Hibiscus covered by blackfly

My Pink Hibiscus is absolutely covered by blackfly, along with tiny 3cm (1cm when they scruch up)whiteish caterpillars which turn black when squashed! They rear-up their head ends like leaches, some of them have black patches on thier backs. What type are they?? I have seen ladybirds with them,could they be their larvae, althought they look a bit too large.Am I right in killing them?

 

Me again, just in case anyone out there is interested, I found out that these caterpillers are friendly (after I panicked and killed a few) and they eat black and greenkfly.That's why they went black when squashed!! Durrr I gather they are the larvae of the hoverfly.