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Evolution Of The Major Arthropod Taxa Biology Essay

Arthropods dominate the terrestrial kingdom and are members of the phylum arthropoda. Major classes of arthropods include crustecea, insects and arachnids. Crustecea are a marine group, whereas insects and arachnids are usually terrestrial. Insects usually have a sharp partition between the head and thorax, and the thorax and abdomen. Starting at the anterior end of a typical insect, the head is usually in one piece, and then the thorax has three segments, each bearing a pair of appendages, usually walking legs. On each of the second and third thoracic segment of most insects is a pair of wings. The segmented abdomen usually does not have appendages attached and has spiracles used for gaseous exchange. However, there are differences which can distinguish between different orders of insect. The main aim of the experiment is to study and be familiar in with these differences.

There are three main classes of arthropods - crustecea, insects and arachnids. Crustecea include crabs, shrimp and barnacles. They have several body segments with variable amount of legs and two pairs of antennae. Arthropods are highly segmented which is achieved during development by cooperating pattern systems, mediated by genes, for example Hox genes. Arachnids include mites, ticks, spiders and scorpions. They have two body segments; a cephalothorax; and abdomen, eight legs, a pair of chelicerae and no antennae. Spiders and scorpions are carnivorous organisms. Arachnids have both a head and abdomen region with jointed legs connected to the head in the subphylum Chelicerata. All arachnids have eight legs and shed their exoskeleton. Scorpions include pseudoscorpions which live in soil or leaf litter in temperate ecosystems. All arthropods, apart from scorpions lay eggs.

Insects have three body segments: the head, thorax and abdomen. They usually have distinct divisions between the head and thorax and between the thorax and abdomen. They have six legs, one pair of antennae and diverse modifications to appendages. Also, they possess an exoskeleton; have a segmented body, bilateral symmetry, jointed limbs and mouthparts, a ventral nerve cord, and a dorsal blood pump. There are about 32 different orders of insects. However there are six important mega orders- Orthoptera, Hemiptera, Diptera, Hepidoptera, Coleoptera and Hymenoptera. Orthoptera and Hemiptera are both hemimetabolous, therefore development is incomplete. The egg forms into a nymph stage before the development of wings and turning into an adult. Diptera, Hepidoptera, Coleopteran and Hymenoptera occurred later in the evolutionary timeline and are all holometabolous, therefore undergo complete development. The egg goes to a larva stage, then a pupa stage before forming an adult and feed on sugar water.

Orthoptera (strong wing) include grasshoppers, crickets and locusts. The two suborders are Caelifera and Ensifera. There are two pairs of wings. At rest, the forewings are held at the abdomen overlapping the hindwings. The forewings, also known as tegmina are hardened at the base, leathery in texture, and spread out in flight. The hindwings are wider than the forewings and are membranous, with straight veins and many cross-veins. Nymphal wing rudiments reverse their orientation in later instars. In most species, the females mount the males during mating.

Their hind legs are enlarged for jumping and they usually have a cylindrical body. They have mandibulate mouthparts, large compound eyes and some species have ocelli. Antennae are segmented and of variable lengths.

The second segment of the thorax is much shorter than the first and third segments. The few segments at the end of the abdomen are condensed and have unsegmented cerci. The hind coxae are small and spaced out. Usually a pronotum is present with large descending lateral lobes on the sides. The hind tibiae normally have two dorsal rows of teeth.

Caelifera are the grasshoppers and other related families. They can be identified by having large legs, modified for jumping, antennae shorter than their body with fewer than thirty segments and a short ovipositor. Sometimes a tympanum is present on its abdomen. Stridulation involves rubbing serration of inner surface of hind femur across veins of front wing.

Ensifera are the long horned Orthoptera, such as crickets and katydids. The hind femora are not as enlarged as in Caelifera, and the antennae are thread like, with more than thirty segments. The ovipositor is long and tarsi are present with about three or four segments. The tympanum is on the front of the tibia in singing groups. Also in singing groups, the forewings are customized for stridulation, having a toothed vein and scraper, in addition to membranous areas that vibrate and amplify the sounds.

Hemiptera (half wing) include aphids, greenfly, bugs and true bugs, such as bedbugs. They have a piercing proboscis on their underside. The forewings are hardened at the base and membranous at the ends, whereas the hindwings are totally membranous and shorter than the forewings. Hemiptera have segmented antennae and legs. They also possess mouthparts where the mandibles and maxillae have evolved into a proboscis, by which alimentary and salivary glands are enclosed in a segmented labium. A modified rostrum is present in the mouth, which is able to perforate tissues and suck out the fluid. Hemiptera also have holocentric chromosomes.

Diptera (two wings) are the true flies, including midges, fruit flies, and house flies. They can be distinguished by having a head region attached to three thoracic segments. The third thoracic segment has reduced wings called halteres attached to it, which are used for changing the pitch in flight. The mesothorax is enlarged to contain huge flight muscles. They have large compound eyes.

