Spiked pea galls on the underside of leaves of the dog rose, Rosa canina. The galls are caused by the gall wasp Diplolepis nervosa, which lays its eggs inside the leaf. The plant responds by producing a hollow chamber around the developing larva. Each gall is unilocular - it contains one larva only. The larva overwinters within the chamber, pupates and emerges in the following Spring. Such galls appear to be sequestered safe spaces, but they are often found to contain inquilines ('lodgers') that include gall wasps and chalcid wasps. Ungrateful larvae of one chalcid wasp, Oligosthenus, may attack and kill the larvae of the original architect of the gall. Galls of this type are often seen on roses growing in the wild, but are rare on garden roses or nursery stock.