The Black Hornet is an evolved Skitter capable of flight.
Black Hornets are highly evolved Skitters with the ability to fly. They are massive in size and covered in engorged, tumor-like growths. Unlike their less-evolved kin, they have blue eyes, have four wings, and an elongated tail. Their tails are prehensile and incredibly strong, allowing them to effortlessly carry objects as large as a fully grown man into the sky.
For over two years the humans were unaware of the existence of the Black Hornets. However, after the Second Assault on Boston the Espheni decided to put them to use. They are used to guard Espheni Ghettos and transport escapees to the next Ghetto. After Tom was brought onboard the Eye in the Sky, the Espheni, Scorch, revealed to Tom that Black Hornets are Rebel Skitters that have been genetically engineered. One attacks John Pope while he climbs the ghetto wall in a Faraday suit, but Weaver, Tector and Dingaan hit it with a pressure cannon, knocking it into the wall and vaporizing it. (The Eye) (Exodus)
After the Espheni Power Station was destroyed, Espheni technology was massively effected and a mass exodus of Espheni left Earth, leaving the Black Hornets now uncontrolled. This is shown when a hornet attacks Tom Mason while he is walking back to Chinatown.(Find Your Warrior)
At Pope's base a Black Hornet found the group and attacked, injuring one of the survivors. Anthony mentions that it was like o steroids, as it was unable to be shot down. Later, during a battle between Tom Mason, and John Pope's Crew, a Black Hornet arrived and carried off Tom Mason into the sky.(Non-Essential Personnel)
As the Global Resistance prepares to set out for Washington, D.C., a massive swarm of Black Hornets is seen approaching their base to the horror of Tom Mason and his men. The attack is quickly repelled, but not before one kills Marty. (Reunion) (Reborn)
During the attack on Washington, D.C., Black Hornets can be seen flying in the sky. When Tom Mason, kills the Espheni Queen with the Dornia superweapon, the Black Hornets, like all other Espheni, start exploding all over the planet until they are all dead. (Reborn)