Growth and Metamorphosis

Stages of spermathecal development in Drosophila melanogaster: Images: Mark Mayhew (UQ)

Objectives

At the end of this topic, you will:

Topic outline

Activities

 

Review Minilecture:

Insect Life Histories

A lecture from BIOL2205 “Insect Science” for review purposes with references to Gullan and Cranston's "The Insects, an Outline of Entomology"

Presented by D. Merritt

download video (.m4v) 9MB

 

minilectureMinilecture:

Growth and Metamorphosis

Presented by D. Merritt

download video (.m4v) 25MB

alternative format( .mov)13 MB

lecture notes (pdf)

 

Evolution of Metamorphosis

To grow, an insect must moult. In most insects, the epidermal cells replicate after apolysis, accommodating a larger surface area for the next instar. Moulting accommodates (1) change in size and (2) change in shape. The degree of change between instars varies. The main types of postembryonic development are ametaboly, prometaboly, hemimetaboly, neometaboly and holometaboly.
 

Chapter 15

Apterygotes

Ametaboly: Apterygotes, e.g. silverfish, undergo progressive growth and show no metamorphosis. Moult as adults. No predetermined final instar.

Pterygotes

Prometaboly: Ephemeroptera have two winged stages, the wings develop in the pre-adult stage or subimago. They have 2 “postmetamorphic” stages.
In all other pterygotes only adults have wings

Hemimetaboly: Immatures resemble the adults. Metamorphosis is not as marked as in holometabola. Functional wings and adult genitalia appear in the final stage. Wing pads are visible in earlier stages. No quiescent pupal instar. Usually immatures are called nymphs, but “larvae” is sometimes used.

Holometaboly: External wing rudiments not seen until the pupal instar. The pupa is the intervening, relatively quiescent, instar between larva and imago. The pupa is sometimes capable of movement. Wing rudiments may develop as internal inpocketings, termed imaginal discs. Immatures are known as larvae.

 

In part, from Gullan and Cranston

The intervening step, alate ametaboly, is not present in modern insects.

Neometaboly appears to be a specialisation of one particular group, derived from hemimetabolic development.

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Diagram showing the visible stages of development and stages of apolysis (A) and ecdysis (E), defining the pharate periods, in an exopterygote insect (grasshopper) and three holometabolous insects: butterfly, midge and fruitfly. Note the difference between a pupa and a puparium.

 

Growth and Moulting

Growth Strategies

Many insects or parts thereof grow according to Dyar's rule: the ratio between a given dimension in one instar and the same dimension in the next is constant throughout all instars.
For example head capsule width of caterpillars increases by a ratio of 1.4:1 at each moult
There are however a number of exceptions: it is a " rule of thumb"
Growth is exponential therefore the last instar experiences most growth, measured as increase in weight or size.
Allometric growth (some parts of the body grow at different rates to others) and isometric growth
There is a critical weight depending on size at beginning of instar.
Hormonal control is critical (next lectures).

Cellular basis

Mitotic increase in cell numbers is the most common form of growth. Cell division occurs just after apolysis and the epidermal cells subsequently change shape, flattening out to occupy a larger area than in the previous instar.

Endoreplication is a form of cell growth whereby the cell increases in volume and the chromosomes replicate, but the cells do not divide (common in larval Diptera). It is most common in cells that are metabolically very active, for example secretory cells such as the salivary glands. This process gives rise to the polytene chromosomes, perhaps best known in the salivary glands. Because these cells have lost the ability to divide, they die at metamorphosis.

 

Evolutionary Aspects of Growth and Metamorphosis

Sehnal, Svacha and Zrzavy (1996) argue that there are no fundamental differences between hemi- and holometabola. Periodic moulting has advantages: it allows for growth while affording the protection offered by the possession of an exoskeleton, possibility of regeneration.

Origin of metamorphosis

1

Reduction of wing rudiments in juveniles rendered juveniles less vulnerable and capable of invading new environments.

2

Predetermined final instar. Adult moults repeatedly expose insects to dangers of moulting (Disadvantages of moulting include: time loss during the apolysis-ecdysis-postecdysis phases, vulnerability to predation, risk of unsuccessful ecdysis.)

