Category: Arthropods: other than Insects
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Arthropods are the most diverse and abundant organisms on Earth, and comprise an estimated 90% of animal species living today. The defining features of arthropods include possession of a hard, chitinous exoskeleton (part of a tri-layered cuticle), body segmentation, paired jointed appendages, specialisation of body regions into functional units ('tagma') and growth through periodic moulting of exoskeletal plates, triggered by cycles of ecdysteroid hormone. These features, and body tagmosis in particular, have brought extreme adaptability and ecological success, allowing arthropods to diversify into all available marine, freshwater and terrestrial habitats. Four major groups of living arthropods are recognised, namely the Chelicerata (spiders, mites, scorpions and horse-shoe crabs), Myriapoda (primarily centipedes and millipedes), Crustacea (crabs, shrimps, lobsters etc.) and Hexapoda (insects and their allies). Precise inter-relationships of these groups are contentious, but strong evidence suggests that sea-spiders (pycnogonids) are basal to chelicerates, and the remaining three groups form a derived clade of 'mandibulate' arthropods, with hexapods emerging from within the crustaceans.
Origins
The closest living relatives of arthropods include velvet worms (onychophorans) such as Peripatus and tardigrades ('water bears') both of which are known as fossils from the Cambrian onwards. Onychophora are predators that walk on segmental but un-jointed 'lobopods', whereas tardigrades are tiny creatures (typically 200-500 micrometres long) with a clearly segmented body and four pairs of legs with claws. Tardigrades inhabit moss, lichens, soil and sand grains in terrestrial and aquatic environments, eating fungi, algae, plant cells and minute animals (e.g. rotifers and nematodes). Some (e.g. Milnesium) are capable of forming dormant cysts, and in this state of 'cryptobiosis' can survive the most harsh extremes of temperature, dessication and pressure known for any animal. The features of these arthropod-like taxa, as well as enigmatic Cambrian forms such as Hallucigenia, Ayshaeia, Kerygmachella and Anomalocaris indicate that the common ancestor of the arthropods probably had fairly uniform trunk segments with lobopod-like, unbranched limbs and anterior sensory appendages.
True arthropods first appear as a diverse assemblage of marine animals in the Lower Cambrian (around 520 Ma ago). They included the lace-crab Marrella, various 'arachnomorph' taxa (e.g. the famous trilobites and horseshoe crab-like Aglaspis) and crustaceans. In the Silurian (443-417Ma), terrestrial arachnids, myriapods and crustaceans appear, while hexapods first appeared in the fossil record in the Early Devonian (around 400Ma, Rhynie Chert) with the collembolan Rhyniella praecursor and bristle-tail Leverhulmia mariae.
Among the chelicerates, myriapods and crustaceans we find numerous excellent examples of convergence, a few of which, ranging from mimicry to eusociality and bioluminescence are named below...
Chelicerates
Certain predatory spiders mimic their prey by resembling ants, and spider crabs (crustaceans) are masters of camouflage, fixing algae, sea urchins and other marine organisms to their exoskeleton. Spiders, spider mites and scorpions all produce silk, as do many insects (e.g. ants, wasps, flies, lepidopterans). Silk is used not only for webs but also a range of other functions, such as trip wires, escape lines, cocoons, egg covers and in scytodid spiders as an ingredient of toxic saliva. The spitting spiders' mode of hunting is similar to that of onychophorans, which squirt a venomous slime containing adhesive threads (collagen-like, rather than silken) at its prey. Silk threads have been independently recruited for 'ballooning' in various spiders, spider mites and moth larvae. A few spiders offer nuptial gifts (e.g. nursery web spider Pisaura and Paratrechalea) wrapped in silk, similar to the silken offerings made by male dance flies. Male nursery web spiders feign death if threatened by a female during courtship, and this 'thanatosis' behaviour has also appeared independently in insects (e.g. fire ants and Claviger testaceus beetles) as well as fish (e.g. cichlids) and mammals. Also notable in terms of convergence, adhesive pads (scopula) at the ends of spiders' legs are hairy, dry and remarkably similar in structure to the adhesive toe pads of geckos.
Myriapods
Myriapods teach us much about convergence, especially through their independent evolution of an insect-like tracheal system, allowing them to breathe effectively on land. Arthropods and other animals (e.g. early tetrapods, evolved from lobopodian fish) made the move from sea to land (terrestrialisation) as soon as the plants had colonised it in the Silurian, and myriapods, chelicerates and crustaceans did so independently. Several insect groups, most famously the butterflies and moths produce toxic defence compounds such as alkaloids, and interestingly, many millipedes and centipedes also secrete alkaloids, making them equally distasteful to predators.
