352 GENERAL BIOLOGY 



gall midge of the willow, (fig. 203) in abundance (see appen- 

 dix). Prepared cross-sections of the thorax of old larvae 

 showing wing and leg buds. Cross sections of the thorax of 

 a damsel fly for comparison of fat development. 



The central cavity of the gall may readily be exposed by 

 sinking the end of a knife blade or scalpel lengthwise, 

 through the end of the stem in the base of the gall, and 

 twisting laterally, laying it open. Although the blood is 

 red, the grown larva will appear white, because it is filled 

 with opaque white fat. As the dissolution of the fat pro- 

 ceeds the red color of the blood will reappear. The progress 

 of the pupa in metamorphosis may, therefore, from the 

 first be gauged by the extent of the red color; later, as the 

 end of the pupal period approaches, the black pigmentation 

 of the adult will gradually overspread the surface, beginning 

 with the eyes. 



Sketch the pupa in outline, and make several rapid copies 

 of the sketch by tracing. Then color these with red and 

 black pencils (or with water colors, if preferred) to indicate 

 the external evidence of the internal changes. Show thus 

 the place of beginning and the order of progression in fat 

 solution, and later progress in pigmentation. 



Place a live pupa that is in the midst of metamorphosis on a 

 hollow ground slide in a drop of normal salt solution, and 

 split it lengthwise with fine scissors. Gently wash away 

 the dissolved interior with a little stream of normal salt 

 solution from a fine pipette and examine carefully the extent 

 and the appearance of the solid organs remaining. A like 

 treatment of a larva would disclose the wing buds and 

 leg buds appended to the interior of the body wall. 



Study and diagram a section of the thorax, showing wing 

 and leg buds ; show also the proportion of fat and solid tis- 

 sue; compare this with cross section of thorax of a damselfly. 



The record of this study will consist in the drawings and 

 diagrams suggested above. 



