The infinite variety: the beginning of life
Living descendents of the Trilobites
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Living descendents of the Trilobites Although they radiated throughout the oceans, only one descendent of this group survives today, the horse-shoe crab (Limulus). This animal is larger than its ancestral trilobites, and segmentation of its armour have fused to form a large domed shield. These animals generally live at great depths but each spring they migrate towards the coast and during full moon and high tides they drag themselves onto the beach where they copulate. Today the similarities between the horse-shoe crabs and the trilobites are only evident in the larval stage where segmentation of the armour plates are clearly discernable in the horse-shoe crab larvae. Crustaceans: Arthropod success in the sea Another group of armoured animals also evolved from the original segmented worms the crustaceans which exist today in the form of some 35 000 species. They may prowl around rocks and reefs as crabs, shrimps, prawns, lobsters and crayfish, they may become sessile such as barnacles, or congregate and swim in vast shoals such as krill. The size of the crustacean and the form of the exoskeleton varies considerably from the paper-thin exoskeleton of the almost microscopic water flea (Daphnia) to the carapace of giant Japanese spider crab (Macrocheira kaempferi) which measures 3 m from claw to claw. In the crustaceans the paired legs have become modified for a variety of purposes. At the anterior end they have become modified into pincers or claws, those in the middle are paddles, or walking legs or tweezers. Some have feather branches acting as gills through which oxygen can be absorbed. All limbs are jointed, tubular and operate by way of muscles. Like the primitive trilobites for crustaceans to grow they need to dispose of their calcareous carapace. As time approaches for moulting the animal absorbs as much calcium carbonate from the carapace into the blood stream, and begins to secrete a new soft wrinkled skin under the carapace. The outgrown armour splits and the crustacean swells its body by absorbing water, and wrinkled new skin stretches and hardens into a new carapace. Arthropod Exoskeleton: Evolving to occupy land This exoskeleton may work to advantage for animals to colonize land if a mechanism of breathing in air as opposed to water can be secured. By developing almost closed air chambers lined with folds of moist skin crustaceans are able to absorb oxygen from air. In this way sand shrimps, beach hoppers and wood lice have been able to colonize land that retains a moist environment. The most spectacular of land dwelling crustacean is the big robber crab Birgus which exploits coconuts. Other descendent of the invertebrates have left the sea for a terrestrial life style the first of which were probably derived from segmented marine worms, but more recently included the familiar snails and slugs. These changes started about 400 million years ago and gave rise to the most numerous and diverse of land animals; the insects. Assignments IN YOUR OWN WORDS WRITE A ONE TO TWO PAGE ESSAY ON THE FOLLOWING TOPICS Discuss the variations in shell structure that have occurred in the phylum Mollusca. Describe the water vascular system that characterizes animals that occur in the phylum Echinodermata. Describe the diversity of segmented marine invertebrates that have evolved. THE FIRST FORESTS Plants: Evolving to occupy land The first land available for colonization was inhospitable due to the considerable amounts of volcanic action. Consequently as volcanoes erupted on land, life in the oceans multiplied with a diversity of species with different structures and adaptations, but the land remained unconquered. Marine algae may have secured an existence on the littoral zones of the ocean in the same way they do today. Around 420 million years ago the first waxy layers developed in plants to prevent desiccation, but this did not totally free such plants from an aquatic environment since they required an aquatic medium for reproduction. Algae reproduce through both asexual division and sexual methods. Sexual reproduction involves the production of sex cells which require locomotion in water for the fusion of the cells to take place. Plants: Fertilization and dispersal the first issues Such a problem still exists for primitive plants living today such as the liverworts and mosses. Such plants practice sexual and asexual reproduction in their alternate generations. The familiar green moss is the generation which produces the sex cells. Each large egg cell remains attached to the stem at the top of the moss plant, while the smaller microscopic sperm cells are released into water and thrash their way to fertilize the egg cell. The egg cell develops while still attached to its parent plant and produces the next asexual generation which is composed of a thin stem with, at its tip, a hollow capsule in which a large number of spores are produced. In a dry atmosphere the capsule splits releasing airborne spores. If the spores land in a suitable site they develop into new moss plants. Mosses: Possibly the earliest land plants? Moss plants have no structural strength and rely on close packing to achieve only modest heights. Their tissues are soft and permeable and they can only exist and reproduce under moist environments. Such plants probably represented the earliest colonization of the terrestrial environment, although no fossil evidence for this has been discovered. Fossils of the earliest land plants The earliest fossilized land plants (400 million years ago) were simple leafless branching strand filaments found in rocks and cherts of the United Kingdom. Like mosses no root tissue had differentiated, however, long thick-walled cells enabling water to be conducted along stems had differentiated and represented a major advance which gave plants structural strength to grow bigger. Such plants, together with primitive mosses and liverworts created the first vegetation which permitted animals to colonize from the sea onto the land. What were the earliest land animals? The first land-dwelling animals were segmented and probably represented the ancestors of