DISSECTION OF APHIDES. 193 



are in due position, the mica films are placed over 

 apertures in card, and then gummed papers, similarly 

 perforated, are pressed upon them. This arrange- 

 ment secures all in their places. 



Methods and operations in science, like events in 

 history, repeat themselves. Fifty years ago films of 

 mica were used to cover objects for the microscope, and 

 before the manufacture of the thin glass now so 

 commonly used, it admirably answered its purpose. 

 Under deep magnifying powers, such as -r^th of an 

 inch, it will be found even now of great service. The 

 mineral may be split by the lancet into films much 

 thinner than glass can be blown in a flat state. Small 

 unscratched pieces may be selected which are perfectly 

 transparent and their cost is quite trifling. 



On account of the high refracting power of Canada 

 balsam, the colours of recently immersed Aphides 

 show themselves very brightly; and it sometimes 

 happens that tints, quite lost through irradiation or 

 glance on the surfaces, become distinct by treatment 

 with this resin. 



The bright colours and markings of some species are 

 due to the hue of the internal juices of the insects. 

 These cannot be preserved by balsam, but it is other- 

 wise with the pigments which stain the somewhat 

 horny coverings of the thorax and abdomen. These 

 colours are persistent. 



10.— THE DISSECTION OF APHIDES 



In the dissection of Aphides, much assistance may 

 be often got by a selection of liquids. Some of these 

 are best suited for the purpose of hardening the 

 tissues, so that they may bear separation and tearing 

 asunder without their destruction. Others are used for 

 colouring the transparent organs, so as to make them 

 more visible. These organs of Aphides are so delicate 

 that pure water will in a great measure destroy them, 



vol. iv. 13 



