This is a spider, which was encased in tree sap while in the act of attacking a wasp. The sap turned to amber, leaving an incredible preserved scene, with even individual strands of silk from the spider's web remaining unbroken for 100 million years.

The paper this is taken from (sits behind a paywall, unfortunately)

Learn more about the preservation of bugs in amber at the website for NOVA's "Jewell of the Earth" documentary