the millipedes you encounter today and reached spectacular sizes (up to 2m in length). The exoskeleton inherited from aquatic ancestors needed only minor modification, but the external gills were unsuitable and in its place a network of breathing tubes (tracheae) evolved. Each tube has an exterior opening on the side of the exoskeleton, and the network of tracheae provides each cell with a supply of oxygen. Living of Land: Issues of reproduction Reproduction on land required changes since their aquatic ancestors relied on water to transport the sperm cell to the egg cell. In millipedes the reproductive cells are located close to the base of the second pair of legs. The male and female animals meet and intertwine, the male reaches forward with his seventh leg and collects his sperm and transfers it to the sexual pouch of the female. Such copulation was laborious but safe, but was not suitable for the predatory animals that evolved then but still survive today as centipedes, scorpions and spiders. These three groups of animals have all undergone a reduction in segmentation and all may indulge in cannibalism. As a consequence of this scorpions armed with large poison glands and spiders have evolved ritualized courtship patterns prior to copulation. Land plants: Making their mark During this early period of evolution in the segmented animals, plant were also evolving, with the development of rooting systems which were absent in the mosses. Rooting systems permitted water sources below the ground to be utilized. Consequently root development permitted plants to survive in less moist environments. Three groups of plants, all of which have living descendants evolved root structures (club mosses Lycopodium, horsetails Sphenophyta and ferns Pterophyta) and possessed within their stems strong woody vessels for the transport of water absorbed by the roots. Such adaptations provided the structural rigidity for some of these plants to grow big (up to 30 m) and created the first true forests. A Forest Environment The development of forests would have necessitated changes in habitat (from the ground to arboreal) for some animals. Evolving at this time were the first vertebrate animals which had four legs, a backbone and moist skins and were also carnivorous on the invertebrates. Among the invertebrates bristletails and springtails evolved and remain one of the most numerous of invertebrates with the most familiar being the silverfish. Silverfishes have clear but even more reduced segmentation consisting of a conspicuous head supporting compound eyes and antennae, a thorax bearing three pairs of jointed legs (a result of three segments being fused) and segmented abdomen which has lost its limbs but possesses three filaments at the extreme end. These animals breath much as millipedes do with a tracheae system, they copulate like scorpions do with the female walking over packets of sperm and taking them up into the genital pouch. Insects: The greatest conquerors of all? The characteristics of six legs and a body divided into three parts became numerically the most successful group of animals: the insects. Although ancestral insects probably climbed about the vegetation, one important ingredient for their success was the development of wings and the ability to fly. How wings evolved is unknown but it may have reflected attempts on insects to increase surface area and become more efficient at warming up their bodies so that they can become active (thermoregulation). Winged insects appeared some three hundred million years ago with animals resembling dragonflies. In the absence of early competition, early dragonflies radiated with some species developing enormous sizes (eg wingspan of 700 mm). Dragonflies have two pairs of wings with a simple up and down movement, and consequently cannot be folded back. Today's dragonflies have large compound eyes and catch smaller insects in flight, but are able to hunt only during the day. Consequently today's carnivorous dragonflies must have been preceded by herbivorous animals or carnivorous forms that prey on non-flying insects. Modern dragonflies probably evolved from primitive omnivorous or herbivorous insect forms such as cockroaches, grasshoppers, locusts or crickets. Land Plants: Still working on the reproduction issue The development of flight in insects was to have a major consequence on the evolution of plants. Early plants including tree forms existed in two alternating forms, a sexual and an asexual generation. Becoming tall would have no effect on the transport of spores and may even enhance their wind-dispersal, however, the distribution of sex cells which, hitherto, was achieved by the male cells swimming through a droplet of water and reaching a female cell. This demanded that the sexual generation was small and grew close to the ground, a situation that is found today in ferns, club mosses and horsetails. The spores of such plants develop into a filmy plant called a thallus which produces sex cells on the undersurface where there is permanent moisture. After fertilization of the female egg cells the thallus develops into the tall spore-bearing plants. Cycads: Getting to grips with the reproduction on land A thallus life cycle stage induces considerable vulnerability, since it is small and possesses little or no protection against herbivory or desiccation. A less vulnerable sexual stage appeared about 350 million years ago with the evolution of plants like the cycads which exist today. Cycads superficially resemble ferns, with some species having spores of the archaic form which are distributed by wind. In other species some spores become large and remain attached to the parent plant where they develop into a conical-shaped structure containing egg cells (that is functionally equivalent to a thallus). When a wind-blown spore, now called a pollen lands on these egg bearing cones, no filmy thallus develops, but a pollen tube which burrows its way into the female cone occurs. The large sperm cell is transported down to the bottom of the pollen tube, where it enters a small drop of fluid secreted by the surrounding tissues of the cone, there it swims to the egg cell and fuses with it and thereby completing the fertilization process. Conifers: