DR JEREMY BURGESS / SCIENCE PHOTO LIBRARY DR JEREMY BURGESS / SCIENCE PHOTO LIBRARY
Lighthouse galls on the upper surface of leaves of ground ivy (Glechoma hederacea), The galling is caused by the larva of the gall midge, Rondaniola bursaria, one of which is present inside each gall. The presence of the feeding larva represents a nutrient sink to the plant, which results in increased cell division and localised growth. The precise mechanism by which galls form remains a matter for conjecture.The larva feeds until late summer, at which point the whole gall falls to the ground, leaving a hole in the leaf. The insect pupates and overwinters in the fallen gall.The picture, taken in September in the UK, shows several galls standing clear of the surface of the leaf, the basis of the common name, lighthouse gall. Each is covered with hairs, thought to deter predators. In several places, holes are present in the leaf where mature galls have already detached.
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