Page:Harvesting ants and trap-door spiders. Notes and observations on their habits and dwellings (IA harvestingantstr00mogg).pdf/149

 in the branched nest it has the upper surface concave and the lower slightly rounded, so that when drawn back and not in use it may not obstruct the passage. The sides of this lower door slope a little, so that the crown is smaller than the base; and this is important, because it causes the door to fit more tightly when driven upwards into the tube, acting on the principle of an inverted cork door.

In form this door is somewhat elliptic, but much broader and shorter than the second door of the branched nest, and it is frequently of a nearly horse-*shoe shaped outline. The second door of the branched nest is necessarily longer, having to perform the double function of closing the opening to the branch and the passage of the main tube.

In either case, however, these doors will be found to be more or less elliptic, and this is necessarily so, for, lying as they do when in use in a plane which cuts the subcylindrical tube obliquely, they have to fill a somewhat elliptical area.

I have observed some variation as to the exact proportions of these doors, and it is quite possible that in many cases they are specially adapted to meet peculiarities in the curvature of the tube.