Short answers, pulled from the story.
A nymph is the immature form of many invertebrates, particularly insects that undergo gradual metamorphosis or hemimetabolism. This juvenile stage resembles the adult self from the beginning with the same body plan and limbs but lacks functional wings and mature genitalia. These creatures grow through a series of moults without entering a pupal stage.
Insect orders such as Orthoptera, Hemiptera, Odonata, Ephemeroptera, and Plecoptera include species that develop as nymphs. Arachnids such as spiders, mites, and ticks also possess a nymph stage. This developmental strategy is a fundamental thread woven through the fabric of invertebrate life.
In the year 1628, the English physician William Harvey published a work containing a hypothesis that the pupal stage was the result of imperfect eggs. He proposed that some eggs produced smaller versions of fully matured insects known as nymphs while others created intermediate forms requiring a second egg stage to reach adulthood. Modern science has long discarded this idea.
Aquatic nymphs of the order Odonata live entirely underwater and are sometimes referred to as naiads. These naiads possess gills and often have a completely different body shape compared to their adult forms. The distinction between nymph and naiad is often a matter of environment.
The relationship between humans and the nymph stage of insects is most visible in the sport of fly fishing where imitation of this life stage forms the basis of an entire industry. Patterns such as the Pheasant Tail Nymph account for over half of the fishing flies regularly used in the United States. Trout feed heavily on the immature forms of aquatic insects making the study of nymphs essential for anglers.