I am not an expert on the insects found in Japan, however, I strongly suspect the insects in your photos are a type of woolly sawfly in the genus Eriocampa (Family Tenthredinidae). Sawflies are actually a type of primitive Hymenopteran, and are related to bees and wasps. Their larvae look more like caterpillars and feed on the leaves of plants. A few types, especially those in the genus Eriocampa, secrete a coating of white waxy filamentsthat covers the body as a deterrent to predators and parasites. I was able to find a few scientific articles indicating that there are several species of Eriocampa sawflies found in Japan, but they didn't included photos. However, I have included a link below to a webpage which shows photos of one of the species of Eriocampa which we have here in the United States and you can see the similarity. If you know what the host plant is and do an internet search for sawflies on that host plant, you may be able to come up with a specific name for yours because many of the species are fairly host specific. It is also possible that this could be a sawfly in a different genus that also produces the waxy coating, but you would need an entomologist from Japan to confirm that possibility.

 http://bugguide.net/node/view/32179/bgimage

Larry, Yes, you are right that the larvae in your photos don't look exactly like the photos I sent you which were of a sawfly species found in the US.  I was not aware of any caterpillars of butterflies or moths that secrete a white waxy coating. However, after a little more searching on the internet, I discovered that there is a type of skipper butterfly caterpillar in India as well as at least one of the giant silk moths from SE Asia which have waxy filaments, so maybe a unusual caterpillar is still a possibility. The "filaments" on your larvae do look much more symmetrical, suggesting that perhaps there might be filamentous "appendages" of the exoskeleton underneath the white coating. That might be more typical of a caterpillar than of the sawfly larvae that I am familiar with. And yes, the 2" length would be a bit large for most sawfly larvae, but again I don't know what might be occurring over in Japan. Unfortunately, from your photos I can't see what the head capsule looks like or see how many prolegs these larvae have, either of which might help me determine if this is really a caterpillar or a sawfly larvae. These do not look like skipper or giant silk moth caterpillars, so if this actually is a type of caterpillar, it is something I am completely unfamiliar with. However, if it was a caterpillar, I would think there would be some kind of photos of it somewhere on the web, and I just cant find anything with my searches that matches yours. One other factor, most caterpillars tend to be solitary, while sawfly larvae often live gregariously. Since you have several larvae in close proximity in the photo, I still think it is more likely that these are some type of sawfly. But without better photos or actual specimens, I just can't give you a better identification. 

Larry, I am not a plant person, and I don't know anyone here at MSU who might be able to identify the tree from just a photo. As you indicated this might be some kind of ornamental tree that isn't even native to Japan. If it was an ornamental that was also planted here in the US, then maybe one of the plant people somewhere in the Ask-an-Expert system might know what it is. However, even if we knew what the tree was, that may not lead to us being able to identify what might feed on it. Unfortunately, I just don't have the resources here to identify foreign insects unless they are something common enough to show up in a book or on the internet. I think to solve this mystery, you are going to need to find an entomologist somewhere in Japan that can recognize this from the photo.