Warning: This may be a bit dry for some people .....
There are 2 groups of Hoyas from SE Asia with white, waxy, bell-shaped flowers which has caused confusion amongst growers and taxonomists alike due to their apparent similarities and vendors applying errorneous tags without due diligence. The first is the Hoya campanulata group which appeared widespread in SE Asia and the second are from the wallichii complex which was collected mostly (maybe even exclusively) from Borneo.
Now let us examine the following 3 photos:
Vegetatively and in flower they look so different from other Hoyas and so similar to each other that one can almost be excused for thinking that they are one and the same. Even R.E. Rintz, who published an excellent early paper on Hoyas of Peninsula Malaysia, lumped them into a single species - Hoya campanulata.
Now, let's do some closeups of the corona ....
The lower 2 plants have slender corona and are collectively lumped under an umbrella called Hoya campanulata - although as I will show later, there appear to be more than 1 species there. This group has a wider distribution, from Malay Peninsula, Borneo, Java and Sumatra.
And here's a portrait of the real Hoya campanulata from Curtis Botanical Magazine, renown for accurate taxonomic depictions of many exotic plants collected all over the world during 1800s. The picture clearly shows the slender corona similar to the latter 2 plants here. It is taken from Simones' excellent web site.
So hopefully, we got these 2 rather different species sorted out.
I shall end this post by flashing another Borneo native that looked like a H. wallichii (or danumensis, take your pick) by virtue of the fat corona, and yet looked distinct in its smaller flatter corolla that curl backwards as shown below :
Any idea what is this ?