of JH synthesis in instars of a threshold size leads to ecdysteroid secretion inducing metamorphosis. Experimental studies show that, with the exception of higher Diptera, treatment of the final instar stage with JH causes an additional immature molt and repetition of that stage. 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 Krüppel homolog 1 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. Compared to many other sequences, the level of conservation is high, even between B. germanica and D. melanogaster, a highly derived holometabolan species. The conservation is especially high in the C2H2 Zn finger domain of the homologous transducer, which is the most complex binding site. This high degree of conservation of the C2H2 Zn finger domain in all studied species suggests that the Kr-h1 transducer function, an important part of the metamorphic process, might have been generally conserved across the entire class Insecta. 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. This paper was met with severe criticism, and spurred a heated debate in the literature. Orders The orders that contain holometabolous insects are: Coleoptera – Beetles Diptera – Flies Hymenoptera – Ants, bees, sawflies, and wasps Lepidoptera – Butterflies and moths Mecoptera – Scorpionflies Megaloptera – Alderflies, dobsonflies, and fishflies Miomoptera (extinct) Neuroptera – Lacewings, antlions, etc. Protodiptera (extinct) Raphidioptera – Snakeflies Siphonaptera – Fleas Strepsiptera – Twisted-winged parasites Trichoptera – Caddisflies See also Hypermetamorphosis Metamorphosis References Further reading External links How Did Insect Metamorphosis Evolve? Scientific American Insect developmental biology simple:Complete metamorphosis