Some type of insect or spider has a habit of gluing one detached gum leaf to another living leaf. On the Eucalyptus leucoxylon growing in my garden there are lots of these. It's quite easy to find because the attached leaf dies and so does the bit of the leaf that it is attached to. There must be some chemical in the 'glue' that effects the living leaf. And when I prise them apart there is always a 'cobwebby' material in the middle so I presume it is a shelter or nursery for babies. I have never found any insects there so I must be looking at the wrong time of the year.
The tree is also host to a lot of lerps and scales- the ants seem to find these particularly attractive. They look amazing under a microscope - so many different shapes too. And a large percentage of the leaves show some insect damage so there must be a lot of insects living on the tree. There are also insects living under the bark. Mostly this isn't possible because the bark fits tightly but at this time of the year the bark starts to peel off so there are many crevices to hide behind.