The Constructicons were once Autobots who were friends with the Guardian Omega Supreme. Able to create incredible and beautiful structures, their skills were prized and admired. Megatron, who had created a device capable of turning an Autobot into a Decepticon, used it on the Constructicons. In turn, they lured Omega Supreme away from Crystal City and destroyed it. As a result, Omega held a long-standing grudge against them.
On Earth, Megatron boasted that he had created the Constructicons. However, it is speculated that he was merely exaggerating his accomplishment, which was actually rebuilding the Constructicons' bodies. This may also explain how Teletran-1 was able to so quickly analyze them when they attacked the Ark, and how Wheeljack was able to later control Devastator temporarily by remote control.
In 2005, Devastator was used as the main weapon against Autobot City's big guns. He succeeded, but the intervention of the Dinobots kept him from joining the main battle. After retreating with the rest of the Decepticons in Astrotrain, the Constructicons vied for leadership, but Starscream won in the end. A year later, they were commissioned to build Trypticon, who in turn destroyed the Ark.
With contributions from Matt Kuphaldt.
[Stanley Lui]: Although Rodimus Prime, through the Matrix, saw the Constructicons creating Megatron, I believe this highly suspect evidence to say the least. For one thing, the Matrix clearly was not present during this occassion. Secondly, neither was Hot Rod or any other previous Autobot leader. My guess is that it was just Rodimus's mind filling in the gaps with familiar characters, just like our own minds do in our dreams.
[Matt Kuphaldt]: The Constructicons were originally Decepticons but later defected. Possibly, their first act as Autobots was to build the Crystal City; thus, the flashback events in "The Secret of Omega Supreme" occurred very soon after their defection. This would explain the Decepticon symbols on the then-Autobot Constructicons...could those events have occurred so close to their defection that they hadn't even gotten around to changing their insignia? That would explain why Megatron targeted them with his loyalty-altering device - he wanted them back on his side, and quickly.
[Robin Armstrong]: The Constructicons were originally built by the Quintessons to assist them in building more robots and structures on Cybertron. Among their creations was Megatron, the mightiest of the "military" model line. (Megatron was most likely subsequently rebuilt to be able to fly unaided in robot mode, and led the Decepticon revolt later). When the Constructicons revolted from the Quintessons, they were classified as Decepticons (hence their symbol), but continued thier building of Cybertron peacefully as "Autobots." Megatron's device turned them to true Decepticons. Later, Megatron rebuilt them on Earth, and claimed to have built them.
[JDSTARBAR@aol.com]: The Constructicons were built by an unknown source and as any computer-based item goes, had defects. They were evil but needed a more competent leader, thus building Megatron. And just like any program. Megatron did not know that these were "friends." He attacked the Constructicons and they had a fight and all died. Megatron was rebuilt by a mechanic and the Constructicons were rebuilt by Omega Supreme. Megatron, finally realizing the Constructicons, used the Robo-Smasher and changed them into Decepticons. Megatron, in Heavy Metal War, talks about building the Constructicons but he only changed them.
The Constructicons were created on Shockwave's orders after he captured Optimus Prime and took control of the Creation Matrix. Devastator was first seen protecting a giant communications relay which Soundwave was using to contact Cybertron. Although a human was able to disconnect the relay, a message had already gone through.
Knowing what an advantage gestalt technology was, Optimus led a diversionary assault on the Decepticons, solely for the purpose of tricking the Constructicons into combining. They did, and the nearby Bumblebee was able to record the event with scientific equipment. The technology would eventually be used to create the Aerialbots. After this, Devastator only appeared once more to fight Superion, who won.
The Constructicons were later used as a rescue team, breaking into the Ark during a large-scale battle and freeing several Decepticons who had been captured by Omega Supreme.
With contributions from Matt Kuphaldt .
The Constructicons were under the command of Megatron when the "Classic" Transformers (Wheeljack, Prowl, etc.) were reactivated.