224 Aquatic Organisms 



the contents of the cells, one by one. They are very 

 inert-looking, stick-like, creatures and easily pass 

 unobserved. Of our two common genera one (Pelto- 

 dytes) is shown in figure 132. The body is covered 

 over with very long stiff jointed bristle-like processes, 

 giving it a burr-like appearance. The larva of the 

 other genus (Haliplus) is more stick-like, has merely 

 sharp tubercles upon the back, and has the body ter- 

 minating in a long slender tail. 



The Riffle beetles (Parnidse and Amphizoidse) prefer 

 flowing water. They do not swim, but clamber over 

 the surfaces of logs and stones. They are mostly small 

 beetles of sprawling form, having stout legs that 

 terminate in curved grappling claws. There is great 

 variety of form among their larvae, the better adapted 

 ones that live in swift waters showing a marked ten- 

 dency to assume a limpet-like contour. This cul- 

 minates in the larva of Psephenus, commonly known as 

 the "water penny." This larva was mistaken for a 

 limpet by its original describer. It is very much 

 flattened and broadened and nearly circular in outline, 

 and the flaring lateral margins encircling and inclosing 

 the body fit down all round to the surface of the stone 

 on which it rests (see fig. 160 on page 260). Under- 

 neath its body are tufts of fine filamentous gills, inter- 

 segment ally arranged. 



The flies (order Dipt era) are a vast group of insects. 

 Among them are many families w r hose larvae are wholly 

 or in part aquatic. The changes of form undergone 

 during metamorphosis are at a maximum in this group : 

 the larvae are very different indeed from the adults. 



Dipterous larvae are very diversified in form and 

 details of structure. The entire lack of thoracic legs 

 will distinguish them from all other aquatic larvae. 

 They agree in little else than this, and the general 



