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The hunt for Last Universal Common Insect Ancestor LUCIA (the insect version of LUCA) goes on. It will go on because it will never be found. Rather than looking for one ancestor, why not consider several? All with 6 legs, segmented bodies and mouthparts?
Hunt for LUCIA
Hunt for LUCIA
The concept of the Last Universal Common Insect Ancestor (LUCIA) refers to the hypothetical most recent common ancestor of all insects. Insects are a diverse group of arthropods belonging to the class Insecta, characterized by a segmented body, exoskeleton, and three main body parts: the head, thorax, and abdomen. They are incredibly diverse, with over a million described species and many more yet to be discovered.
Key Points About LUCIA
Key Points About LUCIA
Evolutionary Context:
LUCIA is thought to have lived approximately 480 to 400 million years ago during the early to mid-Paleozoic era, possibly in the Ordovician or Silurian period.
This time frame coincides with significant evolutionary events, including the diversification of early land plants, which likely provided new habitats and food sources for early insects.
Ancestral Traits:
As a hypothetical organism, LUCIA would have had a set of characteristics common to all modern insects, from which various adaptations and divergences evolved.
Basic traits would likely include a segmented body with three distinct regions (head, thorax, abdomen), compound eyes, antennae, and three pairs of jointed legs.
The respiratory system would have consisted of a network of tracheae, a hallmark of modern insects.
Ecological Role:
The ecological role of LUCIA would have been fundamental, potentially involving interactions with early terrestrial plants and other primitive land animals.
Early insects might have been detritivores, herbivores, or predators, playing a crucial role in the nascent terrestrial ecosystems.
Evidence and Research:
Evidence for LUCIA comes from a combination of fossil records and molecular phylogenetics.
Fossils like those of the Devonian Rhyniognatha hirsti, which date back about 400 million years, provide clues to early insect evolution and their morphological characteristics.
Molecular studies compare DNA sequences across various insect groups to infer evolutionary relationships and estimate the timing of divergence from common ancestors.
Significance:
Understanding LUCIA helps scientists trace the evolutionary history of insects, shedding light on how they became one of the most successful and diverse groups of organisms on Earth.
This knowledge contributes to broader insights into the evolution of life on land, including the adaptation mechanisms that have enabled insects to thrive in virtually every terrestrial habitat.
In summary, LUCIA represents a crucial node in the tree of life, offering insights into the origins and early evolution of insects. Studying this ancestor helps illuminate the vast diversification and ecological dominance of insects throughout Earth's history.
Hmm..I personally think the hunt for LUCIA will go on forever..as there is no such thing. There is no evidence of monophyly in insects, it is quite possible/likely to have had several origins
My perspective aligns with some debates in evolutionary biology regarding the monophyly (single common ancestor) versus polyphyly (multiple origins) of certain groups. Let's delve into the nuances further:
Monophyly vs. Polyphyly in Insect Evolution
Monophyly vs. Polyphyly in Insect Evolution
Monophyly:
Definition: Monophyly refers to a group of organisms that consists of all the descendants of a common ancestor. In this view, all insects would have a single common ancestor (LUCIA).
Evidence Supporting Monophyly:
Molecular Phylogenetics: Genetic analyses, such as those using mitochondrial DNA and ribosomal RNA sequences, often support the monophyletic origin of insects. These studies compare genetic sequences across various insect groups to infer a single lineage.
Morphological Traits: Shared anatomical features, like the exoskeleton, segmented body, and three-part division of the body (head, thorax, abdomen), support the idea of a common ancestor.
Polyphyly:
Definition: Polyphyly suggests that a group of organisms arises from multiple ancestral sources rather than a single common ancestor.
Arguments for Polyphyly:
Convergent Evolution: Some researchers argue that similar environmental pressures could lead to the convergent evolution of similar traits in different lineages, resulting in the appearance of insects having multiple origins.
Fossil Record Ambiguities: The fossil record can be sparse and ambiguous, sometimes making it difficult to trace a clear lineage. Early arthropods show a variety of forms, and distinguishing the direct ancestors of modern insects can be challenging.
Current Consensus and Ongoing Debates:
Support for Monophyly: The prevailing scientific consensus still leans towards a monophyletic origin of insects based on extensive genetic and morphological evidence. Studies generally point to insects having diverged from a common ancestor within the larger group of arthropods.
Alternative Views: Some paleontologists and evolutionary biologists continue to explore the possibility of polyphyletic origins, especially when new fossil discoveries and genetic data provide conflicting signals.