3

Fixing flight and reproductive activity into the final instar. Moulting with fully-formed wings is especially difficult.

4

Allowed the adult form to evolve further specialisations without the restrictions imposed by the requirement to moult.

5

This series of evolutionary events could apply to both hemi- and holometabola

Number of Instars

6

Disadvantages of moulting led to a tendency to reduce the number of immature stages in some insects. Combined with point 2, allowed life cycle to proceed more quickly and utilise ephemeral habitats.

7

Selection pressure for a streamlined worm-like larva with reduced wing pads and appendages allowed invasion of new and relatively safe environments such as soil, sand, leaf litter, dung, etc.

Holometaboly

Restriction of wing development to the very latest instars, i.e. wing development in holometabola is retarded by comparison with hemimetabolous insects.
The pupa (holometabola only) evolved as a quiescent phase, primarily to allow the external growth of the wings. With a non-feeding quiescent instar (the pupa) the development of imaginal features such as wings could be “crammed” into this instar.

Imaginal discs of holometabola arose as a way of allowing even more rapid development in the pupa by allowing some of the development of the wings to occur within the body.

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Imaginal Discs

Imaginal discs

Elongation and shaping of appendages
Change in cell shape from cuboidal to squamous.
Cells remain attached by zonula adherens at apical surface

Eversion

Contraction of the peripodial membrane by change in cell shape from squamous to cuboidal
Spreading and fusion of disc tissues

Primordia

Less extreme versions of imaginal discs are seen in Lepidoptera. The imaginal disc associated with the larval antenna (purple) is a basal, inward extension of the larval antenna that becomes everted at metamorphosis. Larval legs have regions of epidermis (coloured) that will proliferate to give rise to the adult structures. Larval components (grey) degenerate.

left: from Svacha (1992)

Below: from Tanaka and Truman (2007)

 

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Origin of the pupa

Berlese and Heslop-Harrison models emphasise a fundamental difference between nymphs and larvae.
The main difference between the Poyarkoff and Hinton models is that Poyarkoff sees the pupa as an extra or interpolated adult stage, that was required for the evolution of holometaboly.
According to Truman and Riddiford’s (1999) interpretation, the pupa is a modified, resting last nymphal stage

From: Sehnal, Svacha and Zrzavy (1996)

Truman and Riddiford (1999) proposed that a stage retaining many embryonic features, called the pronymph, undertook more moults and became a true feeding stage, the larva.
The nymphal stages were reduced in number and the last became the pupa.
Matches the Berlese and Heslop-Harrison models.
Supported by evidence from activity of Juvenile Hormone (black).

From: Truman & Riddiford (1999)

 

References

Erezyilmaz DF (2006). Imperfect eggs and oviform nymphs: a history of ideas about the origins of insect metamorphosis. Integr Comp Biol 46:795-807.

Fristrom and Fristrom, The metamorphic development of the adult epidermis. In: M. Bate and A. Martinez-Arias, Editors, The Development of Drosophila melanogaster vol. 2, Cold Spring Harbor Press, Cold Spring Harbor, NY (1993), pp. 843–897.

Nijhout HF (1994). Insect Hormones, Princeton University Press, Princeton.

Sehnal, Svacha and Zrzavy (1996). Evolution of insect metamorphosis. In: Metamorphosis: postembryonic reprogramming of gene expression in amphibian and insect cells / edited by Gilbert, Tata, Atkinson. pp. 3-58. (QL494.5 .M48)

Svacha P (1992). What are and what are not imaginal discs: reevaluation of some basic concepts (Insecta, Holometabola). Dev Biol 154:101-117.

Tanaka K and Truman JW (2007). Molecular patterning mechanism underlying metamorphosis of the thoracic leg in Manduca sexta. Dev Biol 305:539-550.

Truman JW and Riddiford LM (1999). The origins of insect metamorphosis. Nature 40:447-452.

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TOPIC REVIEW

Do you know…?

  • Reasons for evolution of metamorphosis
  • The main theories of the origin of the pupa
  • What is an imaginal disc
  • The different types of metamorphosis in insects

End of the Module: Metamorphosis

link to hormonesGo on to the next module: Hormones