Katydids are a member of the order Orthoptera (grasshoppers, katydids, crickets, etc.) and the family Tettigoniidae. Orthopteran nymphs, or immatures can be distinguished from adults by the development of their wings. Only adult insects have completely developed wings (think butterflies, beetles, and flies). Immature insects have not yet developed wings (think caterpillars, grubs, and maggots). Katydids, like other orthopterans, have incomplete metamorphosis, meaning they lack a pupal stage (think chrysalis or cocoons). Instead, the immatures generally resemble the adults in appearance, and partially developed stubby wings, called wing pads, may be present in some of the later molts. The nymph above is very young, and has no wings or wing pads present.
Here’s a picture from my own backyard in Texas!
Katydids are also called ‘long-horned grasshoppers’ in reference to their long antennae. Orthopterans of the suborder Ensifera, or ‘sword-bearing’, generally have antennae longer than their body length as well as exserted ovipositors (sword-like egg-laying tubes that allow females to saw their eggs into plants or insert them down into the fround.) Thus, katydids (like crickets) can be distinguished from grasshoppers by their long slender antennae and legs.
Below, you can see a grasshopper nymph, from the suborder Caelifera. This grasshopper is a member of the family Acrididae, the ‘short-horned grasshoppers.’