yUiv. 1900.] Mercer : Development of Wings in Lepidoptera. 3 



In moulting, the cuticle splits down the dorsal line of the first and 

 second thoracic segments. During the latter part of the last larval 

 stage the caterpillar ceases to eat and prepares to hang itself up for 

 the pupal life. The approach of this event may be detected from the 

 red color of the excrement, which is supposed to come from the cast- 

 off cells of the epithelial wall of the alimentary canal. At the same 

 time the caterpillar is searching about for a suitable place to hang 

 itself up. It soon forms a button of silk to which it holds with its 

 caudal prolegs and at once begins its girdle of silk over the thorax, 

 fastening the ends to its support on either side of its body. The cat- 

 erpillar is now shorter and changed in form somewhat as a preparation 

 for i)upation, which will take place in about 24 hours. 



Throughout this discussion the term prepupal period is applied to 

 the time the caterpillar is hung up before pupation. This period is 

 characterized by the fact that the wings are outside of the body wall 

 although beneath the larval cuticle, the change in position taking 

 place while the caterpillar is spinning the silken support for the pupa 

 stage. 



Technique. 



The caterpillars were killed in hot water, small caterpillars being 

 left in one-half minute, and large ones one minute. 



Perenyi's fluid gave the best results as a fixing reagent. In fixing 

 for wings the caterpillars were cut caudad of the third thoracic seg- 

 ment, and also the head was removed before placing them in the fluid. 

 The chitinous coat that covers insects is very hard to penetrate, there- 

 fore to get a good fixation the greatest care is necessary. A large 

 amount of the fluid in proportion to the caterpillars was used, the 

 thoracic segments of from six to ten caterpillars being placed in from 

 30 cc. to 50 cc. of the fluid. The fluid was heated to 50° C, and 

 the caterpillars transferred directly from the hot water and left from 

 8 to 12 hours in the fixing fluid. They w^ere then hardened in suc- 

 cessive grades of alcohol as follows : 



70% Alcohol 24 hrs. 



80% " 24 " 



95% " 24 '' 



Absolute alcohol . . . . • 12 " 



Absolute alcohol and cedar oil, equal parts, 12 hrs. 



