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ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



spun a small mass of silk. In a few seconds this mass took on the 

 form of the tapering end of the cocoon, with five ridges showing 

 almost from the start. As the structure increased in length the 

 caterpillar backed away from it, the rear end of her body going 

 farther and farther off the other end of the carpet. When the 

 cone had widened to the full width of the cocoon an extra pair of 

 ridges was added, one on each side next the base, so that from now 

 on the walls had the regular seven-ribbed construction. The work 

 progressed very rapidly, the weaver being in no way hindered by 

 the fact that never before in her own life had she ever attempted 

 such a task. She needed no apprenticeship; right from the start 

 she knew her trade like an expert workman. Swinging her head 

 regularly from side to side, with an abrupt jerk toward herself where 

 each rib should be, the perfect structure advanced before her. There 

 was no suggestion of cumbrous human methods of putting up a 



Fig. 14. — A, diagram of the structure of the ribbed cocoon of Bucculatrix ; B, two cross- 

 wise strands of thread (a— e), showing how a series of loops to the right (6) and a 

 second set to the left (d) on the return make both network and ribs. 



skeleton of rafters first and then laying on the sheathing. The 

 whole went up together, the seven ridges not only keeping pace with 

 the intervening network, but extending a little in advance all the 

 while. 



The work was fascinating to watch and at first it is rather mysti- 

 fying to see such a complicated edifice grow at such a rate, but by 

 close observation the human eye can grasp the fine technique of the 

 caterpillar, and figure 14 will give the reader an idea both of the 

 cocoon structure and how it is made. A part of the network and its 

 seven ribs is shown at A ; the method of the weaving is more clearly 

 seen at B where only two consecutive, crosswise strands are shown. 

 The thread (a-e) starting to the right at <2, swings forward in an 

 abrupt angle at the point 5, then goes back and again swings for- 

 ward to the right in another corresponding loop, and so on till there 

 are seven of these advancing points. Then the thread turns on itself 

 at c and comes back in the opposite direction, looping, as at d, to 



