Polyunsaturated monoglycerides and a pregnadiene in defensive glands of the water beetle Agabus affinis

In addition to the C₂₁ steroid 15α-hydroxypregna-4,6-dien-3,20-dione, four 1- or 2-acylated polyunsaturated monoglycerides, 1- or 2-(cis-5,8,11,14-eicosatetraenoyl)-glycerol and 1- or 2-(cis-5,8,11,14,17-eicosapentaenoyl)glycerol were identified as constituents of the prothoracic defensive gland secretion of the dytiscid beetle Agabus affinis by gas chromatography-mass spectrometry of trimethylsilylated gland extracts. In a feeding assay with minnows, synthetic samples of the two 2-acylated monoglycerides showed only a weak activity as a feeding deterrent. For that reason, other possible functions of the monoglycerides are discussed, such as roles as emulsifiers of cannabimimetics.