Implications of Polyphyly:
Multiple Origins: If insects were found to have multiple origins, this would significantly alter our understanding of their evolutionary history. It would imply that the remarkable diversity of insects arose independently in different lineages through parallel evolutionary paths.
Adaptation and Evolution: A polyphyletic origin would emphasize the role of convergent evolution and the adaptability of early arthropods to various ecological niches.
Conclusion
Conclusion
The hunt for the Last Universal Common Insect Ancestor (LUCIA) is a dynamic and evolving field of study. While the current evidence supports a monophyletic origin of insects, the possibility of polyphyly cannot be entirely ruled out. The complexity of early arthropod evolution, coupled with ongoing discoveries and advances in genetic research, means that our understanding of insect origins will continue to be refined.
My scepticism encourages further investigation and critical evaluation of prevailing theories. The true nature of insect ancestry remains a fascinating topic, poised to reveal more secrets as research progresses. And perhaps a better understanding of the role in soil in insect evolution will make the difference.
The key seems to be that insects are ectognathus (external mouthparts ) compared with earlier versions being endognathus (protected mouthparts) . I suggest this occurred when the insect predecessors emerged from soil to open air. There could be several body forms. My hypothesis is that they could have come from other 'hexapods (6-legged arthropods) like springtails - classed as insects till the 1990s, diplurans, or proturans (also classed as insects till recently). In time gone by another soil creature with more legs was also suggested - symphylan.
Origin of Insects
Origin of Insects
Many hours have been spent trying to work out, and debating the origin of insects. I am making my pitch knowing that whatever version, there will disagreements. I would like to borrow Ghilarov's suggestion, and develop the role of soil in their evolution.
As Vannier (1987) said: “According to Professor Ghilarov, this double nature of soil insects - the combination of properties common to aquatic dwellers and terrestrial forms - is due to remarkable properties of soil as a transitional medium. Working on the behaviour of microarthropods (Acarina, Collembola) towards soil water....Soil, like other hygroscopic porous bodies, is a medium intermediate between the hydrosphere and atmosphere. This unique ecological milieu permitted the development of amphibious organisms, and I proposed that the term porosphere be used to describe this environment.”
There is some agreement that insects first appeared around this Period. There were earlier versions, but now is the first flight - a key character to define insects. We’ve already come across some primitive insects, bristletails and silverfish, 50 million years earlier. During the carboniferous period there were quite a few winged insects with beaked mouthparts, which have also become extinct since then.
The extract on right is from the most prestigious ‘Evolution of Insects’. There you can see they await the discovery to fill the gap in Mississippian insect fossils. Perhaps that is because there are not many to find. Or they are looking in the wrong place.
Our version here is that there would have been sand beeches and ponds early in this period (Mississippian) and then the muddy floodplains leading to the swamps in late carboniferous times (Pennsylvanian). Do they evolve mainly in these later conditions?
That dragonfly was flying around with no predators in sight to catch its prey. The were also large – now extinct – griffin flies, being ‘early’ dragonflies. Did crawl around ponds, later swamps - eating other creatures, then climb a stem, wait for their wings to emerge and dry out and then take off – the first major aerial creatures.
I had been looking for somebody other than myself who recognised the importance of soils in the great leap forward above ground. I thought I cannot be the only one. And it was when I was chasing down a rather boring paper my supervisor had written about soil mites many years ago, that I found the person, that helped explain how many creatures evolved above ground by coming from below.
"At present, it is difficult to imagine that just 30 years ago, the naively fantastic representations of A. Handlirsh, who painted the path of evolution from trilobites directly to winged insects, such as Palaeodictyoptera (an extinct order of 6 winged insects) were widespread". (MS Ghilarov Regularities in Adaptations of arthropods to the terrestrial life NAUKA Moscow 1970)
Molecular Clock
Molecular Clock
Molecular Clock
Molecular Clock
Perhaps we can turn to the molecules for some help.
“Molecular clocks are widely applied for evolutionary dating, but clocks for the class Insecta have remained elusive” (Gaunt & Miles 2002) Nevertheless, some molecularists say they have ‘resolved insect evolution’. A hundred 1KITE researchers from 10 countries declared that “Their analyses therefore suggest that insects and plants shaped the earliest terrestrial ecosystems together, with insects developing wings to fly 400 million years ago”. Hmm. There is no fossil evidence for that time.
