One interestingly little bug family that I have occasionally stumbled across while peeling back bark is the flat bugs, family Aradidae. As their name suggests, these bugs are notable for their extremely flattened body morphology, an adaptation to their lifestyle under bark and in crevices of wood. These guys are hard to spot, being cryptic both in body and lifestyle, but, as an example of morphology pushed to the limits, they are impressive to observe.
Because these bugs are of little economic importance (they are rarely agricultural pests) little research has been done on them. Nonetheless this is a diverse group which is found worldwide. Most flat bugs feed on fungi in decaying wood, and some are attracted to the pheromones of bark beetles, which may help them to locate food sources. These bugs tend to be found in gregarious groupings. Probably the best gathering of information that I have found on aradids is Steve Taylor’s info page, complete with some great pictures.