Anyone have any idea what these are? I spotted a line of four or five of them on a tree by my house this morning. They're big -- two-three inches -- although one was noticably smaller. I'm especially curious about the substance they're secreting from the abdomen. When I first passed by (without a camera, sadly) two of them had actually formed flat, vertical discs of the stuff on the ends of their tails. When I saw them, they were either secreting the stuff, or applying it (?) to the underside of the thorax, as you can see in the second picture. Though they were all together, they didn't seem to be interacting, and they sort of wandered off one at a time when they were done. I didn't spot any of them flying.
For reference, these picutres were taken in South Hadley, Massachusetts. Weather is hot and quite humid. There's a small pond (Titus Pond Conservation Area) about a block away. Much thanks for any info!
Megarhyssa is a superb ichneumon wasp - you're lucky to be able to find them Mayflies are an entirely more flimsy kind of insect where the 'tails' are not strong enough to drill into wood and, I think, are used more for stability in flight.