The 1KITE (1K Insect Transcriptome Evolution) project is very impressive and studied the entirety of expressed genes of more than 1,000 insect species encompassing all recognized insect orders. For each species, so-called ESTs (Expressed Sequence Tags) are generated using next generation sequencing techniques. Their algorithms are superb at relating creatures with each other. But their dates?
“1KITE scientists present reliable estimates on the dates of origin and relationships of all major insect groups based on the enormous molecular dataset they collected. They show that insects originated at the same time as the earliest terrestrial plants about 480 million years ago. Their analyses therefore suggest that insects and plants shaped the earliest terrestrial ecosystems together, with insects developing wings to fly 400 million years ago, long before any other animal could do so, and at nearly the same time that land plants first grew substantially upwards to form forests”
When we said that molecular clocks generally predict earlier than fossils, we didn’t mean this long - about 150 myr difference There is a presumption that insects and plants evolved together. But plants were barely in existence 400 mya, and then they only covered the surface. The idea that somehow insects co-evolved is part of the old ‘terrestrialisation, of creatures coming on to land to land on plants. Where did they come from – they cannot change from soft crustacea to hard chitin during the flight. Or could they?
They go on “Terrestrialisation and basal diversification of Hexapoda was not possible before the existence of at least local terrestrial ecosystems, which provided the most essential ecological resources for life on land. We therefore set the maximum age of terrestrial Hexapoda to 450 Ma. Although there are putative older remains of land plants, the earliest unequivocal land plant megafossils date back into the mid-Silurian (about 425 Ma). This hypothesis is founded on diverse data from the fossil record and sets a generous and justifiable maximum bound for the early diversification of terrestrial insect”
If I am going to say they were a hundred million years out, I must have some points to back up the proposal. For many years we thought that Rhyniella was the first insect – around 400mya. Perhaps these 1KITE scientists were including this. But Rhyniellais now deemed not to be an insect, as we’ll see soon. Secondly, when attaching their fabulous algorithms to actual dates, it is all very vague. They give no reason or evidence to place insects when they do. Insect did not origin 480 mya - not a single fossil now we’ve ditched Rhyniella. They quote plant remains back 425mya, saying terrestrial food webs pre-dated them. There is no evidence of ‘terrestrialisation’ then. They quote this presence of plants – although they looked very different 400 ma compared with 300mya. But underneath is the presumption that the insects arrived with the plants. The strong relation of insects with plants was to be another couple of hundred million years later. Plant then did not rely on insects, as there were only a few running round.
Another molecular clock for insects also highlights one insect ancestor and plant co-evolution. It dates the origin of insects and ‘accords with the palaeontogical and biogeographic landmarks (Gaunt 2002) and says “Molecular clocks are widely applied for evolutionary dating, but clocks for the class Insecta have remained elusive…we reveal the insects arising from a common ancestor with the Anostraca (fairy shrimps) at around the Silurian-Ordovician boundary (434.2–421.1 MYA) coinciding with the earliest plant megafossil.” This is typical of molecular clock dating predicting earlier dates than fossil, but this about 100 million years earlier and based predominantly on modelling. It also reflects the presumption made in the earlier report – that insect and plants co-evolved. I believe plants and soils co-evolved, and insects came later – helping plants produce flowers. The lack of fossils may be due to lack of insects at that time.
400 my is long before insects made wings. Wings which fold give early advantage for insects. But how? Some say to enable them to get into crevices. You can see that – they could fly and hide. Or were they flaps created, at random in crevices, climbing up trees, which became useful to enable them to take-off. Wings had to come from somewhere, they did not just arrive fully formed and ready to fly. But they would not and could not have evolved in soils.
Basically, it is the same ideology behind the algorithm when making models, and so the same result is churned out. The new characteristic is that this has been taken to an even more detached level from the actual environment where evolution occurred.
Debate
Debate
This question has been the subject of debate and argument for many years. How did they evolve? There has probably been more vitriol exchanged about the origin of insects than any of the great evolutionary debates. In the correct language of the Royal Society in 2016: “A relatively accurate deep phylogeny (family tree)had been produced by 1904. It was not substantially improved in topology until recently when phylogenomics settled many long-standing controversies. Intervening advances came instead through methodological improvement. Early molecular phylogenetic studies (1985–2005), dominated by a few genes, provided datasets that were too small to resolve controversial phylogenetic problems. Adding to the lack of consensus, this period was characterized by a polarisation of philosophies, with individuals belonging to either parsimony (minimum number to explain) or likelihood (most probable) camps; each largely ignoring the insights of the other.” (Kjer et al 2016)
So, I may as well put my big boots on. Precursors must have already been around, living in moist but not aquatic, circumstances. Just where the springtails, proturans, diplurans and symphylans live. Is that a clue to their origins? There is no reason that insects evolved from one source - one glorious ‘ancestor’. Yet a lot of evolutionaries seem to think this is the holy grail – it is called ‘monophylogeny’. There seems to be a goal to find the oldest, single ancestor.
