CONSTRICTION OF TWIGS BY THE BAG WORM. 178 
per square mm. of surface, S equals the strain which the 
band is capable of exerting tangentially; or, in other 
words, the strength of the band which resists tearing, 
r equals the radius of the twig around which the band is 
fastened, and w equals the width of the band. A num- 
ber of microscopic examinations were made to determine 
the variability of thickness in the band, which ought to be 
included as a factor in the above formula. The variation 
was found to be so small, however, that for the purpose of 
the experiment the thickness was left out of consideration. 
Several hundred twigs from which bag worms were sus- 
pended were collected in the spring of the year from shade 
trees in and near the Missouri Botanical Garden. The tree 
upon which the bag worm is most commonly found in St. 
Louis is the soft maple, and the bags found on this tree 
were made the chief subject of study. Another reason for 
go doing was that the bands found on the maple twigs were 
usually of a sufficient size to permit being measured, while 
those on other trees, and particularly on the conifers, were 
usually so small that with the apparatus at hand it was im- 
possible to make exact determinations. The method which 
was used to determine the strength of the bands was as 
follows: With a sharp knife the bands were cut as near to 
their union with the bag as possible; the bag with the band 
was then removed from the twig without difficulty, and a 
second cut was made in such manner that the band was 
separated entirely from the bag. A flat sheet was thus 
obtained, varying from 6 to 12 mm. in length. After the 
band had been thus removed, a careful measurement was 
made of its width and of the diameter of the twig from 
which it was taken. After numerous trials, an apparatus 
was constructed consisting of a framework, from the top 
of which a pair of pliers was suspended, with the jaws 
pointing downward; a similar pair of,pliers with the jaws 
pointing upward was arranged s0 that a scale pan was 
attached to its handles. In both cases the pliers were so 
