The Lightning Bug is a ranged flying enemy type that shoots lightning balls similar to the ones that heroes have from the Beacon of the Storm shard. Their lightning ball works very similar to the shard to the point that is will bounce a feww times between defenses or Cores. They fly slow and have a weakness to Earth damage making them take double damage from those sources.
Placing Anti-Air types of defenses or any defense that has a blue targeting line that shows in the air can target and damage these enemies. Examples of defenses that can hit them are Monk's Skyguard Tower being an Anti-Air defense or things like Squire's Cannons or Huntress' Poison Dart Towers having a line that extends into the air. Some defenses like the Monk's Flame Aura can also hit them since they have a Sphere Area of Effect.