Originally, the Insecticons were three Decepticons that Megatron had left onboard his ship, the Nemesis, which was pursuing the Ark, as back-up. When the Nemesis crashed on Earth, the trio evacuated in an escape pod, which crashed in an Indonesian swamp. In order to survive, they scanned the forms of the surrounding area's insects. The insect forms they adopted enabled them to extract energon from organic life and metals, though they still ingest fossil fuels and energy-based matter (electricity, energon cubes, etc.) as well. Because they took on the forms of insects the size of Transformers, the three Insecticons caused terror for any humans living nearby. They even developed the ability to make clones of themselves out of any metal.
They were eventually found by Megatron centuries later and were re-recruited back to the Decepticons. But they became more independent as a result of their long absence, making them loose cannons: their relationship with the Decepticons proper was one of betrayals and counter-betrayals. Originally appearing in "A Plague of Insecticons", they were discovered raiding human crops. They quickly joined up with the main Decepticons: unfortunately, the Insecticons swiftly decided this wasn't to their advantage and devoured Megatron's store of Energon instead, resulting in a battle breaking out between the two sides.
After this, the Insecticons entered into a free-lancing relationship with the Decepticons, helping Megatron if they had something to gain from it. They were tricked by Mirage into attacking the Decepticons, later using Bombshell's cerebro-shells to control him and lure the Autobots into a trap. Another deal, with Decepticons and Insecticons working together to raid a military power plant, went awry when they overheard Megatron planning to betray them. Munching on a nova power core, they grew in size and strength. They then controlled all the Decepticons bar Megatron with cerebro-shells, forcing the Decepticon leader to team up with the Autobots to stop his mind-controlled troops. Using Megatron in gun mode, Optimus fired the antidote needed to restore the Insecticons size and prevent the nova core they ingested from overloading.
However, even the Insecticons' rebellious streak was overlooked by Megatron, as the Insecticons could create clones of themselves, thus creating an instant army.
The Insecticons took part in the Battle of Autobot City in 2005, but were badly damaged, and were tossed out into space along with the injured Megatron, Skywarp, and Thundercracker. It is unclear if these were the original Insecticons or some of the many clones they were capable of creating. However, since there is no mention of clones being used during The Transformers: The Movie, we can assume that the writers and animators intended these Insecticons to be the genuine articles. Either way, these discarded Insecticons, along with Skywarp and Thundercracker, were used by Unicron to create new Decepticon troops for Galvatron, with Bombshell possibly becoming Cyclonus (although the issue of whether Cyclonus was a recreated Bombshell or Skywarp is unclear), and with Shrapnel and Kickback becoming Sweeps.
The clone question comes into play when, later in the film, Shrapnel harasses Daniel Witwicky alongside Scavenger. In the third season of Transformers, the Insecticons were still displayed in marginal roles (although their third season appearances are usually dismissed as animation errors).
Each of the basic Insecticons had a way to control other robots.
Abilities: Shrapnel can control other robots (as well as almost any electrically run device) by remote with his antennas. In robot mode, his antennae can, in the proper weather conditions, attract massive bolts of lightning from the sky to augment his own electrical generating capacity, and then release that power through his hands. In insect mode, Shrapnel can use his twin antennae to generate an electric burst of up to 40,000 volts.
Weapons: Grenade launcher
Abilities: Mind control (see Cerebro-shells below)
Weapons: Cerebro-shells (humans) - In his insect form, Bombshell uses his stinger to inject a micro-circuited device into a human’s head. It travels through the blood until it reaches the brain, whereupon it opens up and releases tens of thousands of connecting wires. The wires coil around nearby neural dendrites until the brain’s conscious functions are effectively rerouted through the cerebro-shell, which puts the mind of the victim under the remote control of Bombshell.
Weapons: Cerebro-shells (Transformers) - Bombshell’s stinger is used in much the same way against other robots’ high-function mental circuitry. The major difference is that once the stinger is pressed against the metal casing of the victim’s head, the small laser torches on the sides of the stinger rotate rapidly to carve a small hole in the casing, allowing the stinger to inject a cerebro-shell.
Weapons: Head-mounted mortar cannon
Abilities: Kickback is adept at controlling others through blackmail. In his insect mode, Kickback can use his legs to leap up to heights of 40 feet and over distances of a tenth of a mile. Although small in this form, with a single kick he can punch a hole in a one-quarter-inch steel plate.
Weapons: Submachine gun