Lice and cockroaches don’t seem to have any other form of symbiosis than this ‘living together’ coincidence.
Galls, common on gum trees and wattles, are abnormal plant growths that form in response to invasion of plant tissue by a variety of organisms, in this case Psyllids (plant lice). When a female gall-inducing insect deposits her egg in the leaf or stem of a host plant, a biochemical reaction between the egg and the host may produce the gall, inside of which a hatching larva will feed. Or the larva may modify the plant’s normal response to injury with salivary secretions, which stimulates the plant to grow the gall (rather than scar tissue), resulting in food and shelter for itself. Gall-inducing insects (adults and juveniles) are usually very small, rarely more than a few millimeters in length, but the galls they induce are much more obvious.
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The cockroach was sitting on a shaky leaf, moving because of the wind. I took many photos on a rather high f setting, f/13, to make sure that the entire (small) critter was in focus. 1/100 of a second was the resulting shutter speed, which was pretty slow because of the wind and despite the tripod. After many tries I succeeded.