Female giant ichneumon wasps (Megarhyssa macrurus) laying eggs in a dead tree. These four-inch-long wasps, counting the ovipositor, are parasitoids of the wood-boring pigeon tremex: they lay an egg near a larval tremex, which will serve as the host of the parasitic larval ichneumon. The tremex will not survive the process; the ichneumon will emerge from the tree next year as an adult, and will live, without feeding again, for about a month.
The two upright wasps are females. The one flat against the tree is a male, hoping to mate with a female who will soon emerge from the tree.