stough: the iiackberry psylla. 163 



erine. Xylol was a very clear medium, having the advantage 

 that uncleared specimens could be dissected in it, soon becoming 

 clear, but it evaporated very quickly, and, lacking viscosity, al- 

 lowed too much movement of the parts being dissected. Glyc- 

 erine was found to possess the necessary viscosity and was the 

 most satisfactory medium in most cases. 



The nymphal wing pads were removed with fine scissors, 

 holding the nymph the while between the finger-tips ; the pads 

 were mounted in dilute glycerine after being kept in formalin 

 for about twenty-four hours. 



Serial sections were made through the mouth parts and geni- 

 talia, and cross, longitudinal-horizontal and longitudinal-ver- 

 tical sections through the entire insect. 



Killing and Fixing. — Live insects were killed by plunging 

 into a boiling saturated solution of corrosive sublimate con- 

 taining about one per cent of acetic acid, and after being 

 punctured with a piece of glass drawn out into a fine point 

 were left in the solution to fix for twenty-four hours. 



Dehydrating and CleaHng. — The insects were placed in 

 small vials with ends covered with cheesecloth, and run through 

 alcohols of percentages 70, 85 and 95, twenty-four hours in 

 each. They were removed to absolute alcohol for about twelve 

 hours and then run into xylol for about twelve hours. 



hifiltration and Embedding. — Leaving the specimens still in 

 the vials they were infiltrated in a paraffin oven with paraffin 

 of melting-point 50-55" C. for forty-eight hours. They were 

 then embedded in paraflfin in small paper dishes, being oriented 

 properly as the paraffin hardened when the dish was floated on 

 cold water. 



Sections. — Serial sections 10-20 microns thick were cut on a 

 sliding microtome. The ribbons were floated onto Mayer's 

 albumen fixative on a clean slide, the excess of fixative drained 

 off" with filter paper, and the slide dried on top of the paraffin 

 oven for twelve or more hours. The slide was then left for 

 ten to fifteen minutes in xylol to remove the paraffin, when it 

 was run through the following grades of alcohol, five to ten 

 minutes in each : 95 per cent, 85 per cent, 70 per cent, 50 per 

 cent, 35 per cent, 25 per cent, and then into water, after which 

 the sections were stained in Mayer's Carmalum, ten to fifteen 

 minutes, washed in water and run back through the alcohols, 

 beginning with twenty-five per cent, into absolute alcohol, five 



