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Saturday, May 11, 2019

INTRODUCTION TO THE STUDY OF ARTHROPODS

1.0 ENTOMOLOGY (INSECT SCIENCE)

2.0 PHYLUM ARTHROPODA

Aims

To have a basic understanding of what the science of entomology is all about.

To review the classification and features of significant groups in the Phylum Arthropoda.

1.0 ENTOMOLOGY (INSECT SCIENCE)

1.1 WHAT IS ENTOMOLOGY?

Entomology is a basic and applied science that is dedicated to the study of insects and other related arthropods.

1.2 IMPORTANCE OF ENTOMOLOGY?

For more than 350 million years, insects have survived on Earth, adapting and evolving to become the creatures we see and know today. Through the millennia, insects have become integral parts of almost all ecosystems imaginable. Insects are the most numerous and diverse form of life on the planet. While some insects cause harm by spreading diseases, destroying plants, or damaging structures, others are beneficial in that they pollinate our crops, kill and eat harmful insects, and help recycle nutrients in nature. The study of insects helps us understand how to protect the lives and property of people from those insects that are harmful. Furthermore, applied entomology, which is the practical application of insect science, helps use the skills and knowledge gained through entomology research to teach farmers how to increase their yields through sound pest management strategies, to protect and conserve endangered species and fragile habitats, and to prevent insects from spreading devastating diseases like malaria and sleeping sickness. Insects are also studied as models to help us understand the physiological and biological processes of other animals even man. For example, the fruit fly Drosophila melanogaster has brought significant advances to research in neurodegenerative diseases in humans.

1.3 CAREER OPPORTUNITIES IN ENTOMOLOGY

Agricultural entomologists: These group of entomologists study insects of Agricultural importance. While some insects are beneficial to Agriculture in that they aid pollination and recycle nutrients in the soil others are Agricultural pests that cause devastating losses to crops on the field and in storage. Up to 40 percent of world’s food production is lost every year to insect pests. Agricultural entomologists thus work with farmers and other agricultural stakeholders like Food and Agricultural Organization (FAO) to manage insect pests while protecting the beneficial ones.




Medical entomologists: Each year millions of people die from diseases that are spread by insects. According to the World Health Organization (WHO), a child dies every minute from malaria, which is spread by mosquitoes. In Africa and other parts of the world, diseases such as malaria, river blindness and sleeping sickness reduce the agricultural productivity of farmers and their households and by extension, the food production of their country. Medical entomologists in collaboration with health organizations like WHO thus devote themselves to understanding how vectors transmit pathogens and how to effectively control them.




Veterinary entomologists: Several insects and arthropods are pests of livestock, wildlife and pets. These insect pests like flies, fleas, ticks and even mosquitoes are capable of causing diseases and irritation to farm animals, wildlife and domestic pets. Veterinary entomologists thus work with zookeepers, ranchers, forest rangers, and veterinarians to help them protect animals from dangerous insects.




Taxonomic entomologists: Of the over 30 million species of insects on Earth, only about one million have actually been discovered and described. Identification and description of insect species are essential to effective control, management and study of any insect. Taxonomic entomologists are continuously conducting studies that will help discover the several unidentified insect species.




Forensic entomologists: The study of entomology also plays a significant role in the fight against crime. Forensic entomologists help the police in solving murder cases. They do this by examining the bodies to find insects, which may provide clues about the time of death and how the victim died. Using this information, the police are able to arrest suspects and probably convict the criminal.




Forest entomologists: While some insects are helpful to trees, shrubs, and bushes, others may harm them. Some insects damage trees by boring through them, eating essential parts or by spreading diseases. Forest entomologists specialize in insects that harm wild plants and forests and study ways to prevent and control them.




