56 



Lattin: Introduction 



Intro, fig. 80. Submerged, inverted cone emergence trap used 

 to sample Chironomidae (Brundin, 1949). 



cone-shaped end. The trap is anchored to the bottom 

 by ropes and weights, and can be lowered or raised 

 to any depth in order to sample different levels. The 

 apex of the cone is open and permits the insects to 

 swim into the cage but prevents them from escaping 

 once they are inside. These traps have been used 

 with some success to sample the beetle and bug 

 populations in oxidation ponds (Usinger and Kellen, 

 1955). Certain quantitative information can be drawn 

 from the results, since the trap is left in the water 

 for definite periods of time. However, as it is not 

 known what area or volume of water outside the trap 

 is sampled the results must be used with caution. 



Intro, fig. 81. Floating inverted con* emergence trap 

 8 (W. R. Kellen). 



Intro, fig. 82. Wisconsin type plankton net (Ward's Nat. 

 Sci. Est., 1949). 



An underwater light trap has been developed by 

 J. G. Needham (1924) and others. The light source 

 can be either a large flashlight or a light bulb and 

 reflector connected by an insulated wire to some 

 source of electricity on shore. A surprising variety 

 of aquatic insects has been collected in this manner. 



Plankton nets. — Plankton forms the chief source of 

 food for some fish and many invertebrates; hence it 

 may be an important part of a comprehensive insect 

 survey. For more detailed treatment of plankton, the 

 reader is referred to Welch (1935, 1948). A plankton 

 net (intro. fig. 82) is used to sample the small-sized 

 floating organisms of open water. The particular net 

 shown is called the Wisconsin plankton net. It con- 

 sists of a fine silk cone that may have a special 

 filter trap at the tip. The net is attached to a tow 

 rope and dragged through the water for a known period 

 of time. The plankters accumulate on the sides of 

 the net and are washed down into the filter trap at 

 the bottom. The trap is removed and the contents 

 flushed in a vial of 5 per cent formalin solution for 

 preservation. Later this material is identified in the 

 laboratory. More accurate results as to the amount 

 of water sampled can be obtained if a known volume 

 of water is poured or pumped through the net. By 

 usjng samples of known quantity and a special counts 

 ing cell slide, relative numbers or densities of various 



