A Comparison of the Catches of Emerging Aquatic Insects in Floating Box and Submerged Funnel Traps

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Abstract
1. Two floating box emergence traps and a submerged funnel trap, each covering an area of 5 ft2 (0.46 m2), were set close together over a uniform substrate at Loch Dunmore, Perthshire, and the catches of emerging insects compared over a period of 8 weeks. Out of sixty species caught only half were taken in the funnel trap whereas fifty-two were caught in one box trap and forty-eight in the other. 2. Each box trap caught similar total numbers of insects but the funnel trap caught less than a third as many. The box traps responded in the same way to seasonal variations in emergence rate but the funnel trap responded differently on different occasions. 3. The weekly catches of Caenis horaria (L.), Procladius choreus (Meig.) Lauterborniella orophilus Edw., Tanytarsus gregarius Kieff., T. (?) glabrescens Edw. and Cladotanytarsus mancus (Walk.) are compared. The funnel trap caught significantly fewer of each than the box traps (P<0.001) and for four species the box traps differed from each other but at a lower level of significance. The box traps recorded similar seasonal fluctuations in numbers for each species but the funnel trap responded differently for four species. 4. Comparison of the percentages of the first five species listed above showed that for the three most abundant there was no significant difference between traps. 5. Similar sex ratios were obtained in the box traps for the most abundant Chironomidae. 6. Box traps could be emptied every 2 days without loss of insects but this was not true of the funnel trap. 7. Box traps of 0.37, 0.46 and 0.70 m2 were compared. They responded similarly to seasonal and diurnal fluctuations in the catch but the density of the catch recorded differed. A trap covering 0.46 m2 (5 ft2) was the optimum size. 8. The relative merits of the floating box trap and the submerged funnel trap and the reasons for the low catches in the latter are discussed. Certain changes in design are proposed.

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