This must be one of the cutest galls, wouldn't you agree?
Here is a photo of a gall-inducing caterpillar in its hollow, green home...
I suspect it is rather dark and claustrophobic inside that gall, as there is no exit hole until the larva leaves the gall to pupate on the ground. What about waste elimination? Exchange of gases? Temperature regulation? Well, amazingly, the set up works. The larva will someday turn into a minute moth and repeat the process next year.
I found some Poplar Petiole Galls when I expectantly walked up to some young aspen trees in the fencerow at a friend's farm (unfortunately the aspens in my backyard don't have any of these galls). Sure enough, there on the first tree, at eye level, were a number of these caterpillar galls.
See: [A petiole-galling insect herbivore decelerates leaf lamina litter decomposition
rates. Functional Ecology, Vol. 26, issue 3, June 2012 pgs. 628-636.] One of the authors recently showed me one of these galls and piqued my interest in searching for these galls.
I wouldn't mind a few of these cute galls on my backyard aspens.