Structural entomologists: Several insects like ants, cockroaches, termites and flies are peridomestic. Many of them are capable of causing serious damage to structures and foodstuffs in homes, hotels or elsewhere. Structural entomologists study and discover ways to keep insects out of buildings, and ways to get rid of them. These entomologists work with pest-management professionals and households to teach them the best ways to deal with insects that invade buildings.




Military entomologists: The outcome of any battle will be determined by the physical health of the soldiers amongst other factors. Though military entomologists do not participate in active combat, they help to identify, before the arrival of the army, the presence of any insect pest in the camp area that will be deleterious to the health and food resources of the army.




2.0 PHYLUM ARTHROPODA

2.1 WHAT ARE ARTHROPODS?

The name “arthropod” comes from two Greek words, arthros, “jointed,” and podes, “feet.” The Arthropods are the most successful of all animals. They are made up of over a million species. Members of this phylum range in adult size from microscopic to minute to very large. Generally all arthropods have segmented bodies with jointed appendages. They possess a hard outer or external skeleton or covering made of chitin termed the exoskeleton. Arthropods are bilaterally symmetrical with a ventrally situated nervous system and a dorsally positioned brain and heart. All arthropods have a distinct head, sometimes fused with the thorax to form a tagma called the cephalothorax. Some arthropods have many body segments. In others, the segments have become fused together into functional groups, or tagmata (singular, tagma), such as the head and thorax of an insect. This fusing process is known as tagmatization or tagmosis.

2.2 SUB-GROUPS IN THE PHYLUM ARTHROPODA

A. Subphylum Trilobitomorpha: This sub-phylum is made up of the earliest known members of the arthropods – the trilobites which belong to the Class Trilobita. They occurred some 600 million years ago and have been extinct for about 200 millions years. Their bodies were divided sideways into three “lobes”. The central lobe consists of a cephalon (head), thorax (body) and pygidium (tail). Trilobites, were the first animals whose eyes were capable of a high degree of resolution.

B. Subphylum Chelicerata (cheli = a claw, hoof): The Chelicerates have their bodies divided into a cephalothorax (cephalo = head) and an abdomen. The first pair of appendages of their bodies is mouthparts called chelicerae that function in feeding. The chelicerae may be modified into pincers or fangs. These arthropods lack mandibles or jaws. Classes under this sub-phylum include

1) Class Merostomata or Xiphosura (xipho = a sword; ura = tail): Members of this class have a horseshoe-shaped carapace covering their cephalothorax. Their abdomen is thin, long, and pointed, hence the class name. They are marine, typically found in intertidal areas. Example includes the horseshoe crabs.

2) Class Arachnida (arachni = spider): Members have simple eyes on top of their cephalothorax. The first pair of appendages is modified as chelicerae while the second pair of appendages is modified as pedipalps (pedi = foot; palpi = a feeler). Arachnids have four pairs of walking legs. Occasionally there are other appendages, like the spinnerets in spiders. Arachnids breathe via book lungs. Orders under this class include

a. Order Scorpionida: this includes scorpions. They are arachnids whose pedipalps are modified into pincers. Scorpions use these pincers to handle and tear apart their food.

b. Order Phalangida (phalang = finger, toe): this includes the daddy-long-legs and harvestmen, which have long, slender legs. They are not spiders and do not produce webs.

c. Order Araneida or Araneae (aranea, aranei = spider): This includes the spiders, which have the chelicerae modified as fangs with poison glands (used to paralyze prey), have spinnerets on the abdomen from which they produce silk for their webs, and are beneficial predators.

3) Class Pycnogonida: This includes the sea spiders. Sea spiders are common in coastal waters. Adult sea spiders are mostly external parasites or predators of other animals like sea anemones.

