FAUNA OF A SMALL BROOK 
367 
larvae of the May flies and of the Dytiscid beetles form excellent 
examples of animals of this class which do not have to come to 
the surface for breathing. The immature stages of the Mosqui- 
toes and the adult Dytiscid beetles both play an important part, in 
this association but each of these is so constituted anatomically 
that it must rise to the surface at intervals to renew its supply 
of oxygen. 
The transition zone association. — The transition zone associa- 
tion is characterized by those forms the distribution of which is 
limited to that area included in the margin of the brook — ^the 
edge of the water itself and the immediate banks of the stream. 
The vegetation of this region consists for the most part of semi- 
aquatic and mesophytic plants. A great abundance of vegetable 
debris is usually to be found. The mudflats occasionally found 
at the edge of the stream contain the burrows of Heterocerid 
beetles. Spiders crawl about on the vegetation and make excur- 
sions out over the water. Staphylinid beetles were taken from 
partly submerged decaying leaves. Frogs, while they may be 
found temporarily in any of the associations, must receive dis- 
tributional classification with the transitional zone forms. Ex- 
aminations of the stomach contents of Bana pipiens Sch., taken 
from Butcher’s Run show that their food supply comes partly 
from forms that have aquatic habits during at least a part of 
their lifetime. The air above the brook at times fairly swarms 
with the adults of the May fly and Simulium or Buffalo gnats. 
Craneflies were taken from the vegetation near the edge of the 
water. 
CATALOGUE OF THE FORMS TAKEN. 
In the foregoing discussion of the various associations of ani- 
mals afforded by the brook only such forms were mentioned as 
were eonsidered particularly characteristic of each of those as- 
sociations. The following is a catalogue of those forms of animal 
life taken in this rather superficial survey, with merely enough 
reference to their habits and life history to account for their 
presence in such a brook as that under observation. The table 
on page 373 presents much of the same data in condensed form. 
Platyhelmmthes.—T'he flat worms were represented by the 
fresh-water Tricads in the form of Planaria sp. These were ob- 
served crawling about on the muddy bottom of the shallow stream 
near its source. They were also taken frequently in dredging 
the larger pools. At the time of the first observations, April 10, 