A successful formula Similar morphological changes resulted in the evolution of the conifer group (pines, larches, cedars and firs). These plants, unlike cycads produce pollen and egg-bearing cones on the same plant individual, however, fertilization and the development of the seed takes longer, but the seeds are equipped with a rich supply of food and a hard, water-proof coat that permits the seed to remain dormant until conditions are right for germination and the establishment of the seedlings. Conifers are successful, even today, with one-third of global forests being composed of them. Both the biggest and most long-lived individual organism in the world are conifers (the redwoods and the bristle-cones respectively). Earliest plant defences against herbivores Conifers are also able to repel insect damage with a gummy substance called resin. Insects are often caught in the resin which has proved to be a good fossilizing medium called amber. The first amber containing flying insects appeared 100 million years ago and includes representatives of all major insect groups known today. Each group has developed its own characteristic way of flying. Dragonflies have two pairs of wings which flap up and down synchronously, bees and wasps have linked the fore and hind wings together with hooks, butterflies have overlapped the wings, hawkmoths have reduced the hind wings considerably in size and latched them onto long narrow fore-wings with a curved bristle, beetles have the front pair modified into thick covers which protect the rear flying wings, and flies use only the front pair of wings for flight with the hind wings reduced to tiny knobs. Arachnids: Insects Nemesis Although insects were the first animals to invade the air, they nevertheless fell prey to their arachnid adversaries, the spiders, who evolved the ability to spin webs between branches and thereby trap and consume flying insects. Plants and Insects find “mutual benefit” Plants also responded to the flying skills of insects by using such mobility for the distribution of the male reproductive cells (pollen). Unlike spores in the lower plants, pollen needs to reach the female cell for the development of more adult plants. Wind-dispersal of pollen which is typical in the pines (Gymnosperms), requires vast quantities of pollen for even moderate pollination success. Alternatively if insects could be used to carry pollen to the female cells by using a small incentive (e.g. food), much less pollen would be required to achieve similar levels of pollination success. Such incentives for insect pollination evolved with the earliest of the flowering plants; the magnolias which appeared about one hundred million years ago. In these plants the egg cells are clustered in the centre, each protected by a green coat with a receptive spike on the top called a stigma with which it receives pollen and is necessary for fertilization. Grouped around the egg cells with their stigmas are stamens which produce the pollen. In order to bring these organs to the notice of insects, the whole structure is surrounded by brightly coloured modified leaves called petals. Beetle pollination Beetles had already learnt to feed on the pollen of cycads, and were one of the first to transfer their attentions to the early flowers like those of the magnolias and waterlilies. As they moved from one to another flower, beetles collected meals of pollen and paid for them by becoming covered in excess pollen which they involuntarily delivered to the next flower they visited. One danger of having both eggs and pollen in the same structure is that the plant may pollinate itself, however, this is overcome by egg and pollen cells being mature at different times. Plants learn to manipulate Other flowers developed alternative bribes to pollen this being nectar, a completely specialized adaptation to recruit even more potential pollination agents which included bees, flies, butterflies and moths. Even brighter signals were used to draw attention to the nectar being offered and attractive scented chemicals evolved as additional means of soliciting the services of insects for pollen transportation. The services of flies were enlisted with the evolution of flowers that mimicked the scent of rotting flesh, the usual food of such animals. Some stepelia plants have taken this deception further by producing brown, wrinkled petals covered with hairs which resemble the decaying skin of a dead animal. To complete the illusion, the plant generates heat to mimic the warmth generated by decomposition of flesh. Flies not only visit and transport the pollen of the stepelia plants, but they even lay their eggs in the flower as if it were carrion. The most bizarre pollination systems? Possibly the most bizarre imitations are those occurring in orchids which attract insects through sexual impersonation. One orchid species produces a flower that closely resembles the form of a female wasp including eyes, antennae and wings and an odour (pheromone) that is emitted by the female wasp during the mating period. Male wasps are deceived into copulating with the flower and so doing get covered with pollen before carrying on to the next bogus female wasp which will receive and deposit more pollen. Total dependence: Yuccas and Moths Sometimes plant and insect become totally independent on each other. Yucca plants which produce rosettes of cream flowers attract a small moth with a specially curved proboscis that enables it to gather pollen from the yucca stamens. It moulds the pollen into a ball and the carries it off to another yucca flower. First it goes to the bottom of the flower, pierces the base of the ovary with its ovipositor and lays several eggs on some of the ovules that lie within. Then it climbs back up to the stigma rising from the ovary and rams the pollen ball into the top. The plant has now been fertilized and in due course, ovules in the base of the chamber develop into seeds. Those that carry the moth's eggs will grow particularly large and be eaten by the developing caterpillars. Those ovaries without caterpillars will not be eaten and permit the yucca to propagate itself. Assignments IN YOUR OWN WORDS WRITE A ONE TO TWO PAGE ESSAY ON THE FOLLOWING TOPICS Discuss