There are significant weaknesses in the insect fossil record for many fossil insects particularly before 323 MYA. Insect palaeontology depends on assigning fossils to extant taxa usually on the basis of wing characters. Consequently, no fossil has been identified that could represent a ‘‘missing link’’ between insect orders, and more recent ancestors of existing insects are seldom classified with certainty.
It could be that there were several sources – perhaps from animals already running round in the soil. These could have had most of those characteristics, but cannot stick their heads above ground. When they did, they found that exposed mouthparts were better at consuming food. The one key characteristic that defines insects from several soil creatures is their eating habits. Let’s not rule that out.
I had looked at soil creatures, seemingly similar to insects for years, thinking they must be related. But I seemed to be on my own. For decades, I thought I couldn’t possibly be the only person who has though of this.
Through Soil
Through Soil
I quote Ghilarov extensively (with my explanations not in italics) as he is virtually unknown in Western science. There are few translations despite his work being in Russian Academy of Science. I kept thinking evolution of insects must have come through soils, but couldn’t find anybody who agreed. I could not believe I was the only one who reckons that soils must have played a crucial part in insect evolution – the development of 6 legged creatures that conquered the outside world. I was trained as an entomologist and surprised there were so few insects in soils. He spent 30 years of his life trying to tell people the importance of soils in evolution of many – not just insects - of our small creatures. He could make the case for many other ‘clades’ of creatures.
If you thus believe soil is some sort of ‘amphibiotic state’, as Bosch depicted, that enabled insects and other creatures to evolve in the great leap forward, this raises the even bigger question. When and where was the soil for this to happen? The soil must have been – at least in part – already there? Insects could not have evolved from soil animals until there was soil for them to live in. And that period was around 350-300mya. It is this phase of soil growing that now provides that springboard.
Insects came from the soil, they did not construct it. Many people think insects are a key component of soils. There is no evidence that any insects were involved with soil development at the time. They were not around 350mya and really only came on to the scene in great numbers over the next hundred million years. So hopefully we may be able to follow developments as we go along.
Many people presume that insects would be involved with this important phase of soil evolution. Yet insects are noticeable by their absence inerly soil formation. Sometimes the carboniferous is known as the age of cockroaches. But there was not a great ‘radiance’ of insects around at this time. There were loads of other ‘primitive’ arthropods, but few insects.
Among the early insect groups found in this period are the huge predatory Griffin flies (Protodonata), among which was Meganeura, the giant dragonfly-like insect and with a wingspan of ca. 75 cm (30 in)—the largest flying insect ever to roam the planet. Further groups are the relatives of mayflies (Syntonopterodea), the abundant large beaked insects (Palaeodictyopteroidea) the diverse herbivorous Protorthoptera and numerous basal Dictyoptera, both ancestors of cockroaches.
Terrestrial Invasion
Terrestrial Invasion
So where do insects come from? According to 'The Terrestrial Invasion’ by Colin Little, Diplura, Protura and Collembola, along with Symphyla, all developed similar adaptations in movement from marine to 'land'. We have seen their vital roles in the soil, so their evolution to life on – rather than in – the ground, would be possible. Is it likely that these creatures were the ancestors of insects?
The same book says "Early terrestrial tracks document the presence of various arthropod lineages and support the view that insects themselves originated in a terrestrial environment. It is interesting to note that the arthropods comprised the earliest known terrestrial animals." They confirm that the creatures that are the progenitors of insects are springtails, proturans and symphylans - but they see them as part of 'Invasion of the land’, witness the -sub head 'Invasion of the land'. They identify the apterygote (no wings) but give no explanation of their role, either in the soil or in how they may have changed to evolve into insects.
“With regard to the origin of the insects, while many details remain unclear, a consensus is developing that the hexapods arose from an aquatic crustacean ancestor. The primitive condition in the insects, however, was a terrestrial lifestyle.” (Bradley et al 2009)
That is because they still seem to think that all the springtails, diplurans and insects walked out of the sea together, rather than insects evolving from springtails and their soil mates.
