The origins of the Constructicons are obscure at best. When they first appeared alongside the Decepticons, Megatron claimed to have built them. Discrepancies here include the fact that they have perfectly functional brains and personalities, unlike the unstable Dinobots, but were not programmed by Vector Sigma. Furthermore, it was later indicated that they had been friends with the Autobot Omega Supreme on Cybertron, indicating they had existed before the Decepticons came to Earth. Later reports suggested that they had built Megatron! What follows is basically an attempt at linking the various obscurities together.
The Constructicons, we know, were creators. It is possible that they built the robot who became Megatron, who sowed seeds of rebellion amongst others. As the Decepticon army grew, it is documented fact that Megatron used a device known as the Robo-Smasher on the Constructicons, making them evil in nature. Now evil, the Constructicons betrayed their friend, Omega Supreme, the guardian of the beautiful Crystal City, by luring him away from it, and then demolishing it. Omega Supreme, furious, nearly terminated the Constructicons, and while he served loyally in the Great War, he was obsessed with a need for revenge. The most likely scenario is that Omega finally managed to track down the Constructicons on Cybertron, and destroyed them. Then, Megatron could have taken their personality components, and taken them with him during the Decepticon attack on the Autobot ship, the Ark. Omega was not on board the Ark at the time. Four million years later, when the Decepticons and Autobots were revived on Earth, Megatron would have had Earth bodies built for the Constructicons - hence prompting his claim that he "built" them. Learning that the Constructicons were still active on Earth, Omega Supreme would have then arrived on Earth shortly thereafter, and joined Optimus Prime's forces.
In any case, the Constructicons first mission on Earth was to steal energy collector components, which their leader, Scrapper, then placed in a machine he and the other five Constructicons, Mixmaster, Bonecrusher, Long Haul, Scavenger and Hook, built. The machine transferred all of the other Decepticons' powers to Megatron, and he engaged Optimus Prime in battle, while the Constructicons were sent to destroy Teletran One. However, they met resistance in the form of the Dinobots, who beat them back, until they revealed their hidden power - and merged into the titan, Devastator! While the other Autobots arrived and saved the day, this was the beginning of a bitter rivalry between the Constructicons and the Dinobots.
Throughout the remainder of the 20th century, the Constructicons' primary function was to build machines and devices to further Megatron's plans. They, like the Dinobots, did not require Cybertronium to function (perhaps a further indicator that they were built on Earth), and guarded the Decepticon Spacebridge on occasion. The six Constructicons each have their own powers and abilities, as detailed below, as well as expert knowledge of building and construction, and can merge their forms into one gigantic robot known as Devastator.
Leader of the Constructicons, Scrapper transforms into the likeness of an Earthen tractor shovel. He is ruthless and cunning, with vast knowledge of almost all kinds of machines. In vehicle mode, Scrapper's scoop can pierce solid steel. He forms the right leg of Devastator.
Bonecrusher is the primary muscle of the Constructicons, with the ability to transform into a bulldozer. Specialising in demolitions, he is exceptionally strong, and relatively fast for his size in vehicle mode, but sometimes a little rash in his actions. He forms the left arm of Devastator.
Hook is the narcissist of the Constructicons, always bragging about his work and it's quality. He transforms into an Earthen truck crane. In this mode, his hook can be used as a welding iron, capable of perfection to "one-third of a Cybertronic mini-inch" (ref: "The Core"). He forms Devastator's upper torso.
Long Haul is used to transport materials, and is frequently ordered around by his teammates. As he has put it himself: "Fetch this, bring that - I thought I was actually gonna get to build something for once!" (Ref: "The Master Builders"). Able to transform into a dump truck, Long Haul can shunt his cargo into subspace when he transforms. He forms Devastator's lower torso.
Mixmaster transforms into the likeness of an Earthen cement mixer. He specialises in fabrication of materials, producing any concoction imagniable from the vast storage of chemicals in his drum. He has a cab-mounted barrel to project any of these mixtures, should circumstances arise. He forms the left leg of Devastator.
Scavenger transforms into a steam shovel. In this mode, he is equipped with geologic analyser circuits, which act like a form of radar, enabling him to read the lay of the land. Scavenger is somewhat jumpy and nervous at the best of times. He forms the right arm of Devastator.
Devastator was the first of the gestalt Transformers, created when all six Constructicons merge into one. Devastator has near-impenatrable armour, and great strength. Sometimes, however, he has displayed a lack of intelligence, and was once even taken over by the Autobot, Wheeljack (Ref: "The Core"). He is armed with a massive laser blaster, although he rarely uses it.
The Constructicons were present at the Autobot City Massacre of 2005, where Devastator was used as the Decepticons' primary weapon against the Autobots. When Optimus Prime arrived with reinforcements, the Dinobots took on Devastator, but he managed to beat them down, until Optimus Prime and Megatron had their final battle, and the Constructicons and all the other Decepticons retreated. After the wounded Decepticons, Megatron among them, were dumped into space, Scrapper, Hook, and Bonecrusher attempted to take leadership of the Decepticons for the Constructicons. The team formed Devastator, but were somehow shaken apart by Rumble's piledrivers. Later, the Constructicons provided musical accompaniment for Starscream's coronation ceremony, which was interrupted by Galvatron, who killed Starscream and became leader.
In the Earth year 2006, the Constructicons, with the help of the Quintessons, built the Decepticon City of Trypticon to counter the Autobot, Metroplex. The Constructicons fought loyally alongside Galvatron throughout 2006, but their final fate following the restoration of Cybertron's Golden Age is unknown.