the form of adaptations required by the first invertebrate animals which made the transition from life in the sea to life on land. Describe how the first plants and animals evolved and became dependant on each other. Describe the diversity of flying insect life that has evolved. THE SWARMING HORDES Insects: Almost three-quarters of the animal diversity It is estimated that there are three-times as many insects as all other species of animal put together. Too date more than 700 000 species have been described, probably only a fraction of those still waiting to be discovered and described. Insects have invaded all aspects of terrestrial life. There is no known species of plant that is not attacked by an insect species. Insects may still remove up to three-quarters of crops grown by people in Africa. A Tripartite body plan This success, diversity and variation is all achieved with a tripartite body plan consisting of a head bearing a mouth, mouthparts (modified jointed appendages) and most of the sense organs; a thorax filled with muscles which operate three pairs of legs, and usually one or two pairs of wings and an abdomen which contains the organs for digestion and reproduction. All three sections are enclosed within an external skeleton made principally of chitin, a substance that is chemically similar to cellulose but has both flexibility and permeability. Chitin: A secrete ingredient for success? Insects may cover this chitin with sclerotin to make it hard so as to create armour (e.g. beetles) and produce mouthparts sharp and tough enough to gnaw wood and cut metals. It is the responsiveness of this chitinous exoskeleton to evolutionary change that has permitted insects to diversify. Leg morphology is easily modified to propel an animal for more than two-hundred times its own length, or to create broad oars to row across the water or thin hair tipped stilts to stride across the surface of water. Many limbs may carry special tools moulded from chitin such as pouches to hold pollen, combs to clean compound eyes, spikes to act as grappling irons and notches to create sounds. Issues with an Exoskeleton This exoskeleton still restricts growth and needs to be shed periodically, and a new shell created to replace it. Primitive insect forms like bristle tails and springtails do not change their shape significantly with successive changes of the exoskeleton, but this does permit them to increase their size. The early winged-insect forms (cockroaches, cicadas, crickets and dragonflies) similarly moult without significant changes to the body shape with the exception of acquiring wings in the final moult (although damsel flies take two moults to perfect their wing structure. Even when insects adopt significantly different environments for their early and later lives, their body structure is recognizably similar. A “Larval Stage” leads to success The more advanced insects, undergo structural changes that make it impossible to link the larvae with the adult forms without observing the changes for oneself. In this way maggots change to flies, grubs to beetles, caterpillars to butterflies. Since the earlier form is not required to breed, it has no sex organs and does not need to attract a mate, it needs no wings to fly, since it has probably been placed in environment that is near optimal for its development. Such larvae consume great quantities of food and therefore need efficient jaws and digestive systems. Since these larvae have no exoskeleton, the locomotion is generally slow and they have little protection against predators. This is of little consequence to grubs and maggots which live inside the tissues of plants and animals, but caterpillars which feed in the open frequently use camouflage techniques to resemble a twig, a bit of leaf or a bird dropping. Other defences may exist including squirting formic acid, having an unpleasant taste, or covering the body with unpalatable or even poisonous hairs. Some animals possessing chemical defences advertise this with a conspicuous coloration which warns potential predators of this fact. Other species with no such defences mimic the colours of those species possessing chemical defences and thereby avoid predation. The larval stage of some insects may last a considerable length of time, with grubs of beetles boring through wood for up to seven years before developing into adult forms. Larva: Clothed in silk Only the larvae of insects possess silk glands which have been used to construct communal tents, to extrude life-lines guiding them over plants and getting them from one twig to another. These silk glands are also used to construct a cocoon in which further development takes place (e.g. moths). Metamorphosis Caterpillar larvae undergo one final development before becoming adults (metamorphosis). The larvae sheds its skin and develops a hard shell around itself and is now called a pupa. The pupa has spiracles for breathing, and its tip may twitch sporadically. When the larva first developed from the egg cells it was segregated into two groups. Some of these cells divided after a few hours but remained generalized in form, whereas other cells continued to build the caterpillar body. After the larva hatch these cells enlarge with no further cell division. Within the pupa the original giant cells of the caterpillar are used to feed cell division of those other group of cells which are re-organizing the new body of the butterfly. An insect’s first flying lessons The butterfly exits from its pupa head-first and immediately pumps blood into the network of veins, and the limp wings begin to take their shape. Now the blood is withdrawn from the veins of the wing and the veins harden to create rigid struts, at which point the wings are ready for their maiden flight. All further growth has ceased, and they use food collected when they were larvae and stored as body tissue. Some species like Mayflies do not even have mouthparts. In this adult stage their primary function is to find a mate. However, unlike the larvae, butterflies have large compound eyes, that are sensitive to most wavelengths. The colours and patterns on their wings are created by tiny scales which have pigments and microscopic structures that split light, reflecting back