BOTH THE SEXES IN DIPTERA. 



381 



lamp and let the preparations gradually cool, never letting them get dry, or air will 

 spoil them. (5) When cool, remove the superfluous turpentine with hlotting-paper ; and 

 as quickly as possibly (6) place Canada balsam and a smaller and thinner cover-glass on 

 the dissections. 



(7) This f-glass, when dry, can be mounted between three slips of cardboard, the upper 

 and lower punched with a circular hole (fig. 139), and the middle cut to the sbape of the 

 larger cover-glass (fig. 138). They can then be gummed together and placed in a press. 



(8) For permanently sealing both these preparations I recommend spirit-varnish. 

 This last preparation mounted thus can be examined on both sides with high powei's, 



which is necessary for a proper understanding of the mechanism and of the real shape 

 of the parts. 



Fig. 138. 



FiK. 139. 



Middle piece. 



Lower and upper piece. 



In many cases it will now be necessary to take a fresh insect, dissect out the penis, 

 and then undertake the still more difficult task of separating that into component parts, 

 proceeding to mount it exactly as in the foregoing case. 



The dissections can be done Avith an inch objective on a strong microscope with a flat 

 stage; a telescopic tube is a convenience, as it permits of a quick amplification of 

 magnitude. 



The insects which have been preserved in spirit only need to be thoroughly soaked 

 in water before dissection, but they will be found more brittle, and consequently more 

 difficult to handle, though, on the other hand, muscles will detach more readily. 



Dry specimens. — In the case of dried specimens, they may be placed in hot water for 

 half an hour before dissection, or soaked for a few hours in cold. 



Air-huhhles. — Air-bubbles can be driven to the side of the cover-glass by means of 

 heated needles (not red-hot, or they will crack the cover-glass), or a nail is better, as it 

 retains the heat longer. 



55* 



