Thinking that lerps were all leaf sucking insects that hid under translucent shell-shaped cases, I googled lerp to find that this refers to the sugary case, and not the insect beneath that is called a psyllid; and that psyllids also cause galls. Adult psyllids look a bit like small cicadas but are related to aphids and scale insects.
click here for more psyllid information.
Galls develop when leaves are sucked by the juveniles; these change into adults that erupt out of the gall: (remember the movie ‘Alien’?)
Alan Kerrigan took images of horn-shaped lerps and also psyllids below that he found in a round acacia gall.
See also round type lerps on a eucalypt, and winged adult psyllids that are being tended by ants.
When curiosity overcame me and I opened a few to have a look, my eyes encountered maggots instead of psyllids. The maggots are larvae of the gall-midge (in actual fact a Cecidomyidae fly) that produces a salivary substance which forces acacias to produce gall cells. These have replaced flowers making the plant look like it has fruit.