I'm a fan of Moto Rumi, their machines combine two loves of mine: two stroke twins and quirky engineering.
The Formichino really is a symphony of the unusual. No frame to speak of, a rear casting for the aft of the bike and a vertically split front casting to carry the front end with the engine in the middle holding the two together. The quirkiness even extends to Rumi making their own fasteners with their own pattern of heads for holding the frame members together.
All very unusual but also highly effective. Rumis were a quality product, notably smooth and notably fast. One famous marketing gambit for Rumis in the UK was to balance a 'threepenny bit' (a wide 12 sided coin) on the crankcase and rev the engine, the coin would allegedly stay stock still. The Formichino was even available in production racing trim as the 'Bol d'Or' model commemorating Rumi's victory in that event.
Rumis never sold in great numbers in the UK being very expensive (in part from import tax and in part from just being bloody expensive bikes to make compared to lesser machines) and only available in 125cc size. They were moderately successful however through the rest of Europe and were made under license by Sarolea in Belgium and there was a subsudiary company in Argentina.