I think like a lot of species if you are in the right place at the right time, you can see an incredible amount at one time. I live in California so I only have collected D grantii in Arizona. Some years it's amazing, some years ok, some years horrible. There are times that I have seen over a hundred at one light. I have a friend who collects D tityus in the east and each year he finds a few at gas station lights, etc but never a lot. One year someone called him and said there was a bunch of D tityus on a tree in their backyard. So he went there and I think he found over 30 on that one tree. I guess they had just emerged. The more years you go out collecting, the more often you hit it a big. I'm an old time collector (been going out for over 40 years) and have many friends that are the same. So we often sit around and talk about the times where hundreds of some amazing species emerged. For some species, it's a once in a lifetime experience and you never see it like that again. Other species, like D grantii, can be every 5 years or so.