Topic: Beetles: insights into convergence
The beetles are probably the most diverse animal group on earth, so it is not at all surprising that they provide many fascinating insights into convergence.
Infrared detection of fire
Snakes are famous for their capacity to perceive infrared, but some beetles (Melanophila acuminata, Merimna atrata and Acanthocnemus nigricans) have independently evolved this rather astonishing sensory system. However, while it helps the snakes in capturing ‘warm-blooded’ birds and mammals, the beetles employ it for a very different purpose: to detect forest fires. Not in order to escape (Run away! Run away!), but because of a reproductive cycle in which females need to lay their eggs on freshly burnt wood, where the larvae then develop. A clever trick. Everybody else has run away, the heat has helped to drive toxins out of the wood, and as the larvae emerge the first growth of fungi etc. are all there to be eaten. Despite these different functions and the widely differing origins of the respective organs in the two groups, the overall convergence is striking, even at the ultrastructural level. The infrared detector of Merimna is analogous to the infrared-sensing facial pits of boid snakes, while that of Acanthocnemus shows strong similarities to the bolometer-like arrangement found in pit vipers. An infrared sense has furthermore evolved in other insects (hemipterans and some parasitoid wasps) as well as vampire bats.
Another remarkable convergence in the beetles is the independent evolution of agriculture. Agriculture is best known in the form of fungus cultivation by attine ants (leafcutter ants) and termites, but has also arisen in animals such as dugongs and snails that “farm” fields of seagrass and algae, respectively. The so-called ambrosia beetles are a diverse group of derived weevils that have evolved a symbiotic relationship with ascomycete fungi at least six times. They cultivate the fungi inside tunnels or galleries within dead or decaying wood and feed on their mycelium, which is called ‘ambrosia’. In order to transport the fungal spores, the beetles have developed specialised pockets (the mycetangia), which are rampantly convergent among the different groups. As in attine ants, the risk of infection by pathogens is controlled not only by oral secretions but also by a symbiotic bacterium (an actinomycete). Whilst the ambrosia beetles tend to steal the show when it comes to coleopteran agriculture, such cultivation has evolved at least two more times, in the Lymexylidae (Hylecoetus dermestoides) and Euops weevils.
Eusociality, an advanced and complex social system that is characterised by reproductive castes (i.e. a reproductive division of labour with one or several fertile queens and sterile workers), has evolved at least 20 times in various animal groups. The most important eusocial animals, in terms of numbers as well as ecological significance, are hymenopteran insects (ants, bees and wasps), but eusociality is also found in termites and aphids as well as in some more unusual groups, such as alpheid shrimps and mole rats. Perhaps surprisingly, only a single species of beetle has evolved this social system. The Australian ambrosia beetle Austroplatypus incompertus lives in galleries in Eucalyptus trees. A single fertilised female initiates a gallery system, and her daughters then stay to help and remain unfertilised. Mature colonies fulfil the three criteria of eusociality. They show cooperative brood care, an overlap of two or more generations and a division into reproductive and effectively sterile castes. However, in contrast to some groups, colony size remains small, probably due to limited growth of the symbiotic fungi.
Many organisms display bioluminescence, where cold light is produced and emitted by either the organism itself or by symbionts (usually bacteria).
Light production is based on a chemical reaction involving a luciferin (photon-emitting molecule) and an enzyme, either a luciferase or a photoprotein. Luciferases have been recruited from pre-existing enzymes such as oxidases and synthetases at least 30 times and the light reaction they mediate often occurs in specialised organs. Bioluminescence is particularly common in marine animals such as cnidarians, crustaceans and molluscs and is found in most deep sea dwellers. Its functions are manifold, ranging from counterillumination camouflage in squid to the luring of prey in anglerfish. On land, bioluminescence is probably best known from the fireflies (Lampyridae) and glowworms (Phengodidae). But fireflies aren’t flies and glowworms aren’t worms – both are in fact beetles. Many (but not all) lampyrids produce light in light-emitting organs on their lower abdomen. In the often-toxic larvae, light seems to serve as a warning signal to predators, while most adults use it for mate attraction. Here, light is often emitted in distinct patterns, and individuals in groups have been observed to synchronise their flashes. Female fireflies in the genus Photuris, however, use the light to attract prey, mimicking the mating flashes of other species. The larvae and larva-like females of phengodid beetles have independently evolved to be bioluminescent. So-called railroad worms (larvae of the phengodid beetle Phrixothrix) are unusual in being multicoloured: on each segment they have paired green glowing organs to deter predators (they are toxic to eat) and organs on the head that, uniquely among land living species, emit a red light. Eggs, larvae and adults of Pyrophorus click beetles (family Elateridae) are all bioluminescent and interestingly as adults it appears that these insects emit an orange light as a sexual attractant but a green light for defence.
