Metaldehyde pesticide. Polarised light micrograph of Meta, a metaldehyde-containing pesticide against slugs, snails and other gastropods. When the agricultural pest consumes the substance it reacts by secreting large amounts of mucus. This dehydrates it and causes it to exhaust its energy reserves. The mucous cells collapse and this leads to the death of the animal. Metaldehyde is classed as a 'moderately hazardous' pesticide by the World Health Organisation (WHO) because of its relative toxicity, but does not pollute the soil since it degrades into natural substances. Magnification: x33 when printed 10 centimetres wide.