Direction Field of Lotka-Volterra ODE

Dear PTC Community,

for my studies I have to analyse the Lotka-Volterra ODE System. Now I want to use some plot like the following:

There you can see the Direction Field of the ODE in a phase plot. The standard equations are like:

Also I found the invariant of the System:

Question is: how to make such nice looking direction field plot? From which equation is this going out? I am using Prime 2

Thanks so far

Patrick

PS: I come across this

http://communities.ptc.com/docs/DOC-3503

but dont know anything to do...