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

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Robert Downey Jr. Welcomes Tom Hiddleston to Instagram

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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He's back!

A photo posted by Tom Hiddleston (@twhiddleston) on

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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!"

Join me in welcoming the biggest T. Stark fan of them all to Instagram! @twhiddleston

A photo posted by Robert Downey Jr. (@robertdowneyjr) on

Check out more from Tom Hiddleston in the gallery below!

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Tom Hiddleston
Tom Hiddleston poses for a portrait in promotion of his upcoming role in "I Saw the Light" at the 2015 Toronto International Film Festival on Saturday, Sept. 12, 2015 in Toronto. (Photo by Victoria Will/Invision/AP)
Tom Hiddleston arrives at the LA Premiere of "I Saw the Light" at the Egyptian Theatre on Tuesday, March 22, 2016, in Los Angeles. (Photo by Rich Fury/Invision/AP)
Tom Hiddleston, left, and Elizabeth Debicki participate in a panel for "The Night Manager" during the AMC 2016 Winter TCA on Friday, Jan. 8, 2016, in Pasadena, Calif. (Photo by Richard Shotwell/Invision/AP)
Tom Hiddleston participates in a panel for "The Night Manager" during the AMC 2016 Winter TCA on Friday, Jan. 8, 2016, in Pasadena, Calif. (Photo by Richard Shotwell/Invision/AP)
Actors Tom Hiddleston and Jessica Chastain participate in AOL's BUILD Speaker Series to discuss their new film "Crimson Peak" at AOL Studios on Friday, Oct. 16, 2015, in New York. (Photo by Evan Agostini/Invision/AP)
Actor Tom Hiddleston participates in AOL's BUILD Speaker Series to discuss their new film "Crimson Peak" at AOL Studios on Friday, Oct. 16, 2015, in New York. (Photo by Evan Agostini/Invision/AP)
Actor Tom Hiddleston poses for photographers upon arrival at the Premiere of the film High-Rise, showing as part of the London Film Festival, in central London, Friday, Oct. 9, 2015. (Photo by Grant Pollard/Invision/AP)
Actors Tom Hiddleston, right, and Mia Wasikowska pose during a photo-call to promote the film "Crimson Peak" in Berlin, Germany, Wednesday, Sept. 30, 2015. (AP Photo/Markus Schreiber)
British actor Tom Hiddleston reacts with fans, after arriving at the Maria Cristina Hotel, during the 63rd San Sebastian Film Festival, in San Sebastian, northern Spain, Monday, Sept. 21, 2015. (AP Photo/Alvaro Barrientos)
Actor Tom Hiddleston appears at the press conference for "High-Rise" on day 5 of the Toronto International Film Festival at the TIFF Bell Lightbox on Monday, Sept. 14, 2015, in Toronto. (Photo by Richard Shotwell/Invision/AP)
This image released by Sony Pictures Classics shows Elizabeth Olsen as Audrey Williams, left, and Tom Hiddleston as Hank Williams in a scene from, "I Saw The Light." (Sam Emerson/Sony Pictures Classics via AP)
Tom Hiddleston arrives at the U.S. premiere of "Thor: The Dark World" at the El Capitan Theatre on Monday, Nov. 4, 2013, in Los Angeles. (Photo by John Shearer/Invision/AP)
Actor Tom Hiddleston poses for photographers as he arrives on the red carpet for the World Premiere of âThor: The Dark Worldâ in Leicester Square, London, on Tuesday Oct. 22, 2013. (Photo by Jon Furniss/Invision/AP Images)
From left, British actor Tom Hiddleston and Producer Kevin Feige at a 'Meet the Filmmakers' session, held at the Apple Store in central London, Friday, Oct. 18, 2013. (Photo by Jonathan Short/Invision/AP)
Actor Tom Hiddleston takes pictures with fans during a promotional event for his latest movie "Thor: The Dark World" in Seoul, South Korea, Monday, Oct. 14, 2013. (AP Photo/Ahn Young-joon)
Actress Mia Wasikowska, left, and actor Tom Hiddleston attend the premiere of "Only Lovers Left Alive" on day 1 of the 2013 Toronto International Film Festival at the Ryerson Theatre on Thursday, Sept. 5, 2013 in Toronto. (Photo by Arthur Mola/Invision/AP)
Actors Tilda Swinton, right, and Tom Hiddleston pose for photographers during a photo call for the film Only Lovers Left Alive at the 66th international film festival, in Cannes, southern France, Saturday, May 25, 2013. (AP Photo/Francois Mori)
Tom Hiddleston accepts the award for best villain for "Marvel's The Avengers" at the MTV Movie Awards in Sony Pictures Studio Lot in Culver City, Calif., on Sunday April 14, 2013. (Photo by Matt Sayles/Invision /AP)
Tom Hiddleston poses with the award for best villain for "Marvel's The Avengers" backstage at the MTV Movie Awards in Sony Pictures Studio Lot in Culver City, Calif., on Sunday April 14, 2013. (Photo by Chris Pizzello/Invision /AP)
British actor Tom Hiddleston poses during a photocall of the movie "The Avengers", in Rome, Saturday April 21, 2012(AP Photo/Alessandra Tarantino)
Actor Tom Hiddleston signs autographs while walking on the red carpet during the Russian premiere of the film "Marvel" in Moscow, Russia, Tuesday, April 17, 2012. (AP Photo/Alexander Zemlianichenko)
arrives at the premiere of "The Avengers" in Los Angeles, Wednesday, April 11, 2012. "The Avengers" will be released in theaters May 4, 2012. (AP Photo/Matt Sayles)
Actor Tom Hiddleston and wife Susannah Fielding attend a gala screening for the film "The Deep Blue Sea" during the Toronto International Film Festival on Sunday, Sept. 11, 2011 in Toronto. (AP Photo/Evan Agostini)
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