Am i hallucinating seeing insects?

Sometimes I might see a cockroach, fly, spider or ant somewhere - on my bed, on the floor, on a wall, or somewhere in public. It can be close to me or far off. I look at it intently, and then suddenly it vanishes. As in 'now you see it, now you don't'. I look for where it might have crawled/flown off while I supposedly blinked... but it isn't anywhere.

Very occasionally it's revealed to be a piece of fluff, a dirty mark. But sometimes it's there one minute and not there the next. I see the insects very vividly, in all the detail you'd expect to see in a real one! This has been happening for about 6 months. Am I hallucinating? What gives?