Nymphs, Naiads and Larvae
So there is a difference in the terminology when talking about the young of animals, particularly insects, and they can be called Nymphs, or Larvae (and a Naiad is a form of nymph, as you will find out below). So what is the difference?
Here is the shedded skin of a grasshopper. You can find these ghostly shells around as you walk, from many of the insects listed below, as this is how they grow, without a skeleton.
A nymph - when referring to animals, and particularly insects - means the immature form of an insect which undergoes gradual metamorphosis before reaching its adult stage. A nymph's overall form already resembles that of the adult, except for a lack of wings (in winged species). Here is a planthopper nymph on the right.
In addition, while a nymph moults it never enters a pupal stage. Instead, the final moult results in an adult insect. Nymphs undergo multiple stages of development called instars. Here a Red -Nosed Cicada emerges from its final moult in a fully grown adult, with wings.
This is the case, for example, in Orthoptera (crickets and grasshoppers), Hemiptera (cicadas, shield bugs, etc.), mayflies, termites, cockroaches, mantises, stoneflies and Odonata (dragonflies and damselflies).
Nymphs of aquatic insects, as in the Odonata (Dragonflies), Ephemeroptera (mayflies), and Plecoptera (Stoneflies), are also called naiads, an Ancient Greek name for mythological water nymphs.
Here above you can see the dragonly nymph on the right, in water, with its wing buds. It then has a final moult and emerges as a dragonfly, on the right.
In older literature, these were sometimes referred to as the heterometabolous insects, as their adult and immature stages live in different environments (terrestrial vs. aquatic).
A larva on the other hand is a juvenile form that undergo holometabolous, or complete metamorphosisis with a distinct juvenile form, and some larva enter a transitional stage called a pupa (like a caterpillar to a chrysalis to a butterfly). Many animals with indirect development such as insects, amphibians, or cnidarians (basically jelly fish) typically have a larval phase of their life cycle, where they simply rest, and do not eat, or move, as they undergo a complete transformation.
A larva's appearance is normally very different from the adult form (like the example above of caterpillars and butterflies) with completely different forms, structures and organs and their diet can also be completely different.
Larvae can be found in completely different environments to the adults, like tadpoles, which live almost exclusively in water, but as frogs they can live outside water.
As you can see here, the birdwing Larva (caterpillar) and final adult (butterfly), are completely different.