Crustaceans
Crustaceans offer a rich window into convergence, most strikingly perhaps in the evolution of eusociality in alpheid shrimps. True eusociality has evolved independently elsewhere in several insect groups (e.g. ants, bees, termites) and among mammals, in the naked mole rats. Mantid shrimps are notable in having a complex compound eye and the capacity for colour vision, as well as raptorial limbs that are convergent with those of many insects - mantids and mantaspids being the most well known. Barnacles are intriguing crustaceans with shell plates in one taxon structured like the lamellar bone of vertebrates, while their vestigial abdomen is formed through the same genetic mechanism as abdomen-less mites and ticks. Fire-flies, fungus gnats and marine organisms including squid, fish and jellyfish emit bioluminescent signals, as do certain myodocopid ostracodes, that produce luminescent flashes to confuse deep sea predators. To keep their balance most crustaceans use statoliths (also found in cephalopods and cnidarians), but some crabs have a semi-circular canal system that is astonishingly similar to that of vertebrates. Finally, male fiddler crabs, with their huge dimorphic claw, display to females through lekking, a behaviour evolved independently in other animals, from birds to frogs, deer and even squid.
| Topic title | Teaser text | Availability |
|---|---|---|
| Gliding in spiders, ants and other arthropods | n/a | Unavailable |
| Light producing chemicals: how to make bioluminescence | The most remarkable luciferin in terms of its distribution is known as�coelenterazine. This nitrogen-ring based molecule is found in nine separate groups, ranging from radiolarians to fish. | Available |
| Bioluminescence | Flying through the air on a summer's evening or sparkling in the ocean you may see magical flashes of light that signal some of nature's most enchanting creatures, those that are bioluminescent. | Available |
| Defensive enrolment | Curling up into a ball has evolved many times as an excellent anti-predator defence. | Unavailable |
| Vibrational communication in insects and spiders | Some spiders have evolved a most remarkable method of capturing other spiders – they imitate the vibrations of insects caught in their victim’s web. And this is only one of numerous intriguing examples of vibrational communication in arthropods… | Available |
| Pheromone use in animals, fungi and plants | n/a | Unavailable |
| Bioluminescence in arthropods | n/a | Unavailable |
| Bioluminescence in marine animals | n/a | Unavailable |
| Mangrove swamp ecology | n/a | Unavailable |
| Vibrational communication in animals | What on earth could an elephant or treehoppers have in common with a seismometer? | Available |
| Pressure sensitivity and the tactile sense (excluding the lateral line) | The star-nosed mole is famous for, well, its nose, but do you have any idea what these peculiar 'tentacles' are for? The answer is rather touching and, of course, convergent... | Available |
| Lateral line system in fish and other animals | Some cavefish are completely blind, so how do they manage to navigate through their environment with astonishing ease? | Available |
| Myelinated nerves in vertebrates, annelids and crustaceans | Myelinated nerves are an excellent biological solution and needless to say have evolved independently in several groups other than vertebrates. In each case myelination is associated with very rapid nervous conduction and often escape reactions. | Available |
| Mimicry in fish and other marine animals | n/a | Unavailable |
| Eusociality in alpheid shrimps | A group of coral-dwelling shrimps, the alpheids, have not only evolved eusociality, but managed it several times independently. | Unavailable |
| Crabs: insights into convergence | You might think of crabs mainly as food, but this group is also highly instructive in terms of convergence… | Available |
| Independent eye movement in fish, chameleons and frogmouths | One of the most surprising convergences amongst animals is that seen between a small fish that lives in coral sands, known as the sandlance, and the lizards known as chameleons. | Available |
| Thanatosis (feigning death) in spiders and insects | Beetles that "play possum"? A rather interesting example of convergence… | Available |
| Nuptial gifts in insects and spiders | Male dance flies lure females with a dead insect. Not very romantic, you might think, but it certainly does the trick. Hence, such nuptial gifts have evolved in numerous other arthropods... | Available |
| Taste in arthropods and mammals | The ability to taste is obviously an essential component in the life of any animal, both to assess the potential quality of food, its nutrient capacities and also to detect toxins or other dangers. | Available |
| Camouflage in arthropods | Some insects make a “back-pack” of dead ants that evidently deters the attention of jumping spiders, while even more remarkably a weevil living in Papua hosts a garden on its back, complete with moss, algae and other organisms. | Unavailable |
| Vision in echinoderms | Among brittlestars and sea urchins we find visual systems that in some ways rival the arthropods in the form of compound eye-like structures. | Available |
