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Pronymph Hypothesis

Metamorphosis of Holometabolous (Left) and Hemimetablous (Right) species compared.

Hypothesis for the origin of the holometabolous pupa proposing that holometabolan larvae are an extended version of the hemimetabolous pronymph. A pronymph (hatchling or pre-hatching stage, distinct from subsequent nymphal stages) is the evolutionary precursor to the holometabolous larva, and the holometabolous pupa is the sole nymphal stage.

Based on endocrine studies and morphological comparisons of the development of insect species with and without metamorphosis, support for this hypothesis has been gathered. The three stages of ancestral insects: the pronymph, nymph and adult are proposed to be equivalent to the larva, pupa and adult stages of complete metamorphosis insects.

See also

• Holometabolism
• Hemimetabolism
• Metamorphosis

Support for Hypothesis

The proposed relationship between pronymphs and larvae is reflected in a number of similarities between the two stages that support the legitimacy of the pronymph hypothesis.

Sclerotization

Sclerotization is the biological process of cuticle hardening and stabilizing that occurs periodically during each stage of insect development. The pronymph cuticle has a distinctive ultrastructure and is lacking rigid tanned plates (sclerites) that characterize nymphal and adult cuticles. This unique characteristic is shared with the larval body cuticle which is typically soft and lacks the same sclerites as that of pronymphs.

Juvenile Hormone

The sesquiterpenoid juvenile hormone (JH) is a blood borne 'inhibitory hormone' that determines the juvenile character of insect moults. JH ensures that a larva is succeeded by another larval stage or phase. Only when larvae attain adequate size will JH levels subside allowing for a metamorphic moult into a pupa or different stage. High levels of JH prevents premature metamorphosis equally in hemimetabolans and holometabolans, suggesting that their juvenile stages (larva and pronymph) are equivalent.

Morphological

Pronymphs and larvae both lack wing buds, flattened structures on the bodies of nymphs that form the wings in adult insects. These structures are present however in nymphs and pupae.

Reduced Nervous System

The larval sensory nervous system also shows similarities to that of the pronymph. At the beginning of a pronymphal stage early-born neurons known as 'pioneer' neurons provide pathways that are later used for the growth of nymphal sensory axons into the central nervous system. Similarly a small number of sensory neurons can be found in the appendages of developing larvae of holometabolans. These neurons are considered to be homologous to these sets of pioneer neurons that are present at the start of the pronymphal stage of hemimetabolous species. Making this a primitive condition shared with the pronymph rather than a derived condition that reduces the sensory systems of larvae.

Why did Larval Stage Form?

The evolutionary development of larvae derived from primitive hemimetabolan pronymph stages is also supported by why and how this seperation occured.

Niche Partitioning of Adult and Young

The larval stage of holometobolous species are specialized feeding stages allowing for growth and preparation for development. This specialized stage makes it possible for them to live in inside beneficial environments such as plants or water. Adult holometabolans are specialized mating stages allowing for the production of offspring when ready.

Overall holometabolous insects are provided with more control over their development through distinct stages of metamorphosis. They have the ability to pupate quickly or enter a 'dormant' stage (diapause) based on the environment they are currently inhabiting (many insects enter diapause as pupae during the winter). These abilities provide greater fitness to holometabolans resulting in greater reproductive success.