Muttkowski—The Fauna of Lake Mendota. 
431 
It will be seen that the decisive factor here is respiration. 
Unless the animal can escape or withstand the periodic anaero- 
biosis it is obviously barred. Once having adjusted itself to this 
condition the composition of the bottom will determine its hori¬ 
zontal distribution. Food will be of tertiary importance. 
If, bearing this in mind, we examine the macrofauna of the 
aphytal area we find that adjustment to anaerobiosis is effected 
by two methods: migration and aestivation. Migration ap¬ 
plies to pelagic forms like Corethra and Limnesia histrionica, 
which swim directly upward to oxygenated water; and to 
Sialis infumata, which retreats as the hypolimnion progresses 
outward and upward. 
Aestivation, coupled with the storage of oxygen, is the more 
common adjustment. Species like the Chironomids Chironomus 
tentans and Protenthes choreus, are able to store oxygen. 
Since the aphytal area adjoins the dysphytal area (here the 
shell zone), the fauna of the two are related. That is, the 
species of the aphytal area and the shell zone are alike to a 
certain extent, but there are more species in the latter; for quite 
a number of these have been unable to adjust themselves to the 
deoxygenation of the summer months. Although the shell zone 
marks the lower limit of wave action, this activity is too little 
to affect the composition in any other way than through its 
selective heaping of debris. 
DISCUSSION. 
Notwithstanding the fact that the lake taken as a unit is a 
place of relatively slight and slow changes, it contains within 
itself a series of well defined habitats, each with its own con¬ 
trolling factors and its own biota. For the lake as a whole molar 
agents through their infiuence on respiration are undoubtedly 
most decisive. But for the species established in the lake 
certain additional factors must be taken into consideration. 
For example, why should certain Chironomid species prefer 
plants as a substratum, others mud, and still others sand and 
rocks? Why should certain species, especially of the genus 
Tanytarsus, be extremely localized in their distribution? Of 
Tanytarus I know of four species in the lake, and five or six 
elsewhere, all of which are found in numbers in a very definite 
spot and nowhere else. Again, why should certain forms be 