a narrower range of wavelengths. These colourful wing patterns may be useful for species recognition and mating. Insects: Finding your soul mate Other insects use sound to summon prospective mates (e.g. cicadas, crickets and grasshoppers). Sound in Grasshoppers is produced by sawing the notched edge of their hindlimb against the strengthened vein of the wing. Cicadas have an abdomen which contains two chambers, the inner wall of each chamber is stiff and when it moves in or out it makes a click. In the abdomen behind there is a large muscle which can pull the wall back 600 times a second and the noise created is amplified in the abdomen using a hollow vibrating plate and two hollow rectangular resonators. Sound is received from eardrums on either side of the thorax in cicadas, but grasshoppers use a membrane situated between two deep slits along their first pair of thighs. With each species having a unique sound, they can recognize and attract appropriate mates of the same species. Moths use a third sense, smell to attract mates. Females produce chemical compounds called pheromones which male moths are able to detect with their large, feathery antennae. An Insect’s approach to rearing your young Using sight, sound and smell adult insects attract their mates and copulation can take place. The female then lays her fertilized eggs in an environment suitable for her larvae to exploit. Butterflies seek suitable plants for the young caterpillars, beetles lay eggs in pellets of buried dung, flies deposit eggs in carrion, wasps catch and paralyse spiders and lay their eggs on them so that the young larvae can feed on the spiders. Ichneumon wasps use a beetle grub to lay her eggs, with the hatched larvae eating the grub alive. Insects: Limitations for size The only apparent limitation to insect forms appears to be size, the largest moth is 300 mm in wingspan, the heaviest beetle is 100 g in mass. This reflects insects reliance on tracheae and spiracles, without an effective pumping system to force the air down. Some insects do use contractions of the abdomen to improve circulation and have tracheae that swells into thin-walled balloons which can be depressed and expanded. Insect’s approach to size matters Insects have, however, transcended even these limits in size, by creating highly social community living, an example of which is the termite hill. The termites that inhabit these colonies in effect all belong to the same family and were derived from the same parents. The body plan of these animals is so modified that they are incapable of an independent life, the workers are blind and sterile, the soldiers are armed with jaws so large that they cannot forage and have to be fed by workers. At the centre of the colony is the queen who is encaserated within earthwalls and has an abdomen that is distended to 120 mm and produces eggs at a rate of 30 000 per day. She is fed by workers and her eggs collected for incubation elsewhere in the termitaria. The only other sexually active male is the wasp-sized king who stays by the queen and is also fed by the workers. Chemical Communication An effective communication co-ordinates these individuals and is generally induced by chemicals, although soldier termites sound an alarm by beating their large hard heads on the passage walls. Other chemical hormones (also called pheromones) in effect circulate instructions and dictate both actions and the development of the colony. All members of the colony exchange food and saliva with each other by way of the workers who also gather the excrement in order to reprocess it for food to obtain the maximum nutrition from it. The queen produces a pheromone, which is collected by workers and circulated through the colony. Although the queen termite produces both sexes, the queen's pheromones inhibit development maintaining them as sterile, wingless and blind (=workers). How soldier termites are produced is unknown (either specialized eggs or preferential treatment of larvae). Soldier termites have their own unique pheromone which is circulated through the colony and reaches the queen who probably regulates their numbers. Establishing a new colony At certain times, however, the queen does not suppress larval development and sexually mature winged termites of both sexes are produced and leave the colony by way of splits in the termitaria and take-off ramps. With the commencement of the first rains the flying termites pour out. Following dispersal and pairing the wings fall off and the male and female termites excavate a new nest. These become the royal pair for a new termite colony. Within the small royal cell they copulate and produce the first larvae which have to be feed by the parents until they are able to forage independently and continue with the construction of the new nest and founding of a new colony. The termite towers Termites construct fortresses that may contain several tons of mud and contain several million inhabitants. Ventilation and temperature control are therefore critical for survival of these communities. Around the margins of these termitaria are tall, thin-walled chimneys. As the sun warms the walls of these chimneys air becomes hotter than the air inside of these nests, the air in the chimneys therefore rises and with it draws air from the termitaria. Since the chimney walls are thin and porous, oxygen from the outside diffuses in. This air rises to the top of the nest and re-oxygenates the colony. In very hot weather the workers descend in tunnels that go deep into the ground water, and carries back a crop full of water that wets the wall and lowers the temperature through evaporation. Wasp and Bee nests Wasps and bees also have a colonial lifestyle comparable to termites. Wasps show transitions in degrees of colonialism. Some hunting wasps are entirely solitary, with a female wasp constructing a nest of mud in which she lays her eggs and stores a provision of parasitized wasps. In other species the female wasps remain by the nest and brings daily food to the larvae. In other wasps the females construct nests next to one another, some of the nests are abandoned and wasps may join other wasps in constructing theirs. Eventually one female wasp assumes dominance and lays eggs