Entry 1 sense Webster, 1913 Melolonthidian /(mĕl`ō̇*lŏn*thĭd"ĭ*an)/ · Mel·o·lon·thid·i·an · IPA 01 n. A beetle of the genus Melolontha, and allied genera. See May beetle, under May. 1. A beetle of the genus Melolontha, and allied genera. See May beetle, under May.(Zool.) See: May