[MikeHardman]

I see several of these each year, scattered around the countryside.
Any ideas? (If not, I'll have to take my scalpel to one...)

[oron peri]

Yes Mike these are galls cauesed by an oranged colored aphid named Baizongia pistaciae.

[MikeHardman]

Thanks Oron.
Because these galls are big (the one in the photo was about 12cm long), I was going to ask how big those aphids were - so I could prepare to defend myself!
But I see they are normal size, just lots of them:
"These galls form perfectly closed pouches with a volume of tens of milliliters, which may contain thousands of aphids, representing four generations of descendants of a single female fundatrix."
(http://knol.google.com/k/baizongia-pistaciae-l-a-gall-inducer-aphid-in-pistacia-palaestina-boiss-trees#)
That page gives a good explanation of their life cycle.