Weird Shadow thingy in Water?

[Kaiserin]

I spotted something weird in the river a few minutes ago, There was a small shadow, I couldn't catch it with my Fishing Rod.(No, It wasn't from the balloon in the sky.), And This thing looking like a scorpion?I dunno..But I couldn't catch that either...can anyone tell me how to catch both of them & what's the name of it?

 

[Peter]

Sounds like a diving beetle, you catch it with your bug net
http://animalcrossing.wikia.com/wiki/Diving_Beetle

 

[ams]

There are 2 bugs that appear in the river. The shadow would have been a diving beetle. If you follow it for a bit it will eventually come to the surface and you can catch it with a net. The other bug is called a pond skater and it never goes underwater. Hope that helped!