New meaning of TG

I've been doing some preparatory work/ study for some surveys I'm doing and also some teaching I'm doing next week. As you probably know I have an interest in freshwater macroinvertebrates (Mayfly larvae etc) and I came across TG being used in a dictionary of entomology - it stood for tracheal gills. There will be more of those than transgender people, so should obviously spring to the ordinary person's mind (or what passes for one). I knew you'd be glad to read that.