Mnttkowski—The Fauna of Lake Mendota. 
421 
offer as little resistance as possible. This means that the animals 
have a flattened body, or that they enclose themselves in flat 
structures, or that they apply appression methods. In addition, 
they must be supplied with strong clinging devices. 
The mayfly nymphs Heptagenia interpunctata, Ecdyurus 
maculipennis, and Siphlurus alternatus, and the Odonate nymph 
Argia putrida offer good examples of depressed body structure. 
Depression in a secondary sense is attained by Psephenus 
lecontei and a Parnid larva in that the segments of the body 
are supplied with lateral extensions, which are very efficient in 
permitting the animals to appress themselves. Compression 
laterally is again the means by which the Amphipods Hyalella 
azteca, Dikerogammarus fasciatus, and Gammarus limnaeus 
adapt themselves; however, these belong to the category of mi¬ 
grants or visitors. 
Caddis-flies form a characteristic element of the petrophilous 
fauna, but their submission to conditions is shown more in the 
structure of their protective eases than in their own morphology. 
Thus, the flat, Ancylus-like case of Leptocerus ancylus forms 
one of the flnest instances of shore adaptation. Leptocerus 
dilutus usually seeks some tiny depression in a rock in which it 
anchors its curved, hooded cases. Similar in character to Lep¬ 
tocerus ancylus are the flat leech cocoons which are numerous 
under the shore rocks. 
A number of petrophilous species meet the factors by other 
types of appressed structures. Thus the tubes of Chironomids, 
especially of Chironomus digitatus and C. lobiferus, are quite 
common on the underside of rocks, where they are invariably 
built of sand granules and firmly anchored by means of some 
secretion. A more marked instance is that of Polycentropus sp. 
which weaves a funnel-like tube and over this spreads a broad 
sheet of webbing. Appression, too, is the growth method of the 
stone sponges and bryozoans. All of these forms are equipped 
with strong clinging devices, in the way of claws, suckers, cement¬ 
ing substances, and minor adaptations. 
During certain months of the year the fauna among the 
Cladophora plumes is quite abundant. It consists of the cad- 
disworms Hydroptila sp., Ithytrichia clavata, and Agraylea 
multipunctata, and occasionally an Oxyethira sp., which feed 
on Cladophora. Chironomus lobiferus may frequently be found 
in cases built of algal filaments. Still more marked is the pres- 
