362 MANirULATION. 



if large and transparent, will exhibit the minute ramifications 

 the best; for this purpose, after being slit open and well 

 washed, they should either be mounted in fluid, or placed 

 upon a slide to dry ; if care be taken in the mounting, they 

 will show very well in balsam. The best plan to pursue in 

 these cases, in order to prevent the balsam from entering tlie 

 tube, is to drop a little of it when warm upon the preparation, 

 and before it gets quite cold, to lay on the cover (with its 

 under surface heated), and to press it to exclude all the air 

 bubbles and the excess of balsam. When the entire tracheal 

 system is required to be dissected from the larva of an insect, 

 all the viscera should be taken out, the main trunks, with their 

 tufts of branches, wiU be then seen running down on either 

 side of the body ; and if care be taken in the dissection, the 

 whole system may be removed from the visceral cavity, and 

 laid out on a slide to dry previous to being mounted in 

 balsam. By far the most simple method of procuring a perfect 

 system of tracheal tubes from the larva of an insect, is to 

 make a small opening in its body, and then to place it in 

 strong acetic acid; this will soften or decompose all the viscera, 

 and the tracheae may then be well washed with the syringe, 

 and removed from the body with the greatest facihty, by 

 cutting away the connections of the main tubes with the 

 spiracles, by means of the fine-pointed scissors; in order to 

 get them upon the slide, this must be put into the fluid and 

 the tracheae floated upon it, after which they may be laid out 

 in their proper position, then dried a,nd mounted in balsam. 



The Spiracles require very little dissection, they may be 

 cut from the body with a scalpel or a pair of scissors, and 

 mounted either in fluid or in balsam; very beautiful examples 

 may be seen in the Dyticus marginalis, in the larvae of the 

 Blow-Jiy, and the Cockchafer, and other equally common insects. 

 Large tracheal vessels, when cut across transversely, will 

 sometimes exhibit the fibre unrolling, as is often seen in the 

 spiral vessels of plants ; but the two differ in this respect, in 

 plants the spiral fibre is situated within a membrane, whilst in 

 insects it is between two membranes. 



