Ordinary Foragers 



369 



"water-penny" (see fig. 160 on p. 260). It is nearly 

 circular and very flat with flaring margins that fit down 

 closely to the stone. It adheres closely and is easiest 

 picked up by first slipping the edge of a knife under it. 

 Viewed from above, it has little likeness to an ordinary 

 beetle larvae, but removed from the stone and over- 

 turned, one sees under the shell a free head, a thorax 

 with three short legs, an 

 abdomen and some minute 

 soft white segmentally 

 arranged tracheal gills on 

 each side. 



Other insect larvae that 

 have taken on a more or 

 less limpet-like form, are 

 the nymphs of certain May- 

 flies and of many stoneflies 

 (fig. Ill on p. 204). The 

 body is strongly depressed. 

 The lateral margins of the 

 head and thorax are ex- 

 tended to rest down on the 

 supporting surface. The 

 legs are broadened and are 

 laid down flat so as to 

 offer less resistance to the 

 currents, and stout grap- 

 pling claws are developed 

 upon all the feet. Such is 

 Heptagenia whose nymphs 

 abound in every riffle and 

 on every rocky shore. 

 One may hardly lift a stone from swift water and 

 invert and examine it without seeing them run with 

 sidelong gait across its surface, outspread flat, and 

 when at rest appearing as if engraven on the stone. 



Fig. 218. The nymph of a may- 

 fly (Heptagenia) from the 

 rapids, showing depressed form 

 of the body and legs. 

 (Photo by Anna H. Morgan.) 



