APPENDIX TO CHAPTERS VI. TO IX. 349 



I then make a drawing of the external form of the embryo 

 or nymph, which is subsequently a great aid in the interpreta- 

 tion of sections, and transfer the preparation to 75 per cent., 

 and after an hour or two to absolute alcohol. 



In a week or ten days the preparation should be transferred 

 to the staining solution. I think it is advantageous to place 

 specimens intended to be stained with picro-carmine in a solu- 

 tion of picric acid in 50 per cent, alcohol for a few days, before 

 transferring them to alcohol. 



In preparing sections of the imago, it is necessary to cut the 

 insect into two or more parts, or to remove a portion of the 

 integument, and then to fix the tissues either in absolute 

 alcohol with ten to twenty drops of a solution of osmic acid to 

 half an ounce of alcohol, or to place them in a solution of picric 

 acid in 75 per cent, alcohol. Fixation by heat is inadmissible, 

 as the tracheae swell and displace and vacuolate all the tissues. 

 It is most important to bear in mind that specimens hardened 

 in picric acid must never be wetted with water subsequently. 



I have not found that such dilute osmic solution as I have 

 recommended prevents subsequent staining with hsematoxylin. 



For logwood-staining after cutting, I consider Miiller's fluid 

 far the best fixative for imagos. A week or ten days is suffi- 

 cient before transferring to alcohol. Such specimens must be 

 washed in large quantities of dilute alcohol, 50 per cent., to 

 remove the chromates. 



Whenever it is possible, young imagos should be used, as the 

 chitin of the adult is most difficult to cut. I have, however, 

 succeeded in cutting egg-bearing females without serious frac- 

 ture of the integument. To do this it is necessary to be very 

 careful that the temperature of the paraffin used for imbedding 

 does not rise even for a few minutes much above its melting- 

 point, and any such rise of temperature is always most destruc- 

 tive. Neither must the insects be kept in absolute alcohol more 

 than a few days previously. Such insects should, I think, 

 always be fixed with Miiller's fluid. I have found eau-de-Javelle 

 and eau-de-Labarraque, so much extolled, exceedingly destruc- 

 tive to the internal organs, so that they are quite inadmissible. 



