional facts and a resumd of such work as did not strictly 

 come within the scope of the monograph; there are also 

 included certain matters of practical importance. My 

 thanks are due to the Press Committee for overlooking the 

 clumsiness of the form in its wish to make my work more 

 accessible to students of the subject. In view of the foregoing 

 facts I feel that my readers will pardon the defects of 

 appearance, such as the numbering of the figures, etc., which 

 are the inherent drawbacks of the parts not being immediately 

 consecutive to each other at the time of their original 