| Compound eyes in arthropods | It is clear that amongst the arthropods as a whole the compound eye has evolved at least twice, and possibly even more times. | Available |
| Scanning eyes in molluscs and arthropods | Some sea snails have a linear retina. What a hopeless arrangement, to see the world through just a narrow slit! Not quite, because they have come up with a rather intriguing trick to extend their visual field - and it's a trick too good to use only once. | Available |
| Telephoto eyes in animals | Pursued by the paparazzi? Watch out for those animals equipped with telephoto lenses... | Available |
| Camera-like eyes in arthropods | Arthropods are famous for their compound eyes, but some groups have had a fair crack at evolving the optically superior camera eye… | Available |
| Adhesive pads: from geckos to spiders | In terms of adhesive pads we find they have a remarkably wide distribution evolving in at least four distinct groups, including members of the reptiles, amphibians, arthropods and mammals, with tentative parallels in sea urchins. | Available |
| Silk production and use in arthropods | Remarkably, fossil silk is known, especially from amber of Cretaceous age. Material includes both silk with trapped insects, possibly from an orb-web, and strands with the characteristic viscid droplets that are the key in trapping prey. | Available |
| Spitting in spiders and velvet worms | Scytodid spiders forcibly eject a mixture of saliva, silk and venom in a glutinous mass over a distance of c. 1cm to entrap prey. | Available |
| Biological uses of silk: from webs to ballooning | What material is so versatile that it can be used for capturing prey, building nests, communication and even cleaning? The answer: that most remarkable of biomaterials - silk. | Available |
| Terrestrialization by arthropods | n/a | Unavailable |
| Respiration in myriapods | n/a | Unavailable |
| Cavitation: bubble formation in plants, reptiles and shrimps | The formation of bubbles in a fluid is known as cavitation. Typically this occurs at low pressures, and is perhaps best known in the xylem of plants where embolisms can be destructive to the surrounding tissues. | Available |
| Myriapods (centipedes and millipedes): defence and terrestrialisation | This group of arthropods is also important because they show independent invasion of the land (terrestrialization), which not surprisingly has led to important convergences. | Unavailable |
| Raptorial appendages in mantids and other arthropods | The praying mantises exercise a peculiar fascination, not only because of their lunging predatory habits, but also because on occasion the process of copulation ends with a decapitated male being chewed to pieces by the female while the reproductive movements continue. | Unavailable |
| Sleep in animals | Suffering from insomnia? Fruit flies do as well... | Available |
| Eusociality in arthropods and mammals | Eusociality is most familiar in the insects, where it has evolved several times, notably in bees, wasps, ants and termites, as well as in thrips and aphids. | Unavailable |
| Insecticide production: from plants to primates | Application of insecticides, such as against mosquitoes, has been documented in several primates and birds. | Unavailable |
| Pheromones in arthropods | Not surprisingly this is a rich area of insights into evolutionary convergence because if an animal, such as a spider, can independently evolve the pheromone then a sexual lure is turned into a metaphorical honey trap. | Unavailable |
| Crustaceans: insights into convergence | Whilst predominantly marine, quite a number of crustaceans have invaded freshwater habitats and even more interestingly a few demonstrate terrestrialization, effectively freeing themselves from their aquatic ancestry. | Available |
| Haemocyanin in arthropods and molluscs | The degree of similarity between the active sites in arthropod and molluscan haemocyanin has been called “remarkable” and “startling”, but actually suggests that wherever in the universe life employs copper for aerobic respiration it will call upon haemocyanin. | Available |
| Transparent tissues: eyes, bodies and reflective surfaces | Read on if you want to know about the numerous animal equivalents to the invisible man... | Available |
| Statoliths and balance in animals | An almost universal, but convergent, method to detect changes in orientation is for small grains (statoliths) to be attached to fine hairs, whose movement triggers nervous impulses. | Unavailable |
| Polarized light detection in arthropods, fish and cephalopods | In bees detection of polarized light from different quadrants of the sky is an important component in their navigation. | Unavailable |
| Moulting in arthopods, annelids and other animals | Moulting has, however, evolved independently in other groups, including the annelids where some polychaetes shed their jaws. | Unavailable |
| Mimicry in insects and other arthropods | Defensive mimicry is usually Batesian, where an innocuous species adopts the colouration of a toxic species, but Mullerian mimicry is also known whereby one species, already toxic, converges on the colouration of a more common toxic species. | Unavailable |
