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Population models have suggested that host regulation by natural enemies is achieved by the densitydependent response of natural enemies to temporal and spatial changes in host density. However, surveys of insect host-parasitoid systems (Morrison and Strong 1980, Dempster 1983, Stiling 1987) indicate that few field studies provide unambiguous evidence of densitydependent relationships. It has been argued that density dependence may be obscured by various behavioral and stochastic elements involved in the interaction, rendering our methods for its detection inadequate (Morrison and Strong 1980, Stiling and Strong 1982, Hassell 1985, Lessels 1985, Morrison 1986, Stiling 1987, Price 1988). Hassell (1985) has suggested that the regulatory role of natural enemies may be detected by examining the relationship between the per capita searching efficiency and parasitoid density. The per capita searching efficiency is calculated as

A thin-film technique has been developed for the quantitative analysis of grindable solids such as combustion chamber and spark-plug deposits, coal and fuel-oil ashes, sediments and catalysts. Only 20 mg of sample are required and results are virtually independent of the matrix for all elements of atomic number 11 (Na) and heavier. Detection limits for the 25 elements so far calibrated range betweenf 0.05 and 1.0% by weight. The technique was proved using 22 certified materials, the precision being within 10% relative over much of the calibration ranges.

Slides were prepared from blood of channel catfish, Ictalurus punctatus, and evaluated. The cellular blood elements observed were thrombocytes, lymphocytes, neutrophils, monocytes, eosinophils, basophils, granular anucleate bodies, erythrocytes. In channel catfish, thrombocytes, lymphocytes and neutrophils were the predominate leucocytes. Erythrocytes were observed in various stages of development. Smudge cells were observed in all smear preparations. The variations found between the observations in this study and information provided in the literature suggest the need for establishing a standardized terminology.