C. Subphylum Crustacea (crusta = crust, rind): These are mainly aquatic animals that include several species of crayfish, crabs, lobsters etc. Crustaceans have the head and thorax combined into one body region, the cephalothorax, as well as an abdomen. Most crustaceans have gills, two pairs of antennae, three types of chewing appendages, and various numbers of pairs of legs. Class in sub-phylum Crustacea include -

1) Class Malacostraca:

a) Order Decapoda: The decapods consist of large, primarily marine crustaceans such as shrimps, lobsters, and crabs, along with their freshwater relatives, the crayfish. These decapod crustaceans have five pairs of walking legs including a large pincer, the cheliped.

b) Order Isopoda: Although most crustaceans are marine, many occur in fresh water and a few, like the pillbugs and sowbugs, have become terrestrial.

D. Subphylum Atelocerata (formerly Mandibulata): These include those arthropods with biting jaws, called mandibles. In mandibulates, the most anterior appendages are one or more pairs of sensory antennae, and the next appendages are the mandibles. Among the mandibulates, insects have traditionally been set apart from the crustaceans, grouped instead with the myriapods (centipedes and millipedes) in a taxon called Tracheata. This phylogeny, still widely employed, dates back to benchmark work by the great comparative biologist Robert Snodgrass in the 1930s. He pointed out that insects, centipedes, and millipedes are united by several seemingly powerful attributes:

• A tracheal respiratory system.

• Use of Malpighian tubules for excretion.

• Uniramous (single-branched) legs.

In recent years, a mass of accumulating morphological and molecular data has led many taxonomists to suggest new arthropod taxonomies. The most recent morphological study of arthropod phylogeny, reported in 1998, was based on 100 conserved anatomical features of the central nervous system. It concluded insects were more closely related to crustaceans than to any other arthropod group. This closeness is also supported by molecular evidences.

Classes under the sub-phylum Atelocerata include -

1) Class Chilopoda (chilo = lip). These are the centipedes. Adult centipedes do not have hundred legs as the name suggests. They may however have 30 or more legs. Their bodies are divided into head and trunk regions with the first pair of legs on the trunk modified into poison jaws to capture and kill prey. On the trunk region, centipedes have one pair of appendages per segment. All known species of Centipedes are carnivorous and feed mainly on insects

2) Class Diplopoda (diplo = double, two): These are the millipedes. The adult millipedes also have head and trunk regions like the centipedes but with two pairs of legs on each body segment. Each segment of a millipede is actually a tagma. In contrast to centipedes, most millipedes are herbivores, feeding mainly on decaying vegetation. A few millipedes are carnivorous. Millipedes live primarily in damp, protected places, such as under leaf litter, in rotting logs, under bark or stones, or in the soil.

3) Class Hexapoda or Insecta (hexa = 6): These are the insects. Insects have three body regions: head, thorax and abdomen. Most also have both compound and simple eyes, mouthparts (which include a labrum, mandibles, maxillae, and a labium), 3 pairs of walking legs, one on each of the three thoracic segments. Many insects have wings, which are not modified appendages, merely flaps of the exoskeleton of the meso- and metathoracic segments. The chief internal organs of insects include a tubular digestive tract, a long valvular heart for pumping blood, an intricate muscular system, a system of pipe-like tracheae for respiration, paired reproductive organs opening at the posterior end of the body, a nervous system consisting of a brain, paired segmental ganglia and connectives.

2.3 SUB-GROUPS IN THE CLASS INSECTA

1.0 Subclass Entognatha (ento = within; gnatho = jaw/mouth): They have their mouthparts within the head, are primarily wingless and exhibit simple metamorphosis. They are no longer considered to be insects.

i. Order Collembola: They have no wings. Examples- springtails

ii. Order Protura: They have no wings. Example- Acerentomon spp

2.0 Subclass Ectognatha: (ecto = outside): These have their mouthparts stick out from head. Under this sub class we have

a) Super Order Apterygota (a- = without; ptero = wing): They are primarily wingless and exhibit simple metamorphosis.

i. Order Thysanura: They have a flattened body with three tail-like structures on the posterior end. Their body is often covered with scales. Examples -silverfish and firebrats

b) Super Order Pterygota: they are winged and a few are secondarily wingless. This super Order is futher divided into -

