53 



Lattin: Introduction 



nets and even kitchen strainers make good collecting 

 tools in streams and ponds. 



Intro, figure 736 shows a Needham net that can be 

 used in areas where weeds are thick. The coarse 

 screen on top permits the insects to enter but keeps 

 out most of the vegetation that would foul the net. 

 The Needham scraper (intro. fig. 73a) is used to 

 sample the bottom fauna. It is extended and dragged 

 back to shore along the bottom, and helps to increase 

 the collecting range beyond the depth possible in 

 hip boots. Although less efficient, the common garden 

 rake can be used to drag in large masses of weeds 

 that often contain many insects. 



Miscellaneous methods. — There are many unusual 

 or highly specialized methods of collecting for partic- 

 ular purposes. In bogs, for example, sphagnum is 

 pressed down, and the insects are floated out into 

 open water where they may be netted with ease. Tree 

 holes that hold water may be collected by siphoning 

 with a short length of hose. Mosquito larvae may be 

 readily collected from just under the surface film 

 with a hand dipper. As the edge of the dipper is 

 tipped below the surface, water enters and with it 

 the larvae; specimens may then be taken up with a 



pipette or eyo dropper. Mosquito Larvae arid, in fact, 

 most aquatic insects ma) be washed into a shallow 

 enamel pan from nets, trash, dippers, and BO on, and 

 picked at Leisure with forceps <>r a pipette. During 

 the sorting process, whether from the drying surface 

 of net, screen, or trash, or from a pan, individual 

 specimens are placed in vials containing 70 per cent 

 ethyl alcohol. If the collector is a specialist he ma) 

 sort specimens to species or higher groups b) placing 

 them in separate vials. The general collector usuall) 

 segregates only by locality or habitat and sorts his 

 material later. 



QUANTITATIVE SAMPLING METHODS 



One of the greatest needs in aquatic biology toda) is 



improved methods for sampling populations of insects. 

 All existing techniques are inadequate or impractical 

 for purposes of statistical analysis but in spite of 

 obvious shortcomings, they are the best wo have. 

 Therefore standard sampling devices and some spe- 

 cialized equipment and techniques are described below. 

 Bottom sampling devices. — The Ekman dredge 

 (intro. fig. 74) is the commonest type of bottom 

 sampler used in lake and pond surveys. It is a six- 

 inch (or larger) square brass box equipped with closing 

 jaws. The jaws are cocked at the surface, and 'the 

 dredge is lowered on a rope either by hand or by a 

 winch. When it reaches the bottom a metal messenger 

 is sent down the rope to trip the jaws shut. Then the 

 sampler is hauled to the surface and the bottom 

 material is collected in a bucket and later washed 

 through a sieve or a series of graded screens (intro. 

 fig. 75), the coarsest screen uppermost. After the 

 muck and silt have been removed, the specimens 

 are picked from the sieve and preserved in vials of 

 alcohol for future study. Ekman samples should be 

 taken at several different localities around a pond or 

 lake, wherever the bottom fauna might vary. Transects 

 are frequently run in several directions across a body 

 of water or a random pattern may be followed. The 

 Ekman dredge is usually employed in soft-bottomed 

 lakes and ponds or slow-moving rivers, since the 

 jaws do not bite well into hard clay or rough-stone 

 bottoms. 



o o 



Intro fig. 74. Ekman dredge, note metal messenger at 

 top of picture. 



Intro, fig. 75. Graded screens used with bottom samplers. 