Ghilarov
Ghilarov
Then I found out about a man called Ghilarov, a Russian scientist who had spent 30 years working out how insects evolved. As Ghilarov introduced in his book“The schemes of phylogenetic relations between different groups of organisms were, until very recently, built only on practical materials, without taking into account the possible changes in environmental conditions in which the directions of evolution of certain groups could be realized. Therefore, evolutionary zoologists often came (and come) to diametrically opposed views on the relationships between different groups of animals.” (MS Ghilarov Regularities in Adaptations of arthropods to the terrestrial life NAUKA Moscow 1970)
He was re-inforcing the famous evolutionary scientist Dobzhansky, who said the mid-1960s that "The basic postulate of the modern biological theory of evolution is that adaptation to the environment is the guiding force of evolutionary change” Later he was to add that ‘Nothing in Biology Makes Sense Except in the Light of Evolution’ (Giaiamo 2023)
Ghilarov was writing when most evolutionary zoologists based their theories mainly on the shapes of organisms – their morphology. Since then, there has been much more reliance on the relation of molecules. Yet they still seem to come to opposed views. Is it part of their profession to disagree, or are they missing something?
My favourite quote of his is: “At present, it is difficult to imagine that just 30 years ago, the naively fantastic representations of A.Handlirsh, who painted the path of evolution from trilobites directly to winged insects, such as Palaeodictyoptera, were widespread” He went on.“The concept of the origin of terrestrial arthropods from aquatic ancestors is also generally accepted; However, the specific paths of evolution of individual groups of opinion are very contradictory.Suffice it to recall that, for example, insects have been for different researchers in the last 40 years, bred directly from trilobites (Handlirsch, 1925; Heegard, 1945), crustaceans (Crampton, 1928), from Symphyla (Tiegs a. Manton, 1947) other centipedes (de Beer, 1930), and from Onychophora(velvet worms) (Du Porte, 1965)!“ (MS Ghilarov Regularities in Adaptations of arthropods to the terrestrial life NAUKA Moscow 1970)
A few years later Cloudsley Thompson outlined five main theories of insect ancestors including Campodea, Sympyhla twice, Crustacea, and Trilobites (Cloudsley Thompson 1988)
And 50 years later, we can still hear people make similar claims to tracking down the ‘common ancestor’ of insects.
Ghilarov researched and explained how he believed that insects had evolved through the soil. To develop their organs, and metabolism, like their respiratory system, excretion, water retention, reproduction, egg protection & nitrogen metabolism they need the stable soil conditions to evolve slowly from aqueous to terrestrial life. It did not just happen.
“M.S. Ghilarov considered ways of animals' adaptation to the terrestrial mode of life through soil as the transit environment. He developed new approaches to the interpretation of aromorphic and idioadaptive(wherethe structure and function of organs alter but the ancestral form is preserved) changes in animals' organization and to the origin of the insect metamorphosis. Patterns, being common for all groups of animals, were revealed with regard to the processes of phylogeny (family tree) and mechanisms of their control by the principle of positive feedbacks.” (Striganova 2014)
"Insects are the most completely adapted to life on land. In the insect complex, there are at least two independent phylogenetic branches of the transition from the hidden (soil, in the wide sense of the word) way of life to living in open air - in collembolans and in ectogatous (external mouthparts) insects. At the same time, in different groups of insects, there is an unequal degree of connection with the original wet habitats.
Rows of emerging transitions of release from constant communication with a wet environment can be traced in different classes of centipedes. Among helixes, such series are outlined in general in a complex of different orders of arachnids and even within smaller taxa (for example, in ticks). We can analyse the whole range of transition from life in the water to the terrestrial existence.
If the currently distinguishing selection of scorpions from the arachnid class to the merostom class, then scorpions give another parallel branch of evolution from aquatic forms (fossil Palaeophonus) to terrestrial.
Finally, phylogenetically young and ecologically very complete series of transitions illustrate crustaceans such as isopods. The secondary transitions to life in the water of insect larvae (and the secondary transitions of secondary-water larvae to life on land!) make it possible to isolate more plastic adaptation features when changing habitats...
Regarding the lower taxonomic categories, the patterns of possible variations of characters and their combinations are expressed in the ideas of N. I. Vavilov (1922), who formulated the “law of homologous series” of variation(characters in different organisms that are similar because they were inherited from a common ancestor that also had that character). (Kupzow 1975) Knowledge of the patterns of evolution of groups allows us to foresee the direction of their further evolution, as pointed out by V.V. Popov (1939) with respect to some bees.”
