Snakes in the gate! Or insects?

I'm trying to figure out the kanji on the bottom of a mug I inherited from some person terminated by my employer several years ago—a pretty mug, wave-patterned red with a gold dragon on each side, facing one another opposite the handle, with an inset tea infuser and a lid/stand and saucer. (I'm sorry for the person who left it, but I'm happy to have it. It's pleasant to drink coffee from dragons.)

The logo on the bottom is an encircled dotted double mountain, with the letters MTPI, and below that two kanji. The second of the kanji is 通 つう tsuu; that's easy. The first should be easy—it's just 門 with 虫 inside, 14畫—but I can't find any trace of it, in either Japanese or Chinese kanji resources. You'd think it would show up as a variant, at least, or a simplification of something older.

Fun: The older (pre-simplified) form of 虫 apparently is 蟲—just the same kanji, but three times—insect, insect, insect!!! Jisho.org defines it as an insect but also as "temper"; there must be more of a story behind that. Henshall says little but that the basic kanji represents a partially coiled snake and that anciently snakes and insects were not differentiated; I find the latter part hard to believe. If one or the other were in your bed, you'd know the difference.

The Book includes the kanji 門+虫—examples of it as written by eight calligraphers—but doesn't give a reading for it. (I'm sure it's indexed by sound, but that's not helpful.) My suspicion, given that 虫 can suggest a serpent or worm, is that the kanji mean something like "dragon pattern". Jisho gives variants for 竜, but nothing seems likely. Mysterious....