in the amalgamated nests with other wasps building more cells to house larvae and collect food. Dance of the bees The evolution of community living is also elaborate in bees. A single queen bee is also a specialist egg-layer, that is supported by worker bees. The community is also bound by a system of chemical messages (pheromones) but they also use a dance behavioural pattern to communicate to each other. When a worker bee returns from a new nectar laden flower a dance behavioural sequence is initiated. If the source is nearby, the bee performs a simple round dance, alternatively circling in clockwise and counter clockwise directions. The other bees are excited by the dancing scout and follow it outside, and they find the food by orientating to chemical signals present on the scouts body. If the food source is more than 80 m from the hive, the scout expresses this in its dance with a distance and direction of the source. A waggle dance traces two semi-circles with a straight run between them. The food's distance is described by sounds and wagging movements executed during the straight run. The further away the food lies the longer the sounds last and the more slowly the dancing bee waggles its abdomen. The angle of the straight run describes the direction of the food source in relation to the sun. A run straight up the hive wall denotes a location directly towards the sun. When food exists at an angle to the left or right of the sun, the bee runs at the same angle to the left or right of the vertical. Even on cloudy days these dances are effective, because bees detect the sun's location by the analysis of polarized light. The interpretation of these behavioural patterns have been debated, since inexperienced workers do not seem to be as efficient at foraging for pollen whereas experienced workers are almost always successful. The returning scout bee is usually covered with pollen and some researchers feel that the bees respond to olfactory signals rather than the interpretation of behavioural patterns. Insect and plant cohabit The most complex and highly evolved forms of colonialism in the insect world are those created where the organisms (wasps, bees and ants) live within plants, stimulating the tissue of their host to provide them with custom-built homes, by growing special galls, hollow stems or thorns with swollen bases. The leaf-cutting ants of South America build vast underground nests and have expeditions via long tunnels. They may remove entire trees (leaves, roots and stems) converting the material to pulp in their chambers which forms a compost for cultivating edible fungi. Imperialism- Insect style Most ants, unlike termites and leaf-cutting ants are carnivorous. Such ants may prey on termites, devouring the workers and larvae. Yet other ant species make slaves of other ant species, by raiding a nest and collecting the pupae and rearing them to be slaves. Yet other carnivorous ants do not make nests, but march in great masses. Such an army of ants may forage on animals caught in its wake for several weeks. When the larvae produce pheromones they are circulated within the army and keep it on the move, when the larvae pupate, no pheromones are produced and the army clusters around roots of a tree. Individuals clinging to each other create a living nest of tunnels and chambers. The queen starts producing eggs which hatch into larva, while soldier ants emerge from their pupae. The next generation of larvae produce pheromones which stimulate the army to move-off. Assignments IN YOUR OWN WORDS WRITE A ONE TO TWO PAGE ESSAY ON THE FOLLOWING TOPICS Describe the forms of social life that occur in insects. Describe the signals used by insects to attract a mate for sexual reproduction. Many insects have long larval stages and have larvae that differ significantly in morphology from the adult forms. Discuss this statement giving suitable examples. THE CONQUEST OF THE WATER AND THE BIRTH OF THE VERTEBRATES Although animals without back-bones (invertebrates) are more abundant numerically and more diverse in species variety, they have never been able to reach the sizes that animals possessing a backbone can (vertebrates). All animals with a back-bone and some with a stiffened cartilage rod called a notochord belong to the phylum Chordata. One of the most primitive members of this group are the Tunicates or sea-squirts. Although the sessile adult phase bears a superficial resemblance to a sea-anemone (Coelenterata), the rest of the vertebrate fauna was derived from such a simple organism. Evidences for this ancestry is in the tunicate's larval stage which resembles a tadpole and has the following features which are shared with all other vertebrates: 1. Perforations in the wall of the pharynx, or pouches that suggest ancestral perforations. 2. A nerve cord dorsal to the gut that is tubular and reflects its embryonic development from a tough piece of ectoderm that became roofed over. 3. A stiff rod called a notochord that supports the nerve cord from below. The larva is short and the animal attaches itself to a rock and loses its tail and becomes a sedentary filter-feeder. Free-living chordates The next most advanced animal in the evolutionary tree is the lancelet or amphioxus, which is more fish-like in appearance but also has a stiff rod or notochord. This animal is 50 mm in length has a well-developed segmented muscular system that allows it to bury itself quickly in the sand. This animal has no clearly defined head region, only a light-sensitive spot at the anterior, no heart only a few pulsating arteries, no fins or limbs but only a slight dilation at the hind end. The strong muscles rhythmically contract against the notochord and the animal is propelled forward in a series of waves. These lancelets and the larval tunicates therefore resembled each other and considerable argument arose as to which form was the most direct ancestor for the rest of the vertebrates. The embryonic development of many animals often reflects their phylogeny or ancestry. Consequently, larval termites resemble bristletails and larval horseshoe crabs resemble the segmented trilobites. It was therefore argued that the lancelet was the ancestor to the tunicates, Fossil evidence for the first