The Lightning Bug is a strange device that looks vaguely like a giant beetle on an altar; when the wings are moved into the proper position, it fires lightning bolt-like blasts of energy from its antennae.
Megatron took any excuse he could to hump it.
Generation 1 cartoon
The Lightning Bug was found on the dead Autobot colony of Antilla by a small group of Decepticons, including Megatron and Starscream. They took the Lightning Bug back to Earth with plans to use it against the Autobots.
When Megatron (and supposedly some other Decepticons) was infected by Cosmic Rust, the kidnapped Perceptor determined that the heat from the Lightning Bug was accelerating the spread of the disease. Megatron whined and cried about how he didn't want to get rid of his weapon. But since Perceptor stupidly forked over the Corrostop cure, which he had previously said should never fall into Decepticon hands, Megatron didn't have to. He sent back the rust-infected Perceptor, so the virus spread to all the Autobots. Megatron then used the Lightning Bug to slightly enhance the spread, then stopped and went home to watch Jem or something rather than finishing the weakened Autobots off.
When the repreive allowed the Autobots to find a deus ex machina -- er, a cure, Megatron decided to use the Lighting Bug on the recently-Corrostopped Statue of Liberty. Why? To make sure they didn't get more Corrostop. Menasor was to use the Lightning Bug to destroy the statue (instead of just smashing it to bits like he normally does). The Autobots were there to stop them, however, and Superion blasted the Lightning Bug to tiny bits.