Protelean organisms are those that begin the growing phase of their lives as parasites, and in particular, typically as internal parasites. As a rule they end that phase of their lives parasitoidally by killing or consuming the host, and then they emerge as free-living adults.

Functional aspects

Protelean organisms are widely regarded as a special class of parasites, or more usually parasitoid

Parasitoid

A parasitoid is an organism that spends a significant portion of its life history attached to or within a single host organism in a relationship that is in essence parasitic; unlike a true parasite, however, it ultimately sterilises or kills, and sometimes consumes, the host...

s. The most typical examples of such organisms are the parasitoidal Hymenoptera

Hymenoptera

Hymenoptera is one of the largest orders of insects, comprising the sawflies, wasps, bees and ants. There are over 130,000 recognized species, with many more remaining to be described. The name refers to the heavy wings of the insects, and is derived from the Ancient Greek ὑμήν : membrane and...

, Diptera

Diptera

Diptera , or true flies, is the order of insects possessing only a single pair of wings on the mesothorax; the metathorax bears a pair of drumstick like structures called the halteres, the remnants of the hind wings. It is a large order, containing an estimated 240,000 species, although under half...

, Strepsiptera

Strepsiptera

The Strepsiptera are an order of insects with ten families making up about 600 species...

, and some other insects. Usually such insects are holometabolous

Endopterygota

The Endopterygota, also known as Holometabola, are insects of the subclass Pterygota which go through distinctive larval, pupal, and adult stages. They undergo a radical metamorphosis, with the larval and adult stages differing considerably in their structure and behaviour...

, which preadapts

Exaptation

Exaptation, cooption, and preadaptation are related terms referring to shifts in the function of a trait during evolution. For example, a trait can evolve because it served one particular function, but subsequently it may come to serve another. Exaptations are common in both anatomy and behaviour...

 them for such a life history

Biological life cycle

A life cycle is a period involving all different generations of a species succeeding each other through means of reproduction, whether through asexual reproduction or sexual reproduction...

 because it implies that their larval stage of life differs from the adult stage.

There are however many other ranges of protelean organisms, such as early instar

Instar

An instar is a developmental stage of arthropods, such as insects, between each molt , until sexual maturity is reached. Arthropods must shed the exoskeleton in order to grow or assume a new form. Differences between instars can often be seen in altered body proportions, colors, patterns, or...

s of certain species of mites that attack other Arthropoda.