1930 ] 
Jeivelled Caddis-ivorm Cases 
393 
and under the microscope the particles are seen to be in 
part crystalline (consisting of broken bits of highly refrac- 
tive quartz) and in part dull opaque, usually yellow, brown 
or dark colored, non-crystalline material. The transparent 
particles are all opalized to some extent, although some 
are much more brilliant or “fiery” than others. A count 
of the particles from two cases, after removing them from 
the silken network by immersion in a caustic potash solu- 
tion, shows that more than half are opalized to some extent. 
Thus in one case the ratio of crystalline and opalized par- 
ticles to dull ones is 102:68 and in another 380:222. On 
the other hand, the amount of material from which selec- 
tion is made by the larvae shows a far smaller, though diffi- 
cult to estimate, proportion of opal. 
Several caddis-worms are known to exhibit a considerable 
intra-specific variation in selecting the material for their 
cases, depending usually upon what material is within 
reach. Lloyd (’21) describes a clear-cut example of this 
in Limnephilus combinatus, where he found either shells 
or bits of bark used by the larvae in accordance with the 
availability of the two types of material. Here and in other 
species, e. g., in Phryganea vestita and Limnephilus sub- 
monilifer, such differences are to some extent associated 
with age, as the younger and older larvae build cases of a 
different structure, or again the larvae may migrate as 
they mature and leave the area where certain materials are 
available. Some species are furthermore quite indiscrimi- 
nate in their choice of building material. 
It would seem that the sense of discrimination of caddis- 
worms in general in selecting objects to attach to their 
cases is such that it may be entirely dependent upon a 
tactile sense coupled with an appreciation of weight. Even 
where crystalline sand is chosen and rounded grains dis- 
carded, the same is true. Thus, the almost exclusive use 
of small quartz crystals by some species need not involve 
vision, as the latter may be recognized by their sharp edges. 
In the present case vision must, however, form a neces- 
sary factor in the selection, as the brilliant opalized bits 
are not physically different from the white or slightly opal- 
ized pieces of quartz. All undoubtedly are of the same 
