Thanks for checking into this, Lynette. My local spider expert Lloyd Crim agrees with Enoplagnatha as a possibility but was also wondering if it might be an orb weaver in the Araneae. Admittedly, this is all very hard given the very small size of the specimen, the changes in appearance they undergo through different stages, and of course the less-than-stellar quality of my photo. Unfortunately there won't be a chance for a later photo. Lloyd tried raising the spiderlings, but released them when his effort at feeding was unsuccessful.

Thanks Lynette, Rod, and Lloyd for your help in this ID effort!

... except to note that the only spiders I have seen on trees in my yard to date have been Six-spotted Orbweavers (Araniella displicata). I also see wolf spiders (Lycosidae) in my yard frequently, but have never observed one in a tree.

Iowa spider expert Lloyd Crim now has the spiderlings and will feed them in an attempt to get more mature specimens. We'll post a update to this note if that is successful.

I should also note for the record that the purplish color on the abdomen of the spiderling in the photos is not genuine, but rather an effect of the LCD lights built into my microscope camera; they tend to turn pale areas into a light purple color on digital photos.