I was reading the following explanation:

http://german.stackexchange.com/questions/7510/how-come-there-are-two-ways-to-conjugate-the-same-verb-e-g-erschrecken/7511#7511

however I am still confused with the verb to scare. 

I gather from the above example that if one wanted to say:

>* you are scared of spiders = "du erschrickst vor Spinnen" (which seems to be from the verb erschrecken/erschrak/bin erschrocken)

>* you scare the little child, when you laugh = "du erschreckst das kleine Kind, wenn du lachst" (which seems to be from the verb erschrecken/erschreckte/hat erschrocken from the above link)

but when do you use the reflexive "**sich** erschrecken"? I thought it would have been 
"du erschrickst dich vor Spinnen"? or would that be more if you wanted to say "you scare jmdn with etwas"?

and when do you use "hat erschreckt"?

Please could you give examples for each of the above? 
I'm finding it tricky separating the different scenarios with the correct verb form. 
Thanks in advance.