Sorry, but the original rumour came from the book
Xenology
Really a fantastic read. Go get it!
As far as I can tell this is a completely reliable and fully endorsed Fluff source.
[Spoiler. Highlight if you want to know the rumour]
The Alien, insectiod race is called the Q'Orl, which communicated through diamond shaped Pheromone emitters. The Ethereals have this EXACT organ in their... erm... facial orifice, which upon dissection, looks like it didn't evolve there. The two races are from opposite ends of the galaxy. Not only this but the Q'Orl have a written 'language' that tells of how the 'Dar stole one of their queens
You do the math.
[/Spoiler]