I quote much of his work, especially any reference to perhaps the greatest ever agricultural scientist – Vavilov, who created the first ever seed bank, learning 24 languages so he understood the culture of the seeds not just the composition. Most of Ghilarov’s work has not been translated, and very little is quoted elsewhere, hence my extensive translation here. I came across his work when I found this guy 'Vannier' who I had stumbled upon when looking at academic paper by my old supervisor. Above all, Ghilarov made the case that we need to look at the environment - in his case soil - in oredr to better understand how they evolved, and hence their relation to each other. Here, we have the opportunity to configure this role of soil a lot more clearly than previously. Understanding the evolution of soil helps us understand the evotuion of those involed with it - like insects.
Vannier
Vannier
We came across this guy earlier as he wrote a paper about the poroshere. "I extended the idea to all porous bodies of having an internal surface which fulfils the same role; even before the formation of modern soils in the Cretaceous era with the first appearance of flowering plants (Angiosperms). I acknowledge that Professor Ghilarov's view based on ecophysiological considerations was of great help.”. That confirms my time scale pretty well, and indicates the soil has more development yet.
It was he title of his paper that caught my attention: (Vannier 1987) “The porosphere as an ecological medium emphasized in Professor Ghilarov's work on soil animal adaptations”. He explained:
"M.S. Ghilarov advocated (in Adaptations of arthropods to the terrestrial life NAUKA Moscow 1970) a theory on the evolution of insects through the soil as a special medium: the soil consisting of three phases (solid, liquid and gaseous) has as an environment many peculiar properties. As the air in the soil is almost always saturated with water vapour, and films of water are present around solid particles, the conditions of existence in this medium are a character intermediate between aquatic and epigeion.
So, for many groups of terrestrial invertebrates the soil was a transitional medium in the course of their evolution from aquatic to terrestrial habitats (Ghilarov 1959). Professor Ghilarov based his arguments on the eco-physiological adaptations of soil animals, in particular of insects that were different from those of epigeion forms. These include a high integumental permeability in soil dwellers, skin respiration, in tracheate forms spiracles operating without a closing mechanism, relatively little resistance to cold, little sensitivity to increased CO2 concentration, saprophagy like aquatic animals (detritophagy), and external insemination (spermatophores).
The late Professor Ghilarov was a great entomologist and an enthusiastic pedozoologist. He was also a far-sighted specialist who studied the evolution of terrestrial faunas. In the course of the VIIIth International Colloquium of Soil Zoology at Louvain-laNeuve (Belgium), he reviewed the main physiological adaptations which have permitted soil invertebrates to become independent of aquatic conditions and to engage in the conquest of terrestrial environments (Ghilarov 1983). For more than 35 years Ghilarov advocated a theory on the evolution of insects through the soil as a special medium: the soil consisting of three phases (solid, liquid and gaseous) has as an environment many peculiar properties. As the air in the soil is almost always saturated with water vapour, and films of water are present around solid particles, the conditions of existence in this medium are a character intermediate between aquatic and epigeion. So for many groups of terrestrial invertebrates the soil was a transitional medium in the course of their evolution from aquatic to terrestrial habitats (Ghilarov 1959).”
When we try to work out how insects evolved, the two strong characteristics they have need the closest attention - mouth & wings
Gnashers
Gnashers
The three orders - Collembola, Protura and Diplura are sometimes grouped together within a class called Entognatha. They are in the same subphylum of hexapods, as insects, all having six legs; but the key diving line between the springtails and their near mates is that they are Entognatha, meaning they have internal mandibles while insects are all Ectognatha. While this dividing line was proposed over a 100 years ago by Grassai, it was really defined by Hennig in the around the 1960's and 1970s.
It seems that this particular characteristic was behind the differentiation of , Collembola, Protura and Diplura, previously considered insects, from insects in the 1990s. The springtails and their mates have ‘internal’ or protected mouthparts, whereas insect mouthparts are external and exposed to the world. Up until then, this was not considered that important a distinguishing characteristic. It is now.
