Robert Downey Jr. welcomes Tom Hiddleston to Instagram by trolling his co-star's custom Taylor Swift tank

Robert Downey Jr. is legendary for his on-set pranks. But on Tuesday he took the teasing to Instagram when he trolled his Avengers co-star Tom Hiddleston just hours after the Thor actor announced his debut on the photo-sharing service. "He's back!" Hiddleston wrote on his first (and so far only) post alongside a winking, metal horns-throwing selfie.

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Downey was super psyched for his pal (who already has more than 500,000 followers), so he re-posted the infamous pic of Hiddleston sporting his custom "I Heart T.S." tank top from the 35-year-old British actor's fourth of July getaway with girlfriend Taylor Swift. The caption said it all: "Join me in welcoming the biggest T.Stark fan of them all to Instagram!"

Check out more from Tom Hiddleston in the gallery below!

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