Holometabolism
Holometabolism, also called complete metamorphosis, is a form of insect development which includes four life stages: egg, larva, pupa, and imago (or adult).
In some species the holometabolous life cycle prevents larvae from competing with adults because they inhabit different ecological niches.
Many adult insects lay their eggs directly onto a food source so the larvae may begin eating as soon as they hatch.
Larvae can be classified by their body type: The larval stage is variously adapted to gaining and accumulating the materials and energy necessary for growth and metamorphosis.
According to the latest phylogenetic reconstructions, holometabolan insects are monophyletic,[6][7] which suggests that the evolutionary innovation of complete metamorphosis occurred only once.
These fossil remains show that the primitive Apterygota, and the ancient winged insects were ametabolous (completely lacking metamorphosis).
[citation needed] By the end of the Carboniferous, and into the Permian (approximately 300 Ma), most pterygotes had post-embryonic development which included separated nymphal and adult stages, which shows that hemimetaboly had already evolved.
Harvey suggested that the nutrients contained within the insect egg are so scarce that there was selection for the embryo to be forced to hatch before the completion of development.
During the post-hatch larval life, the "desembryonized" animal would accumulate resources from the external environment and reach the pupal stage, which Harvey viewed as the perfect egg form.
However, Jan Swammerdam conducted a dissection study and showed that pupal forms are not egg-like, but instead more of a transitional stage between larvae and adult.
[10] In 1883, John Lubbock revitalized Harvey's hypothesis and argued that the origin and evolution of holometabolan development can be explained by the precocious eclosion of the embryo.
X. Belles illustrates that the maggot of a fruitfly "cannot be envisaged as a vermiform and apodous (legless) creature that hatched in an early embryonic stage."
[15] More recently, an increased focus on the hormonal control of insect metamorphosis has helped resolve some of the evolutionary links between hemi- and holometabolan groups.
The molecular pathway for metamorphosis is now well described: periodic pulses of ecdysteroids induce molting to another immature instar (nymphal in hemimetabolan and larval in holometabolan species) in the presence of JH, but the programmed cessation of JH synthesis in instars of a threshold size leads to ecdysteroid secretion inducing metamorphosis.
The increased understanding of the hormonal pathway involved in metamorphosis enabled direct comparison between hemimetabolan and holometabolan development.
Most notably, the transcription factor Krüppel homolog 1 (Kr-h1) which is another important antimetamorphic transducer of the JH pathway (initially demonstrated in D. melanogaster and in the beetle Tribolium castaneum) has been used to compare hemimetabolan and holometabolan metamorphosis.
Namely, the Kr-h1 discovered in the cockroach Blattella germanica (a representative hemimatabolan species), "BgKr-h1", was shown to be extremely similar to orthologues in other insects from holometabolan orders.
In 2009, a retired British planktologist, Donald I. Williamson, published a controversial paper in the journal Proceedings of the National Academy of Sciences (via Academy member Lynn Margulis through a unique submission route in PNAS that allowed members to peer review manuscripts submitted by colleagues), wherein Williamson claimed that the caterpillar larval form originated from velvet worms through hybridogenesis with other organisms, giving rising to holometabolan species.
