Wasp eaters (or Wasp weasels in older chapters) are small constructs with eight legs developed from by Doctor Bren for the Wulfenbach Vespiary Squad. Though to hunt slaver wasps, they can also detect the resulting revenants the wasps create. This would not have been considered useful until recently, as one could easily identify revenants by their shambling zombie-like behavior. However, with the discovery of the sneaky, dangerous ones who have the audacity to look and behave like normal human beings until the Other calls on them, the weasels have become even more useful (at least to the Baron).
On this particular occasion, a convenient demonstration becomes available when the weasel discovers that a messenger from Sturmhalten, who's calling on Klaus at the same time, is a revenant (and is taken off to be, um, "treated"). We see from this the basic way the wasp weasel works: it sniffs the interesting person and if he or she is "clean", hums contentedly (“♫”) afterward; while an encounter with a revenant provokes a loud hiss or shriek, raised hackles, and so on. The reaction can be considerably more emphatic, ranging up to full-fledged freakout if they smell an actual wasp. Reactions to non-revenants can vary. If a wasp eater sniffs someone who could really use a bath, the little weasel would gag in disgust. Another of them adversely when briefly crossing paths with a Dreen; whether the latter entity counts as a revenant or is just that scary is currently unknown.
The Baron uses this test to publicly "prove" that Gil is a revenant, but it's almost certain that Klaus rigged the process somehow to produce a false positive.
Physically, the weasels appear similar in coloration and overall aspect to ordinary weasels (Mustelids of genus Mustela, which do hunt primarily by smell and occasionally eat insects), though perhaps larger (30-50cm vs 20-30cm). They appear to have a strange mechanical implant to the left hemisphere of the brain consisting of several small spheres, and perhaps most strikingly (once it is noticed), two extra pairs of legs.
When the Squad escapes Castle Wulfenbach and makes a forced landing in Mechanicsburg, we learn that although most of the constructs have been made individually, eight-legged weasel kits (with less-prominent “implants”) survive the crash. These may be the result of a successful program by Dr. Bren to produce a version capable of breeding.
The weasels on the crashed ship really like Tarvek. Happy weasels make a “dook” noise, or squeal “eeeee” or “squee” when excited. Kits go “neep” and “skeee”. One has taken a shine to Agatha Heterodyne and now accompanies her as well; following the timeskip and the disruption of the Wulfenbach empire, it is the only one of its kind known to be still alive.
On Tarvek's orders, the surviving members, weasels, and materials of the Vespiary Squad secretly spirit themselves out of Mechanicsburg interspersed in the retreating Wulfenbach forces. They plan to go underground due to mistrust of Wulfenbach forces after the Baron ordered them eliminated, and await further orders from Tarvek. However, two and a half years later, there is no sign that any of them have survived, and Agatha's widely-disseminated dissertation on the subject is being used by (at a minimum) Gil and Colette Voltaire to create a new breed of the animals.