170,000-Year-Old Cicada Larva

170,000-Year Magicicada Nymph (17万年ゼミ幼虫, 17 Man-nen Zemi Yōchū) was a large Mysterious Being that attacked Z-City at some point during Saitama's training years.

Appearance
The Magicicada Nymph is a massive cicada, that has the face of a child located at the center of its head, between its eyes.

Recollection Of A Summer
During Saitama's training years, the Magicicada Nymph attacks Z-City and defeats two C-Class heroes Grad School Graduate and Red Muffler and B-Class hero Darkness Blade with ease while the rest of the population barricades itself within Shelter Nr. 7. Sometime later two unnamed A-Class heroes try to fight it, but are also defeated, this causes the Hero Association to declare it as a Demon level threat and they decide to send one of their newly formed subjugation squads to finish it off. Saitama, while running around in search of a bathroom, runs into the Nymph who suddenly attacks him, but falls victim to Saitama's punch.

Abilities & Powers
The 170,000-Year Magicicada Nymph is shown to be strong enough to defeat multiple heroes without taking any visible damage. It was also able to defeat two A-Class heroes which prompted the Hero Association to deem it as a threat with a disaster level of Demon.

It can be inferred from its name that over some period of time, a Magicicada Nymph will grow into a 170,000-Year Magicicada Adult.