In scientific terms, the group of arthropods (animals with exoskeletons) which bear just six legs are sensibly called ‘Hexapods’ -meaning six-footed. Chances are you learned in school that all these animals with six legs are insects. You’d be mostly right, but some of them aren’t.
Once considered wingless insects in the subclass Apterygota , three orders -the Proturans, Two-pronged bristletails and Springtails- have since been split into their own subphylum, the Entognatha. Though opinions are still divided as to the exact relationships between them, or whether they even form a single related group, they differ from insects in two key ways; Their mouthparts are enclosed within a pouch in the head (a condition known as ‘entognathy’, their namesake) and almost every segment of their antennae (if present) contain muscles. Entognathans also lack a waxy covering to their exoskeleton (the cuticle) possessed by insects, so are less able to regulate their body water content. Hence most entognathans live in humid habitats such as soil, leaf mould, deadwoood or moss. They generally feed on decomposing organic matter, microbes and fungi, though many two-pronged bristletails are predators of small invertebrates and some springtails graze on live plants. Most are tiny, less than 5 millimeters long, but they are an important part of the soil fauna, contributing to the formation of soils and the carbon cycle, which we in turn depend on to grow crops.
The separation of the entognathans from the insects increases our appreciation of the evolutionary relationships of hexapods and may yet help us understand how insects arose. Fascinatingly, insects and each of the three entognathan groups have independently evolved to have six legs from their many-limbed common ancestor (though once thought to be related to the centipedes and millipedes, the Hexapod’s closest living relatives are now considered to be the fairy shrimps, made famous as ‘Sea monkeys’). Lets delve into these fascinating, little-known insect relatives.
Arguably the most bizarre of these three groups are the Proturans (Order Protura). They live in the soil or amongst plant debris. Like many animals that dwell in darkness, they are eyeless and lack body pigment, appearing a ghostly white or pale brown. They also lack antennae, instead using their hairy front limbs- held poised above their pointed heads- to sense their surroundings. Their name means “early tail” in greek, since unlike other entognathans they have no appendages on their rear. Their tiny size and cryptic habits mean they went undiscovered by humans until a research project on soil invertebrates in 1907! Since then, over 800 species have been described.
Proturans are considered the oldest group of hexapods. They have two features unique in hexapoda. Firstly, they develop by ‘anamorphosis’. This means that each time a growing proturan moults, it adds a new segment to its abdomen. In this way they grow from a hatching with nine abdominal segments to an adult with twelve. Secondly, as well as their six true-legs, they have limb-like appendages on their abdomen called ‘styli’. These odd features prompt some scientists to question whether they are hexapods at all.
Like the proturans, the Two-pronged bristletails (order Diplura) are blind and are mostly pigmentless, though they sport long antennae and a namesake pair of cerci or ‘tails’ on their rear (‘Diplura’ means two-tailed). Their lifestyle varies from herbivorous detritivores to predators of small invertebrates, with grades in-between.
The form and function of the cerci varies between different groups. The cerci of the family Campodeidae (the only British family) appear to mirror their long antennae and are used to detect vibrations, such as approaching predators. If the cerci are grabbed they can snap off and regrow, making diplurans the only hexapods besides stick insects that are able to regenerate body parts. The Anajapygidae have much shorter cerci which include silk glands, whereas those of the Japygidae have been modified into short, sharp pincers for catching prey.
The Japygidae are monstrous entognathans, reaching 5cm long. They ambush their prey by lying buried in debris with only their rear end exposed. When a small invertebrate walks across the pincers, they snap shut like a bear trap. These pincers are the only part of any dipluran with pigments, which may help camouflage them and shield them from the sun’s rays.
Two-pronged bristletails are believed to be the closest living relatives to the insects. A possible fossil dipluran from over 300 million years ago, Testajapyx, posseses complex eyes and insect-like mouthparts , suggesting that diplurans evolved entognathy separately from the proturans and springtails.
The largest and most familiar group of entognathans are the Springtails (Collembola). They come in three main groups; The Elongate springtails (Entomobryomorpha) which have sausage-shaped bodies, the Poduromorpha which are squat and slightly flattened with stubby legs and antennae and the Globular springtails (Symphypleona), surely the cutest entognathans with a rotund body and bobble head. The latter group includes the grass-eating Lucerne flea (Sminthurus viridis) , perhaps the only entognathan that can be considered a pest to humans.
Left to Right: an Elongate springtail (Isotomurus palustris), a Poduromorph springtail (Hypogastrura sp.) and a Globular springtail (Dicotyrmina minuta). All images (c) Ed Phillips Wildlife.
Springtails are named after the two-pronged ‘spring’ called a furcula which they use to leap away from predators or harsh conditions. At rest, the furcula appendages are folded under the abdomen and fit into a catch developedfrom appendages of the third segment. When threatened the springtail rapidly squeezes body fluids into the abdomen to release the furcula and spring away. The jump can take as little as 15 milliseconds and a 2 millimetre springtail can leap 16cm away. Though it provides an effectiveescape mechanism for most species, some groups of springtails have lost their furcula due to the practicalities of living in soil, on water or possessing a larger body size
We perhaps encounter springtails more than other entognathans because they are hardier and more adaptable. Many species have bodies peppered in waxy bumps, making them better able to retain moisture and live in drier surface habitats. They can also take in extra water using a pair of unique, tentacle-like organs called eversible vesicles which they extend into surface films . Though generally still tied to humid microhabitats, they can be found in such varied places as coastal rockpools, deserts and the summit of Mount Everest. Many species float on the surface of still water-where they are too light to break the surface tension- and feed on floating particles. The Sea springtail (Anurida maritima) floats in rafts on salt water, whereas the Snow flea (Hypogastrura nivicola) can walk amongst snow and ice thanks to anti-freeze compounds in its body.
These tiny yet tough arthropods may also have been amongst the first animals to venture onto land. Fossils show that springtails have been around for at least 410 million years. Having evolved so many unique modifications, their relationship to other entognathans and insects remains uncertain.
So when you next venture outdoors on a walk or do a spot of digging in the garden, take an extra-close look at the ground. Even when there is little insect life about in winter, you may still spot the insects forgotten cousins trundling about in their micro-world as they have done for millennia. Sometimes it’s the smaller things in life that are the most satisfying.
A Chaos of Delight: The wonderful world of soil Mesofauna: www.chaosofdelight.org/
Gordon’s Protura page: www.earthlife.net/insects/protura.html
Ed Phillips Flickr collection: Collembola
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