Insects lead two lives, one as a larva and another as an adult. The habitat in which they thrive is always (well, I think this is true) very different and it certainly is in longhorn beetles. They live their larval stage in dead wood eating their way through dead timbers and helping to break them down in to humus and so the beetles are rarely seen in this stage unless you go specifically looking for them. As adults they become pollen feeders and can be found on a variety of flowers, quite often umbellifers, and so are far more visible.The attractive four-banded longhorn (Leptura quadrifasciata) is quoted as being relatively common but the adult beetle lives for only a short time and so they are seen far less often than their numerical status would suggest they should.
Read more: Four-banded Longhorn Beetle: band stands | Nature Notes from Dorset