^ RAP -DOOR SPIDERS. 107 



in the branched nest it has the upper surface con- 

 cave 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 tliey are specially adapted to meet 

 peculiarities in the curvature of the tube. 



* The lower door here, as in the branched nest (see above, p. 100), is some- 

 times united to the silk of the tube below by two nearly triangular gussets 

 of silk, when, instead of Ijeing free except at the hinge, as I have represented 

 it (Plate XII.), it is surrounded on either side by silk and only free at the 

 extremity away from the hinge. Tnis does not, however, alter the function 

 of this door in any way. 



It may be that these lower doors are always attached from below in this 

 way, but it is very dithcult to be sure of this, as they readily break away 

 from the surrounding silk, when they appear quite free, as in my drawing. 

 It was not until I adopted the plan of stuffing the tube full of cotton wool 

 before removing the suriounding earth that I detected this fragile attach- 

 meut. 



