The wing of my 5 metre ASW17 has the same 'construction', Roo!
How do I know?
While on the tow at Middle Wallop a few years ago, just as the '17' got to about 200 feet, the port wing folded just outboard of the wing joiner construction, but luckily it helicoptered down with no damage to the fuselage and the other wing. On inspection of the remains of the port wing, sure enough there was no spar, just balsa leading and trailing edges with lots of white foam inbetween.
Actually I blame it all on Bernie,
, as I bought the '17' from him. When he saw a video of it gently soaring on the South Downs he said it could be flown much more vigorously than that. https://www.youtube.com/watch?v=jSgWW0Xkq60&t=14s
So on the next flight at Thurnham I put it through its paces and sure enough it performed as Bernie said it would! Unfortunately the flight after that was the fateful one at Middle Wallop, and so the failure there was totally down to me over-stretching the envelope!
The next trip it will have will be to Simon WS' bandsaw to slit both wings down the middle to fit a full length vertical web in each wing - that should make the wings somewhat stronger.
Enjoy the rebuild Roo! . . . I hope you didn't mind us letting Graham know where the remains of his incident would have a good home; much better than the bin men on Thursday!