Magnetic minerals. Keystone-like crystals in cells of hornet combs.

The hexagonal brood-rearing cells inside the nest combs of the hornet Vespa orientalis are uniform in both their architecture and orientation. We have discovered that each cell contains a minute crystal that projects down from the centre of its domed roof and has a composition typical of the magnetic mineral ilmenite. These tiny crystals form a network that may act like a surveyor's spirit-level, helping the hornets to assess the symmetry and balance of the cells and the direction of gravity while they are building the comb.