chordates However, fossil evidence in the Burgess shales (550 million years ago) included a finned or backboned swimming animal similar to the living lancelet called a Pikaia and was the predecessor to a group of fish-like animals that were jawless (apart form modified parasitic forms) and consequently could only feed on micro-organisms and small animals. These animals belong to the class Agnatha. A jawless predator Another larva provides evidence for the next step in the vertebrate evolution. This is the Lamprey, class agnatha (= without jaw) which have larvae that are also jawless, blind and without fins except for a fringe around the tail and very similar to lancelets. These larvae were once thought to be adult creatures called ammocoetes. The adult lamprey is very fish-like except being jawless. It possesses the beginnings of a backbone in the form of cartilaginous elements. They also have a clearly defined head, with two small eyes, a single nostril leading to a blind sac, and on either side of the neck a row of gill slits. The mouth is a circular disk and possess a tongue with sharp spines. It is with this disk that the lamprey clamps itself on to fish which it parasitizes. Ostracoderms – an extinct group with heavy armour Within the agnathan group were other small fish-like animals called ostracoderms and possessed heavy armour-plating which may have originated from deposition of salts derived from their food. This marks the first presence of bone, the material that was destined to influence much of the evolution of the vertebrates. These early bony plates may have provided protection against the large (2 m) sea scorpions that co-existed at the same time. Heavy deposition of salts in the head region have permitted remarkable fossilisation of these animals where the structure of the brain and nerve and blood vessels can be identified. In addition a balancing mechanism composed of two arching tubes at right angles to the vertical plane has been recognized. The liquid within these tubes, moved over the sensitive inside surfaces enabled these animals to be aware of their posture in the water. These animals dominated the freshwater streams 500 million years ago and the largest representatives reached 600 mm in length. The single median fins down the midline of their back provided stability in locomotion, but only the group Cephalaspidomorpha had paired lateral appendages that may have had a similar function to the lateral fins of true fishes. All these animals had gills located in pouches Protofish and internal bony skeletons An important development in one group of protofish was the development of bony rods stiffening the pillars of flesh between the gill slits. The first pair of which hinged forward and were supported with muscle tissue, and produced the first jaws. The evolution of jaws permitted fish and their descendants to utilize larger and harder food, and thus enabled them to become adapted to many new and diversified ways of living. This advance was of sufficient importance so that fish and tetrapods (four-legged animals) are together called gnathostomes (= jaw + mouth). Some of the bony scales in the skin which covered these animals enlarged and became the first teeth. Lateral flaps of skin evolved into the first true fins and their swimming skills improved. These animals were called Placoderms and may have pioneered the gas bladder for vertical movement in water and eventually evolved into lungs. The most impressive of the placoderms was the Arthrodira which reached 9 m and possessed large jaws equipped with serrated teeth. Developing some backbone One of these animals (Acanthodii) were acquiring an internal bony skeleton and included the beginnings of a vertebral column running longitudinally through the body and encompassing the primitive notochord. These were the probable ancestor to the bony fish we know today and possessed a streamlined body, large lateral eyes and wide mouths with numerous teeth. Their heads are bony and their small scales are thick and hard, but unlike the placoderms they did not have armour. The numerous lateral fins of these animals are unique in that each has a thin membrane supported at its leading edge by a long stout spine. Re-inventing the cartilage skeleton At this time a pronounced split appeared in the fish dynasty, with one line of animals losing all their bone and developing cartilage, a softer more elastic and lighter material. The descendants of this are the fish belonging to the class Chondrichthyes and represented by sharks (orders Galeomorpha and Squalomorpha), rays (order Batoidea) and chimeras (order Chimaerida). Although this lightened them they would still need to continue to swim or they would sink. Swimming is still accompanied by a powerful thrash of the tail and pectoral fins which prevent them from diving nose down. Since the pectoral fin is stiff these have less mobility than the pectoral fins of the bony fish. Some of these fish rested by sinking to the sea-floor, and one group has adopted such a position on a semi-permanent basis (rays and skates). As a consequence they have become greatly flattened with pectoral fins expanded into undulating lateral triangles which they use for locomotion and the muscle in the tail is almost completely lost (although it may bear a poisonous spine at the end). Rays and skates are not as fast swimming as sharks, but this is of less importance since they feed on molluscs and crustaceans. Sharks and Mantas Sharks have mouths on their undersides and water passes through the mouth and over the gills and out through the slits. With bottom-dwelling mantas and skates this would cause mud to get into the gills, so instead, they have two openings or spiracles on the upper surface of the head that take in water and lead it straight to the gills. It is then expelled on the underside through the gills. One kind of ray, the manta has reverted from bottom-dwelling to surface dwelling, using the large lateral extensions to remain afloat. Swimbladders: refinement The other group of fish which retain bone in its skeleton, also had to overcome weight problems in the water. Early fish with heavy bone-based scales, colonized shallow lagoons and swamps which had warm, poorly oxygenated