412 MAJOR W. S. PATTON. 



of domesiica, we find that the front of the former is at least half as wide as that of the 

 latter. Were this only the case in a few specimens, I should be prepared to accept 

 Dr. Hewitt's statement regarding the world-wide distribution of Mtisca domesiica, 

 but after examining himdreds of specimens of males of this tropical species, 

 and comparing them with typical males of domesiica caught in Great Britain, 

 which I assume are conspecific with the Linnaean species, I find that in not a single 

 specimen from India which I have examined is tlie front as wide as that of the male 

 domesiica. It is true that the width of the front of this form varies to a small 

 extent, and so does that of the t3'pical male domesiica, but on the one hand I have 

 yet to see a specimen of domesiica from Great Britain with a front as narrow as that 

 of this tropical form, and on the other hand a specimen of this tropical form with 

 a front as wide as that of the typical domesiica. 



I am quite prepared to find a typical male of domesiica taken at any of the tro]>ical 

 ports visited by ships from Europe, for everyone who has travelled in the East knows 

 that large numbers of domesiica are regularly carried to such ports, and leave the 

 ships on their arrival at their destinations. In this way it is possible Musca domesiica 

 has been carried to the ports of the world, and has now become established there. 

 But T must admit that the examination of the specimens in the National 

 collection from the localities mentioned b}- Dr. He^\'itt have not convinced me that 

 this is the case. 



It is true that Major Austen and other authorities have drawn attention to this 

 narrow-fronted male house-fly and have determined it as domesiica, L. But when 

 we find a form exhibiting a character which is constant, I think we are justified 

 in regarding it as distinct. And the width of the front in the males of the species 

 of this genus is a constant character, and one of the most rehable for purposes of 

 determination. In using this character it is ver\' necessary to point out that great 

 care must be taken in noting whether the front has collapsed, which it often does 

 in the case of specimens pinned too soon after hatching and before the chitin has 

 hardened. It should be remembered that just below the front there is an opening 

 through which the ptilinum is protruded and later invaginated. If the insect is 

 pinned before the edges of the ptilinal opening have sufficiently hardened, the whole 

 front is very apt to collapse inwards ; it then appears to be very much narrower 

 than it really is, and it is only when such a specimen is macerated in caustic potash 

 that the exact width of the front can be determined with accuracy. Further it is 

 important to note that in comparing the front of one specimen with another it is 

 very necessary to take into account their relative sizes. It is always better to com- 

 pare the width of the front with the entire width of the head, or with that of one 

 eye, rather than give it in linear measurement. 



It may be thought that the question regarding the true identity of this tropical 

 and subtropical house-fl}' is only of academic interest, and not worth further investi- 

 gation, but I may point out that the biological connection of particular species of 

 insects with certain disease-causing bacteria is now regarded as of a much more 

 specific nature than has been thought to be the case hitherto. As a good example, 

 1 may draw attention to the case of the tropical rat-flea, Xenopsylla cheopis, which 

 is the invertebrate host of Bacillus pesiis. Until recently it was thought that there 

 was only one species of the genus Xenopsylla found on rats in India, but a more careful 

 study of rat-fleas has enabled Mr. Charles Rothschild to separate the Indian cheopis 

 into three distinct species, one of which, X. astia, has undoubtedly been confused 

 with cheopis in the past. It is thought b}/ Cragg and Hurst that the absence of 

 plague in certain areas is due to the larger percentage of asiia found on rats as 

 compared with the numbers of cheopis. If this is the case, then we have a clear 

 indication that asiia is not a good host for the extracorporeal life of Bacillus pesiis. 

 In the same way there may be a much more intimate connection between certain 

 disease-causing bacteria and a particular species of Musca rather than with the others 