Perhaps that dividing line helps explain how insect evolution may have occurred. Insects are ectognaths – always with external mouthparts. The soil hexapods had protected mouthparts – inside gnashers, and only extended them when feeding. Insects evolved external mouthparts and this has increasingly been seen as the distinctive trait to distinguish them from springtail and their mates – by taxonomists. So perhaps this feature is what made insects. The ‘inside gnashers’ would enable feeding underground, in dark and difficult conditions. When more time was spent on the surface, their mouthparts could extend out, so they came closer to any feed source.
“An astonishing diversity of mouthparts have been tailored to use different resources of food. For example, dragonflies and crickets use biting-chewing motions of their mandibles to chop food particles, true bugs evolved piercing-sucking mouthparts to suck fluids from plants, flies evolved sponging mouthparts, and moths and butterflies evolved the unique proboscis to siphon mostly nectar of flowers . Although mouthparts functionally interact in nearly all insects to process food, many winged insects evolved a structural interaction resulting in the formation of new mouthpart types. Corresponding structural changes are radical and morphologically very different to each other, a comparable diversity and complexity of mouthparts is not present in any other arthropod group. It is unclear when this major trend in insect evolution – structural mouthpart interaction (SMI) – evolved for the first time.
The presence of a homologous SMI in Collembola and Diplura implies the presence of this basic principle in stemgroup representatives of the entire Hexapoda (insects in the widest sense). It remains unclear until which point SMIs are exhibited in the stemline of early insects before this principle occurs again in more derived winged insects . However, the biting-chewing mouthpart type with mandibles connected to the head via articulations, as shown in jumping bristletails, can no longer be attributed as the plesiomorphic condition in insects. Rather, we suggest that a "light" form of entognathy with an enlarged subgenal area, probably overgrowing the mouthparts laterally to some extend, might be ancestral. In this scenario, the mouthparts probably already interacted with each other due to some form of structural coupling. Exposed (Ectognathous), structurally uncoupled mouthparts are a derived groundplan feature of silverfish and winged insects.”
“In butterflies, bees, flies and true bugs specific mouthparts are in close contact or even fused to enable piercing, sucking or sponging of particular food sources. The common phenomenon behind these mouthpart types is a complex composed of several consecutive mouthparts which structurally interact during food uptake. The single mouthparts are thus only functional in conjunction with other adjacent mouthparts, which is fundamentally different to biting-chewing. It is, however, unclear when structural mouthpart interaction (SMI) evolved since this principle obviously occurred multiple times independently in several extant and extinct winged insect groups. Here, we report a new type of SMI in two of the earliest wingless hexapod lineages--Diplura and Collembola. We found that the mandible and maxilla interact with each other via an articulatory stud at the dorsal side of the maxillary stipes, and they are furthermore supported by structures of the hypopharynx and head capsule. These interactions are crucial stabilizing elements during food uptake. The presence of SMI in these ancestrally wingless insects, and its absence in those crustacean groups probably ancestral to insects, indicates that SMI is a groundplan apomorphy of insects. Our results thus contradict the currently established view of insect mouthpart evolution that biting-chewing mouthparts without any form of SMI are the ancestral configuration.” (Blanke et al 2015)
I think this is saying that the mouthparts of springtails and diplura (earwigs!) did give rise to the external mouthparts of insects..and not from some other biting and chewing relative. Which if true lends substance for the idea that insects evolved from collembola and diplura, strengthening Ghilarov’s theory
Wings
Wings
Of the ectognathus, the first sub group is Apterygota, which used to include , Protura, Diplura and Collembola, but not any more.
Although the three orders are sometimes grouped together in a class called Entognatha because they have internal mouthparts, they do not appear to be any more closely related to one another than they all are to insects, as we have just seen, which have external mouthparts (Ectognatha). It makes some sense that creatures moving through soil would need internal mouthparts to protect them, whereas on the surface and in the air, external mouthparts make for better consumption.
Some are pretty certain that apterygotes are related to insects. The term apterygote came from Greek words meaning ‘without wings’. They are seen as primitive, wingless insects and they hexapods which mean they have six legs. Some of them do not have eyes.
The springtails and their mates were classed as insects until 'recently'. However since the early 1990s, "The three orders were removed from class Apterygota and put into their own class once scientists determined that they were not in the evolutionary lineage of insects. In other words, these three orders did not give rise to modern-day insects". So where did insects come from?
It seems that colonisation and creation of the soil took place by creatures that dominated the soil, that then evolved into insects - characterised by taking to the air.
