34 GNATS OR MOSQUITOES — CHAPTKJI II 



be very permanent, especially in hot climates, so that it 

 will be well to preserve a few slides with Tarrant's solution. 

 The slide should be first prepared by forming on it a 

 shallow cell of Hollis' liquid glue, of such size that the edge 

 of the cover rests on the middle of the cell-wall, leaving an 

 edge of the glue outside it. This should be allowed to set, 

 but not to dry hard before using, so that the edge of the 

 cover can be imbedded in the serai- solid material. When 

 all superfluous preservative has been drawn off with blotting- 

 paper, a ring of the glue is run round the edges of the cover 

 and the preparation set aside to dry. The sectionising of 

 perfect insects is even more difficult than that of larvae, 

 as unless the razor be exceptionally keen, it carries the 

 dense chitinous covering before it instead of cutting, and 

 so crushes the internal parts. 



There are, however, many structures alike in the larva 

 and adult insect which can be demonstrated in no other 

 way than by the method of serial sections, and as already 

 mentioned, there are especial difficulties in applying this 

 plan to animals with a chitinous integument. Thin as it 

 is in most of the species with which we have to deal, it 

 yet is apt to resist anything but the sharpest of razors, and 

 what is even worse, is well-nigh impervious to the entry 

 of preservative and other fluids, so that I have found it 

 quite impracticable to adopt the plan of staining en masse 

 either in the larva or adult insect, and this must hence 

 be done after the sections have been lixed on the slide. 

 After some experimentation I find that the following 

 method may be relied upon to yield satisfactory and well- 

 preserved preparations. For many of its details I am 

 indebted to suggestions from Mr. Allen, the Director of 

 the Marine Biological Laboratory at Plymouth, who has 

 recently been working on certain copepods, which are not 

 altogether dissimilar organisms, as far as consistence is 

 concerned. 



The larva or adult insect, as the case may be, is killed 

 by immersion in a solution consisting of two parts of 

 alcohol (90 per cent.) to one part of aqueous solution of 

 perchloride of mercury (1 per mille.), in a test tube, which 



