I'll try to explain it in english, but it won't be so easy for me... Hope you're gonna understand what I mean guys
Well, let's start with
Orthopeta. It's an order of insects where you can find crickets, grasshopers, weta, katydids, locusts and so on... All those species with legs allowing them to jump.
Then,
Orthoptera are divided in two groups. The first one is the group where you can find locusts, grasshopers... Vegan species, with short antennae: they are called
Caelifera. The second group is the one that is important on this topic: the group of the
Ensifera. They have long antennae (often as long as the body) and they eat plants and other insects. That's the case of katydids, wetas, crickets and so on.
To answer you, Keith, wetas is the name of two families of
Ensifera. The are wingless, nocturnal and live in New-Zealand and Australia. The are principally vegan, it's a kind of fossil group, they don't seem to have evolved a lot.
The katydids is the common name for the
Tettigonidae family (very different that what I supposed... in french, "katydide", wich is the translation of "katydid", is the name we use for the tropical and leave-like ones), so all those
Ensifera that are most evolved (I think) than he weta, often green (despite there are some brown species) more "lateraly compressed"...
So here, in fact, RaZias took a picture of what you call "katydid" in english
Hope you see what I mean... If you don't, check "weta", "katydid" and so on on google