Cecedia, better known as galls, grow on plants infected with bacteria, parasites or insect eggs. Different species' galls are highly distinctive, often providing protection or nourishment for the creature growing within. Though frequently undesirable, they've also been useful to humans, over the years: ink is traditionally made using tannic acid from oak galls.
The long-abandoned gall pictured above, photographed by nutmeg66 in Theddlethorpe St. Helen, England, shows the exit routes of whatever creatures it nurtured.
Photographer Kim Fleming spotted this evacuated wasp gall in 2006.
Neuroterus numismalis galls on a leaf, shot by Mick E. Talbot of Lincoln, U.K.
Flickr's tiny_packages writes: "We found a lot of these weird-looking things just up the track from the house … all different alien shapes and colours, slightly sticky, and if you break them open, you can find the acorn inside."
This gall, spotted by Johann Dréo, hangs from the branch of an oak tree in Chamadelle, France.
Galls take many odd shapes and sizes.
Silk Button Spangle Galls on English Oak, caused by the gall wasp Neuroterus numismalis. Uploaded to flickr with the understated title "Too many galls", anemoneprojectors's photo was featured here at BB early last year. "Here were so many galls on this tree that I actually felt sick," anemoneprojectors wrote. "But not too sick to take a photo!"
The galls covering this poison ivy leaf were caused by the Aculops mite. The holes indicate galls on the other side of the leaf, reports photographer Martin LaBar.
There are hundreds of other photos of similar infestations on flickr; check out the cecidology pool to start.