Thanks for the reply direwolf, I posted a thread like this before about a different bug, it turned out to be a Carpet Beetle. While I was posting this thread I was thinking that it would be great to have a thread with many different bugs so you could find the one you are looking for
About this bug, by the characteristics I would say it's an aphid but it does look a little bit different from other aphids so maybe is from the family like the root aphid you mention.
Let see if anybody knows about it