906 FRESH-WATER BIOLOGY 



ones, are strongly depressed in form, and have flaring lateral mar- 

 gins to the body segments that fit down closely against a stone, 

 limpet-like, to withstand the wash of the current; hence, these 

 are able to maintain a footing in the swiftest waters. The common 

 "water penny," the larva of Psephenus lecontei, illustrates the 

 extreme of flattening; this larva has developed abundant tracheal 

 gills from the thin membrane between the body segments, and these 

 are completely covered over by the projecting lateral margins of 

 the body segments. The adult female Psephenus crawls down on 

 the lee side of a stone and deposits her yellow eggs in broad one- 

 layered patches on its surface. 



The Gyrinidae or whirligig beetles constitute a small group of 

 strictly aquatic forms, very peculiar in structure and habits. They 

 are well known to every one as shining black beetles of oval form, 

 that gather in companies upon the surface of brooks and ponds and 

 glide about in irregular curves with a speed which the eye can hardly 

 follow. When captured they exude a whitish repugnatorial fluid, 

 having a rather disagreeable odor. They hibernate as adult 

 beetles in the mud and in their season of activity they spend much 

 time beneath the water, in which they can dive and swim dextrously. 

 Their fore feet bear hooked claws with which they can cling to the 

 bottom when desiring to remain beneath the surface. They are at 

 once distinguishable from other water beetles by the unusual 

 brevity and peculiar formation of the hind legs, and by the 

 possession of divided eyes, there appearing to be one pair above 

 for vision of objects in air when the beetle lies on the surface, and 

 one below, presumably, for seeing things in the water. 



The larva of the gyrinids is elongate and slender, and possesses 

 at the tip of the abdomen two pairs of backwardly directed grap- 

 pling hooks, and long slender paired filaments arranged segmen tally 

 along its sides somewhat like those of the Neuropterous genus 

 Sialis. Both larvae and adults are carnivorous. The larvae possess 

 long perforate sickle-shaped mandibles well adapted for punctur- 

 ing the skins of soft midge or other dipterous larvae, etc., and for 

 sucking out the fluid content of their bodies. The pupae of the 

 Gyrinidae are formed in thin cocoons attached to the side of verti- 

 cal plant stems above the water. 



