Because it darts about vigorously at high speed, it is very difficult to see. Hearing its distinctive cries for too long induces a headache.
(From Bulbapedia)Ninjask is a small, cicada-like Pokémon with two pairs of wings. Its body is mostly black with accents of yellow and gray. Most of its head is yellow, but there is a solid band of black between its gleaming red eyes. The band expands into a circular pattern on its forehead, between two pointed crests on the sides of its head. The underside of its head is gray and segmented, making it somewhat resemble a veil. On its back is a patch of black with a stretch of yellow, shaped like an upside down letter Y. Most of its carapace and abdomen is black, but there is a ring of yellow near the tip. Its gray insectoid arms extend from its chest, and usually have their small, yellow claws turned inward. There is a similar pair of back legs. It has large, veined wings with triangular, red tipping.
Ninjask is a very fast Pokémon that is said to be able to evade any attack, and was long believed to be invisible due to its high speed. If it is not trained properly, it will refuse to obey its Trainer and cry loudly. Due to this, it is said to put the Trainer's abilities to the test. Hearing its loud cries for too long induces a headache. It lives in forests, where it congregates around tree sap to feed.