368 Aquatic Societies 



2. Ordinary Foragers. — These are the members of 

 lotic societies that lack such specialized means of 

 gathering food from the passing current, and that forage 

 by more ordinary methods. They live for the most 

 part on the sides of stones and underneath them, and 

 not on their upper surfaces. These also live where the 

 water runs swiftly, and, for the most part, out of the 

 reach of those fishes that invade the rapids. There are 

 two principal categories among them: a. Free-living 

 forms that are more or less flattened or limpet -shaped. 

 b. Shelter-building forms, that are in shape of body 

 more like the ordinary members of their respective 

 groups. 



a 



The limpet-shaped forms are members of several 

 orders of insects, worms and snails. Th^rr flattened 

 form and appressed edges are doubtless adaptations to 

 life in currents. v They adhere closely, and are on 

 account of their form, less likely to be washed away; 

 the current presses them against the substratum. 



Not the most limpet-like but yet the best adapted 

 for hanging on to bare stones in torrents is the curious 

 larva of the net-veined midge, Blepharocera (see fig. 159 

 on p. 259), an inhabitant only of clear and rapid streams. 

 The depressed body of this curious little animal is 

 equipped with a row of half a dozen ventral suckers, 

 each of which is capable of powerful and independent 

 attachment to the stone. So important have these 

 suckers become that the major divisions of the body 

 conform to them and not to the original body segments. 

 On these suckers, used as feet, the larva walks over the 

 stones under the swiftest water, foraging in safety where 

 no enemy may follow. 



Most limpet-like in form of all is the larva of the 

 Parnid beetle, Psephenus, commonly known as the 



