cockchafer(n = noun.animal) may beetle, may bug, melolontha melolontha - any of various large European beetles destructive to vegetation as both larvae and adult;
is a kind of melolonthid beetle
is a member of genus melolontha, melolontha

   A beetle of the genus Melolontha (esp. Melolontha vulgaris) and allied genera; -- called also May bug, chafer, or dorbeetle. [1913 Webster]

cockchafer, n. a large nocturnal beetle, Melolontha melolontha, which feeds on leaves and whose larva feeds on roots of crops etc. Also called May-bug.

Etymology:
perh. f. COCK(1) as expressing size or vigour + CHAFER