May I ask what distinguishes the two? Is it the pattern of the spines, or something else I'm not seeing? (I am assuming, by the way, that the image I selected as an example was correctly identified as Pselliopus--or is this not the case?)

First and foremost, the image you selected as an example is definitely a Pselliopus nymph.

In Pselliopus nymphs, you'll notice the coloration is essentially orange, black and white and the legs have rather discrete annuli. In Zelus renardii nymphs, the coloration is paler and softer, there is a hint of green and the legs are more delicately speckled rather than ringed. Additionally the spines of the Z. renardii nymphs remain on the lateral or caudal margins of the developing tergites while the Pselliopus nymphs have their spines fixed in more dorsal positions or more anterior segments on the abdomen though keep in mind the extent of this armature depends on the instar.

A less definite character (owing again to developmental stage) is the length of the rostral segments. Adults of both genera have rostral segment two longer than the first segment. However, Zelus has an extremely short first rostral segment and its apex usually doesn't reach the anterior margin of the eye. Pselliopus has a longer first rostral segment, one that usually reaches the anterior ocular margin. This character can sometimes been seen in the nymphs though it doesn't always hold in the younger ones.

It's an easy identification to confuse and as logged under the thumbnail I linked, I had them confused on Bugguide until someone confined a nymph and photographed the newly adult Z. renardii which emerged.

for taking time to explain the differences so thoroughly. I appreciate it.