Voldemort addressing Pettigrew as "Wormtail"

[stickbook41]

No: HPFGUIDX 54246

Does anyone know why Voldemort addresses Peter Pettigrew 
as "Wormtail"?  This struck me as odd since "Wormtail" was the 
nickname his school friends gave him.  I don't have my canon with me 
right now, but is there any place in GoF where Voldemort calls Peter 
by his name?  In the DE circle, he calls all the other DE's present 
by their names when addressing them.

I thought at first, maybe Voldemort did that to keep his DE's 
compartmentalized, but that can't be true after saying all those 
names in the DE circle.  Plus, he called Peter "Wormtail" when no one 
else (except Frank Bryce) was present--Chap 1, GoF.

Does anyone have any theories on this?

Cheers!

-stickbook