32 MR. K. M. SMITH ON CERTAIN SENSE-ORGANS 



formula (consisting of Absolute Alcohol 6 parts, Glacial Acetic 

 Acid 1 part, Chloroform 3 parts), also Flemuiing's Fluid, Cor- 

 rosive Sublimate, and Bouin's Fluid. The one that gave the 

 best results was Carno3''s Fluid. 



The entire insects, or, in the case of large flies, the head only, 

 were left in this fixative for at least twenty-foiu' hours, and were 

 then washed thoi^oughly in 90 °l^ alcohol. The heads Avere cut 

 into longitudinal and transverse sections by the method of either 

 double embedding in Avax and celloidin or embedding in wax 

 alone ; for most purposes 1 found that wax alone suflficed. In 

 this latter case, after washing well in 90 "/o alcohol to remove 

 all traces of the fixative, the heads were left in absolute alcohol 

 for twenty-four hours ; they were then put into a mixture of 

 equal parts of absolute alcohol and chloroform, thence into pure 

 chloroform, thence into a mixture of chloroform and wax, and 

 finally into pure wax. The changes must be as gradual as 

 possible or shrinkage will occur. Much depends upon the time 

 the object to be cut is left in the wax ; the chitin will become 

 hard and brittle if left in the wax too long, and a similar result 

 occui's if not left in long, enough. ISTo definite time can be given; 

 it varies from two houi-s with fragile insects to twenty-four hours 

 or more with very large files with thick chitin, such as Asilids 

 or Muscids. 6 ^ was found to be a good thickness at which to 

 cut the sections, and all the drawings and photographs are made 

 from sections of this thickness. Various stains were tried, those 

 giving the best results being Delafield's Hpematoxylin, Heiden- 

 hain, and Heematein. Most of the figures are drawn from 

 sections overstained in Delafield's Ha?matoxylin and washed 

 out in Acid Alcohol. All the drawings were made at the level 

 of the microscope stage with the camera lucida at varying 

 magnifications. 



The great difficulty in carrying out this piece of work has lain 

 in the tendency of the chitin. and with the chitin the delicate 

 structures underneath, to break up under the razor ; therefore a 

 oood i'a,zor with a good edge is essential, and a wax of medium 

 hardness, melting-point 56° C, is the best. Owing to this 

 difficulty it is necessary to cut very large numbers of sections 

 and to select the best ; over three hundred slides had to be 

 prepared in order to obtain the results here given. 



The species of flies examined have been exactly determined 

 whenever possible. Owing, however, to various causes complete 

 identification of the material has not always been feasible, and 

 in some cases only the genus or famil}^ can be given. Acknow- 

 ledgments are due to Mr. 0. G. Lamb, to Mr, C. J. Wainwright, 

 Mr. F. "W. Edwards, and Rev. A. E. Eaton, for determining 

 certain species. I desire to thank Mr. Hugh Scott for much 

 useful assistance and advice, and Mr. Cecil Gunns, Chief Labo- 

 ratory Assistant at the Imperial College, for his valuable help in 

 making the photomicrographs. 



