55 



Saw-flies, Gall-flies, Ichneumons, 



sides of the formicarium. The darkness occasioned by the screen leads 

 them to believe that they are working underground, at certain distances 

 from the surface, and thus induces them to construct many tiers of chambers 

 and connecting corridors within the range of practical observation. This 



FIG. 751. A convenient bell-jar formicary. The dish in which the bell jar stands is sur- 

 rounded by water held in the large zinc pan. 



we may judge to our satisfaction when, after a few days, the screen is with- 

 drawn for a short season, and the marvels of the constructive instinct of the 

 little people revealed to our wondering gaze." 



Janet, a distinguished French student of ant life, uses a block of porous 

 earthenware in which several little chambers or hollows have been made, 



FIG 



752. Plan of a Janet nest, o, opening covered by opaque cover, c; we, wet chamber. 



(After Janet.) 



connecting with each other by little surface grooves, the whole covered with 

 a glass plate, and over that an opaque cover (Fig. 753). Into a cavity at 

 one end of the block he puts water which soaks some distance along the 

 length of the block, thus rendering some chambers humid, while others at 