Flies are holometabolous insects; their life cycle involves a major change in form from a soft-bodied, wingless larval stage to a hardened, winged adult. The larval stage of development in diptera is a small, pale, soft bodied maggot. They lack true legs, have a cephalopharyngeal skeleton and move via peristaltic contractions. The two suborders of Dipera are Nematocera and Brachycera. The Nematocera include mosquitoes and midges. Brachycera include horse flies and robber flies.

Lepidoptera include butterflies which are generally day flying, and moths which are usually night flying. Butterflies and skippers are monophyletic groups and moths are paraphyletic.

Butterflies rest with their wings closed on top of their bodies, and make a naked pupa, also known as chrysalis. Their antennae are thin with knobs.

Moths have feathery antennae and usually rest with their wings open. Caterpillars are the larval form of Lepidoptera which have a cylindrical body with a well developed head, mandible mouthparts and usually about eight legs.

The caterpillars of many species of Lepidoptera dig into the ground to pupate to form a protective cocoon around the pupa. The cocoon is made of a combination of silk and other natural materials including leaves to their own body hair. They have four membranous, wings with scales. The forewings are bigger than the hindwings. They undergo complete metamorphosis. The proboscis is the extended mouthpart and is generally in a coiled tube structure.

Coleoptera (sheath wing) are the beetles – the largest order of animals in the animal kingdom. Beetles are holometabolous insects. Their bodies are divided into three sections: the head, thorax and abdomen, each of which is composed of several segments. Beetles are known to have a hard exoskeleton of numerous sclerite plates. This is for defence whilst retaining flexibility. The middle section of the upper body surface is a hard plate known to be the pronotum.

Wing cases (elytra) are usually present, protecting the hind-wings underneath, which have reduced venation and are used for flying. Elytra are formed via the hardening of the forewings which are used to protect the more fragile hindwings and the dorsal surface of the abdomen. The insect opens the elytra and then extends the hind-wings, flying while still holding the elytra open. However, some beetles, such as ground beetles are unable to fly.

The antennae of an adult are used mainly for olfaction and have eleven or fewer articles. There are two hind thoracic segments; mesothorax; and metathorax, which together form the pterothorax. The pterothorax is connected with the abdomen so that the functional units of the body are the head, prothorax, pterothorax and abdomen rather than the typical head, thorax, abdomen in most insects. The genitalia are drawn into to the abdomen. The segmented legs usually bear claws on the last tarsi segment. The legs are primarily used for walking but can be used for swimming or digging or even jumping in some species.

Most species have chewing mouthparts called mandibles. A gula is present on the undersurface of the head. Oxygen is obtained from spiracles along the bodies’ surface. Coleoptera have an open circulation system and haemolymph is used to carry oxygen around the body.

Hymenoptera are the ants, bees and wasps. This mega order has two diagnostic features. There is either a gaster present next to the waist or/ as well as hooks present on their wings known as hamuli, that join them together. This allows the wings to beat as a single unit. Their wings bear fewer veins than other insects.

They have large compound eyes with ocelli. Their mouthparts are well developed and usually form a proboscis to enable them to drink liquids. The ovipositor has become modified as a stinger in some species, with the eggs laid at the base, rather than the tip, which is used only to inject venom. A stinger is used primarily to immobilise prey, however it’s used as defence in some bees and wasps.

They usually possess more than ten segments. Their antennae are longer than the head. Some groups are wingless, however in the winged species the forewings are usually bigger than the hindwings and they usually have few cross-veins.

Larvae usually have mandibles, three thoracic limbs, and abdominal legs. Complete metamorphosis occurs. Females have a prominent ovipositor, modified in some groups to be a stinger enabling prey paralysation and defence.

Arthropoda is evidently a highly diverse phylum. Segmentation of arthropods arose from convergent evolution. As Arthropods have adapted, they have diverged their phylogenetic tree. Horizontal gene transfer passes on some of the genetic material from one organism to another. Recent research has found that horizontal gene transfer from Wolbachia bacteria to arthropods has had a dramatic effect on their evolution. The evolutionary ancestry has been dated back to the Cambrian period. The oldest known arachnid was in the Silurian period, by which shortly after was the development of silk producing organisms in the Devonian. The first appearance of true spiders was in the late Carboniferous period. Later, in the Jurassic and Cretaceous periods, fossils of spiders were found. In the Silurian and Devonian period, aquatic scorpions with gills were fossilised. Early Carboniferous there was a fossil found of a scorpion with book lungs.

Conclusion

Arthropods have jointed exoskeletons which moult on a regular basis. They can be divided into three classes: crustecea, insects and arachnids. They can be distinguished in by having different traits. All insects have three main regions: the head, abdomen and thorax even though there can be many structural differences among them. Also, they all possess compound eyes. It is evident that the amount of segments and wings vary between different species. The main reason for these variations is due to evolving to their environment causing many species to adapt. In both Orthoptera and Hemiptera, development is incomplete. Diptera, Hepidoptera, Coleopteran and Hymenoptera go through complete development, therefore occurred after these two orders through adaption, over evolutionary time.

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