1. In order to clarify the main functions of reducing layers on the integuments of the wireworm, Melanotus caudex LEWIS, comparisons were made on the sensitivity of the reducing layers to ammoniacal silver nitrate between the feeding (spring and autumn) and non-feeding wireworms (summer and winter). Attempts were also made on the effects of pre-treatment with acid solution or hot water on the sensitivity of the reducing layers of the non-feeding wireworms, and on the distribution of the reducing layers in other species of wireworms.<br>2. The sensitivity of the reducing layers was very high at feeding period, while no reducing reaction was detected at non-feeding period. Moderate sensitivity was found at the period during which the wireworms migrated between surface and deeper layers of top soil. The annual changes in sensitivity are in accord with those in depth of creep in soil (YOSHIDA, 1951).<br>3. The reducing layers were found to be sensitive during the short periods from late in March to middle in May and from middle in September to late in October.<br>4. The reducing layers were insensitive when the wireworm crept in deeper layer of top soil for hibernation or estivation. This insensitivity was found to be due to non-reducing substances covering the reducing layers.<br>5. The non-reducing substances were soluble in acid solution. The sensitivity of the feeding wireworms was not lost by the pre-treatment with hot water or potassium cyanide solution. The non-reducing substances of the dormant wireworms could be removed very quickly by the treatment with acid solution when the wireworms had been treated with hot water.<br>6. The wireworms collected from deeper layer of top soil in November were cultured at various temperatures, and estimations were made on the days needed for the reducing layers to become sensitive. The sensitivity became apparent on the 50th day at 25°C and 30°C, but never at 15°C and 20°C.<br>7. In the case of Agrypnus binodulus, in which body water was known to be lost more quickly, the area of the reducing layers was large and their sensitivity was high as compared with Melanotus caudex.<br>8. The sensitivity of the reducing layers was highest in Agrypnus binodulus and wireworms living in decayed woods such as Alaus berus and Ampedus hypogastricus, was moderate in Melanotus caudex and M. senilis which often caused great damage in upland farm, and was relatively low in Melanotus legatus and M. cete which were found in the soil liable to dry such as field in mountainous region and nursery garden.