Metamorphosis means change in form. Most organisms undergo a change in form as they grow from an embryo to an adult. Some changes are radical and the immatures bear little resemblance to the adults; others are more gradual, with the immatures looking very much like the adults.
The term larva has very broad usage in invertebrate zoology, being applied to an assortment of forms (often the dispersive state) in virtually all invertebrate phyla. In Arthropoda other than insects, larvae is most often used for first stages, as it is in the mites and ticks, and for the first-stage hexapod larvae of millipedes, with “nymph” being used for second-stage mites and ticks. However, in continental Europe (especially France) “nymphe” refers to a pupa; in English-speaking countries “pupa” is universally used for the stage between the last instar and the adult of insects with complete metamorphosis (Holometabola).
In the insects, larva has been used in different ways, including such diverse forms as the immatures of the most primitive order Protura and of the most advanced order Hymenoptera (sawflies, ants, wasps, and bees). The termites present an interesting problem: some authors use nymph for all juvenile termites, whereas others use larva for those lacking wingpads, and nymph for those having wingpads. This is further complicated because the supplementary reproductives may be wingless or bear wingpads, even though the two forms are functionally equivalent.
The kind of metamorphosis insects undergo is closely related to which of the subclasses a taxon belongs to. In the Apterygota metamorphosis is either anamorphic in the Protura, in which three abdominal segments are added as the individual develops to an adult, or it is ametabolous, in which the number of molts is indefinite and molting may continue throughout life after sexual maturity (Collembola, Diplura, Archeognatha, and Zygentoma).
In the Pterygota there are two fundamental kinds of metamorphosis: the hemimetabola develop through the egg, larva, and adult stages, and the holometabola develop through the egg, larva, pupa, and adult stages. There are also other terms (defined below) that have been used to describe variations in metamorphosis.
LARVAE VS. NYMPHS AND NAIADS
Defining a larva is also necessary because its use has been highly variable in the Insecta. In 1918, Comstock proposed restricting the term larva to juveniles of the holometabolous orders, nymph to the juveniles of his paurometabolous (nonholometabolous) terrestrial orders, and naiad to the juveniles of his hemimetabolous nonholome-tabolous aquatic orders (Ephemeroptera, Odonata, and Plecoptera). Because these three aquatic orders have a much greater change in form from the last instar to the adult than the terrestrial hemime-tabolous orders, there was some basis for Comstock’s proposal to call them naiads. However, the Ephemeroptera and Odonata are Paleoptera, which cannot fold their wings, whereas the Plecoptera are Neoptera, which can fold their wings over their back, so they are not closely related.
Currently there is a tendency to use larva for all immature insects that are not eggs, pupae, or adults and the term immature insect for all life stages except adults, no matter how many specialized names are applied to the various developmental forms in the different orders.
There is little difficulty in defining an egg or an adult, but naming and defining the instars or stages that may occur between egg and adult can be problematic. Some insects are larviparous, never depositing eggs; some multiply from a single egg by polyembryony, and some are sexually mature as immatures (paedogenesis or neo-teny). Nevertheless, all of them undergo a series of molts as they grow. When larva is used in the comprehensive sense, the subcate-gories “exopterygote larva” (Hemimetabola that have the wingpads developing externally) and ” endopterygote larva ” (Holometabola that have the wingpads appearing externally in the pupal stage but having developed from internal larval histoblasts) are useful for pterygote immatures. A useful term roughly equivalent to larva in the comprehensive sense is “juvenile,” which can be used as a general term for nonadult larvae of all orders.
KINDS OF METAMORPHOSIS
Below are terms that are currently widely used for different types of metamorphosis. Most species are either holometabolous or hemi-metabolous, with more than 85% of them holometabolous and most of the rest hemimetabolous.
Anamorphosis
This term means development with fewer body segments at hatching than when mature, which is found in the Protura, in which three abdominal segments are added anterior to the tail as the individual develops to an adult. Because of this some workers believe proturans are not true insects.
Ametabolous
Ametabolous means development with the major change being an increase in size until sexually mature. The number of molts is indefinite, and molting may continue throughout life; it is found in Apterygota (excluding the Protura).
Simple Metamorphosis
This is a broad term covering all types of metamorphosis except holometabolous.
Hemimetabolous (Gradual, Incomplete, Direct, Paurometabolous)
Development through egg, larva, and adult is covered by this term, which includes everything except ametabolous, anamorphosis, and holometabolous. Among the hemimetabolous insects, most species are found in three orders, the Orthoptera, the Heteroptera, and the Homoptera.
Holometabolous (Complete, Indirect)
This means development through egg, larva, pupa, and adult.
All insects do not fit neatly into Hemimetabola or Holometabola. Some Hemimetabola are intermediate in having one or more non-feeding stages before the adult instar and in having a last instar that forms into a pupa. For example, in the whiteflies (Homoptera: Aleyrodidae) the first instar is active, but subsequent instars are stationary, resembling and feeding like scale insects, and the last instar stops feeding and becomes a pupa, with the wings developing internally. This could be termed holometabolous, but the homopterans as a whole are certainly hemimetabolous. In the Thysanoptera (thrips), there are two feeding instars followed by two or three nonfeeding instars, the propupa and pupa, which may be contained within a cocoon formed by the last feeding instar. This is certainly closer to Holometabola than to Hemimetabola.
