NEST MAKING : ITS ORIGIN AND USE. 333 



The material which is fastened upon the internal silken sack consists 

 of particles of the food plant upon which the caterpillar is reared. These 

 are the stems and other rejected portions of the plant, left when feeding, 

 and which hang to the silken bag on the outside. They are sometimes so 

 thickly placed that the silken sack is entirely covered, as at Fig. 325, 

 which is a specimen from the Southern States in my collection of insect 

 architecture. 



One might extend these comparisons much further and find that the 

 striking resemblances between the protective architecture of spiders and 

 that of the larvae of insects might be carried to the very lowest 

 izo forms of life. Prof. Joseph Leidy, in his monumental work 

 upon the Rhizopods, 1 has presented numerous forms of these 

 creatures, that lie so far down in the scale of animated being, which at 

 once call to mind the habits of the caddis fly larva and the larva of the 

 house builder moth. Fig. 326 represents the Rhizopod, Difflugia urceolata, 

 a common form found in ditches in the neighborhood of Philadelphia. 

 Ordinarily the shell of this Difflugia strikingly resembles the ancient 

 Roman amphora. The body of the shell varies from a globular shape to 

 a more or less ovoid form ; the principal extremity or fundus is more ob- 

 tusely rounded, or more or less acute; and sometimes it is rounded and 

 more or less acuminate. The shell is composed, as is generally the case 

 in other species of the genus, of colorless angular particles of quartz sand, 

 mostly of larger ones scattered with more or less irregularity, while the 

 intervals are occupied with smaller ones. Frequently larger stones occupy 

 the larger shell ; but, passing this, they gradually become smaller, approach- 

 ing the edge of the rim or reflected lip. 2 



Another Rhizopod which suggests at once the architecture of the bag 

 worm is represented at Fig. 327. Difflugia acuminata is one of the most 

 common forms of Rhizopods, and is very generally distributed. Not un- 

 frequently, as in the figure, the shell is composed of colorless, chitinoid 

 membrane incorporated with quartz sand, alone or with this and intermin- 

 gled diatoms. In this the grains of sand are usually closely placed in jux- 

 taposition at and near the mouth of the shell, but are elsewhere scattered 

 and separated by wide intervals. In some cases the shell is more or less 

 covered with large diatoms, which are generally adherent in the length, 

 and diverge upward beyond the boundary of the shell. 3 



Not only do we find these striking resemblances in the external archi- 

 tecture of these widely separated creatures, but apparently we find the 

 same purposes originating the architecture. The house builder moth larva 

 constructs her thatched domicile in order to cover over its soft body ; 



1 "Fresh Water Ehizopods of North America," Washington, 1879. 



2 Op. cit., page 107, and pi. 14, Fig. 3. 



3 Leidy, Idem, page 111, pi. 13, Fig. 21. 