Springtails - not
Springtails - not
The first fossil springtail, a 50 million years earlier may give clue to insect evolution (Whalley & Jarzembowski 1981) Hirst and Maulik published a report in 1926 (online 2009) that dated back to the Rhynie Chert in which they described Rhyniellapraecursoras an insect, stating the specimen to be a “supposed larval insect”. It is now known to be a (poduran) springtail.
Several other pieces, including the Rhyniognatha head, were also described as R. praecursor,but this specimen was correctly identified as a different (insect) species and renamed Rhyniognathahirsti in 1928 by entomologist Robin J. Tillyard” (Greenslade & Whalley 1986)
There it is – right down to the confusion because of the similarity between soil dwelling springtails and seemingly ‘larval’ insects. Again early ‘hemi-metabolic’ insects look like soil arthropods.
If I were to suggest a ‘common ancestor’ for insects, which I don’t believe in, I would suggest springtails. In the Rhynie Chert the two are virtually indistinguishable, evidenced by the original wrong classification. I could add my name with the springtails to the list that Ghilarov collected of ‘different researchers in the last 40 years’ who claimed insects bred directly from a particular group. (Adaptations of arthropods to the terrestrial life NAUKA Moscow 1970) It is about time springtails were there – after all they used to be classed as insects. However, I think we should not rule out several sources.
Proturans
Proturans
There are 800 species found all over the world. Some do not class Protura as hexapods as their two front ‘legs’ do not walk but are thrust forward. They have been round longer than many think, As usual there are all sorts of arguments about where they have come from – possibly chelicerates rather than hexapods, and one suggests a sister group to Diplurans others with Collembola. “Due to their very distinctive and enigmatic morphology, i.e., the primitive lack of antennae and wings, the absence of eyes and tentorium (internal skeleton of the head) and the anamorphic post-embryonic development, their phylogenetic position within the hexapod tree is still debated” (Carapelli et al 2019)) There does not seem to be a common ancestor, just the usual arguments.
In terms of whether they were a forerunner of insect, it would not be a big jump to see how the front non-legs became proper legs as their mouthparts were pushed forward, when on land in search of food. It looks like an insect and a key insect feature – trachea- is present in some Protura, but not all.
“Among the seven or ten families of Protura, only Eosentomidae and Sinentomidae have thoracic spiracles and a tracheal system, whereas the remaining groups exchange gasses using their skin or recta.” They concluded “Our revision of both deep and shallow phylogeny of Protura has highlighted some of the many obstacles that have thus far hampered the possibility of deriving a clear picture of the evolutionary history of the group.” (Carapelli et al 2019). I still think Proturans would look a lot like insects if they were to live more on the soil surface.
Symphylans
Symphylans
As far back as 1873, suggestions have been made that Symphylans could have been the insect ancestors. In 1930 R.J. Tillyard demonstrated that both crustacean and trilobite hypotheses were untenable and almost accepted the symphylan theory, but rejected it as the genitalia didn’t fit; but 10 years later O.W. Tiegs (Whom Ghilarov quoted in his list of people claiming various insect ancestors in his Adaptations of arthropods to the terrestrial life NAUKA Moscow 1970) decided that this was not crucial. According to Cloudsley Thompson in his important work ‘Evolution and Adaptation of the Terrestrial Arthropods’ “The jaw mechanisms of Symphyla are of the myriapod type, and they therefore could not be relicts of stock from which the hexapods had also evolved “. He goes on to show there was further evidence indicating links between the Onychophora and Symphyla, and between the Symphyla and Protura and in 1947, even further evidence was obtained from a study of the Pauropoda. He also said that “Time spent on insect phylogeny is often wasted”. He concludes:
“Taking everything into account, there seems to be little room for doubt but that the insects evolved from terrestrial ancestors which had antennae; endognathous, labiate mouthparts; tracheal respiration; and, probably, 14 post-cephalic segments; 12 pairs of legs; coxal styles; eversible vesicles on most segments; and a pair of terminal cerci.”
The key changes to create insets would appear to be the endo- to ecto-gnashers, the number of legs and perhaps cerci. Does the change from ‘endognathous’ to ‘ectognathous’ tell us more than we have considered before? Insects do not have to come from one stock. Notice there is nothing about wings at this point.
I suggest there may be three main ancestors of insects – Collembola, Diplurans and Proturans – and it would be interesting to put some molecular and morphological analysis to that direction.
Mouthparts of a beetle (Ectognatha, left) and anterior view of a springtail, with mouthparts retracted within the head (Entognatha, right).
There were no holometabolous insects, so none were spending any part of their lives in these early soils - just on and around.
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