water. The bichir (Polypterus)(order Polypteriformes), a heavy scaled fish occurring in Africa indicates how these early fish overcame such problems. These animals rise regularly to the surface and take a gulp of air which goes into a pouch leading off the top part of the gut. A concentration of capillaries in the walls of the pouch absorb the gaseous oxygen. These air-filled pouches which were the first lungs also provided buoyancy and the ability to float without using the tail and eventually evolved into swimbladders. With the ability to absorb gas from the blood systems there was no need to collect air from the surface and the connecting tube to the throat became no more than a solid thread. The diffusion of gases into and the expelling of air out of the swimbladder would permit a precise means of vertical control in the water. The pectoral fins would provide refinement to this control. However, swimming skills were improved still further with increased tapering of the twin-bladed symmetrical tail that is driven by banks of muscles on either side of the backbone. Streamlining was enhanced with reduction of heavy scales into smaller tightly fitting ones that overlap like tiles of a roof and are covered by slippery mucous, and pectoral and pelvic fins being able to fold back into depressions in the lateral sides of the fish. The respiration using gills was further refined with the development of a movable, bony operculum which by inducing negative pressure forces water over the gills and improves respiration. The diversity of morphological forms is testimony to the success of the group. One group, the flying fish (order Atheriniformes) leap out of the water and glide hundreds of metres in the air using the elongated pectoral fins. This may be an anti-predator tactic. Garfish (order Lepisosteiformes) have pectoral fins that have become filmy skulls rotating slowly bach and forth which permits them to hover in water. Dragonfish (order Pegasiformes) have lateral fins modified into defensive mechanisms with each ray barbed with poison. The swimbladder has released fish from weight problems, and therefore, some like the box-fish (family Ostraciontidae) and sea-horse (order Gasterosteiformes) have regained armour. Down the flanks and around the head of fish runs a series of pores, connected by a canal running just below the surface. This is called a lateral line and enables the fish to detect differences of pressure in water. As a fish swims, it creates a pressure wave ahead of it, when this wave meets another surface the fish can detect pressure changes created by this surface. It is this ability that permits them to detect other fish and to polarize themselves into swimming in shoals. Vulnerability to predators is thought to be reduced by shoaling. Fish also have an acute sense of smell and detect minute changes in the chemical composition of water. This sense of smell may guide fish to food. Fish also detect sound with the addition of a third canal (in a horizontal plane and below the sac) which supplements the two semicircular canals that are found on either side of the skull of the lamprey. All three canals and the sac have very sensitive linings and contain small calcium particles which move and vibrate. Sound waves, which travel better in water, penetrate the semicircular canals without the need for passages which are required by terrestrial animals. The eyespot of the lamprey is primitive compared with the bony fishes. The eye of the bony fish and higher vertebrates is a closed chamber with a transparent window and a lens in front and a photosensitive lining at the back (retina). The photosensitive lining contains two kinds of cells, rods for distinguishing light and dark and cones which are sensitive to colour. Sharks and rays lack cones and are unable to perceive colour; this may reflect the lack of highly coloured examples within the group. Bony fish have both types of cells in their retina, and are also characterized by vivid colours and striking patterns. The Butterfly fish (Family Chaetodontidae) showing particularly diverse colours and patterns which permits species recognition. Colour is also an important asset in male fish during spawning. Such displays serve to chase other male fishes away, and to attract female fish. Pigment granules diffuse within the skin as the fish become excited and fights other rivals or to stimulate a female fish to lay her eggs. Eyes of fish have become adapted in various ways to vision below and above water. The archer fish (Toxotes jaculator) squirts fluid at an insect above the water and knocks them into the water where they can be eaten. This required compensation since light bends as it passes from water to air due to differences in density. Anableps has a horizontal division across its pupils which effectively gives it four eyes, the two lower halves for underwater use and the two upper halves for above water. Since fish can occur at great depths (below 750 m) where there is no light, they may posses modified cells producing luminescent chemicals which are activated rhythmically and may represent some form of communication to the rest of the shoal. The whiskery angler fish Antennarius scaber (order Lophiiformes) has a modified dorsal fin spine with an elongated thread at the end of which are cells producing luminescence. This is used to entice other fish to explore the light and be consumed. Water that is covered with floating mats of vegetation is also turbid, and in such an environment some fish have generated electricity from modified muscles in their flanks. Electrical signals are transmitted almost continuously creating flow patterns of current in the immediate vicinity. Any object encountered disrupts these flow patterns and the fish perceives these changes through receptor pores located over the body. The electric eel of South America Electrophorus electricus, although not a true eel, has additional body tissues that produces a massive shock of waves with which it kills or stuns prey items. From the jawless armour-laden prototype fish have evolved some 30 000 different forms to occupy seas, lakes and rivers of the world. Download 2.01 Mb. Share with your friends:
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