Hadda Beetle

Coccinellidae; Henosepilachna vigintioctopunctata (Fabricius, 1775) This Hadda Beetle is mostly orange in colour, with 28 black spots, 14 on each elytron, hence the name "vigintioctopunctata". That's the easy part of the description. I say it is easy because I can understand it. Now I want to mention that every visible surface is "pruinose", or covered in "pruination". That is my problem. I can say it, but I don,t understand it. I know that the word pruinose means covered or frosted with a white powdery substance, but I have no idea how it gets there or what is its purpose or significance. Can anyone spare a moment to explain this to me. I don't mind admitting my ignorance of any particular subject, but I am uncomfortable living with it. Please help if you can.