1937 ] 
Case-building Instinct of Trichoptera 
129 
bits of pine needle ( Pinus Strobus ) , or in part of paper. The 
Molanna larvae built cases of assorted debris when no sand 
was present; they built cases of both sand and plant bits 
when both were present ; and were also able to make a case 
composed only of small pieces of broken glass. 
The method of construction and the form of the case in all 
observed instances depends on the species rather than on 
the type or scarcity of material employed. Our observations 
lead us to regard the modification in the behavior of a larva 
when supplied with insufficient or unusual material as rather 
slight. The animal adheres as closely as possible to its usual 
method of case-building. 
Dembowski, who studied only one species ( Molanna , 
though probably not the same species as that used by us), 
emphasizes the variability and plasticity of the animals’ 
activities under varied circumstances. Comparison of the 
behavior of various species, especially during the early steps 
of case-building when the larvae work rapidly, leads us to 
regard the variations within each species as slight and of 
little significance. 
A few tests were made with limnephilid larvae to deter- 
mine whether they would select short pieces of pine needles 
rather than longer ones which would require cutting by the 
larvae if they were to be used at all. In one test a larva with 
a case partly completed was given 50 bits of pine needle 
3.5 mm. in length (about the length normally used) and 30 
bits 11 mm. in length (a length greater than that of any of 
the pieces in any observed case built by larvae of this 
species) . A few days later there were only 19 short bits left 
in the dish though 25 of the longer ones remained. The larva 
had certainly discarded the long pieces except in a very few 
instances. Since there were no long pieces in the case it is 
clear that the animal must have cut up the missing 5 long 
pieces. In a similar experiment performed later on the same 
animal 3 out of 25 short pieces were unused and 14 out of 15 
long pieces were unused, that is, with but one exception, only 
the short pieces were accepted. 
If a larva which has no case at all is given 11 mm. bits of 
pine needle it joins the pieces and forms a girdle but the 
structure is too cumbersome and inflexible to permit its being 
wrapped around the animal. The larva will again and again 
make the motions of wrapping the chain about itself, and 