Best known from the flies (Diptera), halteres are club-shaped gyroscopic organs that serve to stabilise the insect in flight. However, dipterans are not the only insect group to have evolved such balancing organs, Males of the parasitic Strepsiptera possess halteres that are highly similar in form and function, although they are derived from the forewings and not the hindwings as in the dipterans. A remarkably analogous arrangement has also evolved in the African lymexylid beetle Atractocerus brevicornis, which is an unusually fast flyer. Here, the elytra, modified forewings that are typically hardened and protect the membranous hindwings, have been adapted into haltere-like organs. They oscillate in flight and experiments showed that stable flight became impossible when they were removed.
The larvae of dytiscid diving beetles, such as the sunburst diving beetle (Thermonectus marmoratus), are effective predators that track their prey visually before striking. Surprisingly, the retinae of their small lens eyes are not cup-shaped but form a narrow band, so do they see a potential victim through merely a slit? No, because they perform a smooth scanning motion and thus extend their visual field considerably. As the eyes are immovable, this is achieved through pivoting movements of the head and thorax. A similar capacity to enlarge the visual field through scanning has evolved in some other predatory groups, presumably in relation to prey capture. Heteropod molluscs, jumping spiders and a number of crustaceans are, however, able to move their eyes or part of them, and this is likely to be superior to the body scanning employed by diving beetle larvae.
Thanatosis, the feigning of death, is widespread in animals, but some particularly interesting examples are found in insects (e.g. fire ants) and spiders (e.g. nursery web spiders). While thanatosis is often employed as a defence mechanism, the pselaphid beetle Claviger testaceus goes one step further. It feigns death in order to be picked up by Lasius ants and to be transported into their nest, where the supposed cadaver unexpectedly revives. Not only does the beetle manipulate the ants’ behaviour such that they feed and care for it, but it also preys on its host’s eggs, larvae and pupae – not the ideal guest after all.
Many scarab beetles (Scarabaeoidea) possess horns, rigid projections from the body that are used as weapons. Males fight for access to females, and individuals with bigger horns are more likely to win and thus increase their reproductive success. The horns are extremely diverse in size and form, and those groups that are dominated by large-horned species (the family Geotrupidae and the subfamilies Scarabaeinae and Dynastinae) are relatively distantly related. This has led to the suggestion that horns have evolved independently several times within the scarabs. It has, however, been argued that it seems to be surprisingly easy to gain or lose horns during development and that all extant scarabs might have inherited a developmental capacity to grow as well as suppress horns from a horned ancestor.
Some scarab beetles feed exclusively on faeces and are, accordingly, known as dung beetles. This group shows a number of interesting convergences. The African genus Pachysoma, for example, resembles the Neotropical eucraniines in morphology and behaviour, probably as a result of living in similar arid habitats. Furthermore, dung beetle assemblages in two European mountain chains, the Western Rhodopes Mountains and the Iberian Central System, are convergent as regards altitudinal variation in species richness and compositional turnover. In South Africa, dung beetle assemblages on mined dunes converged on those on natural, unmined dunes with respect to species composition (especially in shade specialists).
Cantharophily, the pollination of plants by beetles, is rampantly convergent. It has evolved independently in 14 eudicotyledon families and in six families of petaloid monocotyledons. Floral presentation in these beetle-pollinated plants can be classified into four modes, two of which are particularly common. Chamber blossoms, where the flowers are vase-shaped and often trap-like, have their highest diversity in the wet tropics. This presentation mode is not only found in magnoliids and basal monocots (e.g. Araceae and Cyclanthaceae), but also in derived eudicot families (e.g. Polemoniaceae and Clusiaceae). Many chamber blossoms are capable of heat production (thermogenesis), which is convergent as well. Painted bowls, where pollen is more easily accessible, occur in petaloid monocots and some eudicot families mainly in Mediterranean zones. For example, corn poppies (Papaver rhoeas), poppy anemones (Anemone coronaria), Persian buttercups (Ranunculus asiaticus) and eyed tulips (Tulipa agenensis) all have large, bowl-shaped flowers that are radially symmetric. They do not produce a strong scent, but are orange-red with a black centre, which attracts scarab beetles in the genus Amphicoma as primary pollinators. Such “poppy guild” species have evolved convergently in the Mediterranean region, in accordance with a high diversity in Amphicoma beetles.
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Map of Life - "Beetles: insights into convergence"
September 21, 2020