Exopterygota: (exo = out, outside): Exopterygotes exhibit gradual metamorphosis, with wing pads that develop externally. Young exopterygotes are called nymphs or naiads if aquatic. Orders under this division include –

i. Order Ephemeroptera (ephemer = temporary): Members include the mayflies.

ii. Order Odonata (odonto = tooth): Members include the dragonfly and damselfly.

iii. Order Orthoptera (ortho = straight): Members include the grasshopper, katydid, camel cricket etc.

iv. Order Phasmida (phasmato = apparition, phantom): Members include the walking stick, leaf insect.

v. Order Mantodea (manti, mantid, mantis = a soothsayer): Members include the praying mantis.

vi. Order Blattaria (blatta = cockroach). Members include the cockroach.

The orders Blattaria, Mantodea, and Phasmida, used to be grouped under one order, Order Dictyoptera.

vii. Order Isoptera (iso = equal): Members include the termites.

viii. Order Dermaptera (derm = skin). Members include the earwig

ix. Order Plecoptera (pleco = twine, twist, braid, twisted, folded): Members include the stoneflies

x. Order Phthiraptera (phthir = lice) — ectoparasites (mostly on birds or mammals). This Order has the following suborders. Suborder Mallophaga e.g. chewing lice and Suborder Anoplura e.g. sucking lice.

xi. Order Hemiptera (hemi = half). The true bugs often have long antennae divided into a small number of segments, and the front wings can be somewhat hardened. Some bugs resemble beetles, but beetles have wing covers that do not overlap, unlike the bugs.

Traditionally, the Order Hemiptera was split into two suborders: the Heteroptera (e.g. shield bugs, bed bugs, assassin bugs etc) and the Homoptera (e.g. aphids, scale insects moss bugs cicadas etc). This distinction was primarily based on the structure of the wings. Recent genetic phylogenic studies have however shown that though the groups within the Homoptera share a common ancestor, they are not as closely related as previously thought. Consequently, the suborder Homoptera has been scrapped. The insects that make up the suborder Homoptera have been regrouped into three new suborders namely: Auchenorrhyncha (e.g. cicadas), Sternorrhyncha (e.g. aphids. scale insects) and Coleorrhyncha (e.g. moss bugs). There are thus four suborders in the Order Hemiptera.

Endopterygota (endo = within, inner): Endopterygotes exhibit complete metamorphosis, with wing pads that develop internally until pupa stage. Young endopterygotes are called larva. Orders under this division include –

i. Order Neuroptera (neuro = nerve, sinew, cord): Members include the dobsonfly, lacewing, antlion etc.

ii. Order Coleoptera (coleo = a sheath): Order Coleoptera has the most species of any insect order. Members include the beetles

iii. Order Mecoptera (meco = long, length): Members include the scorpion fly

iv. Order Siphonaptera (siphon = tube; a- = without): Members include the flea.

v. Order Diptera (di = two): Members include the flies and mosquitoes. Their hind wings are modified as knob-like balance structures called halteres.

vi. Order Lepidoptera (lepido = scale) Members include the butterflies, skippers, and moths.

vii. Order Hymenoptera (hymeno = a membrane): Members include the honeybees, ants, wasps and hornets.

Success of Insects as a Group

Insects have attained the largest number of kinds of any animal group with an estimated 950,000 described species. The total number of described species for all animal groups is 1.6M. The great success of insects colonizing every conceivable habitat on earth can be attributed to the following factors or reasons:

1. High adaptive capacity.

2. High reproductive capacity (high fecundity).

3. Capacity of flight.

4. Dispersability by air current.

5. Ability to consume different kinds and qualities of food.

6. The insect Exoskeleton – the exoskeleton performs a number of functions e.g. protecting insects from desiccation or excessive evapo-transpiration, mechanical injury and invasion by pathogens. Another feature of the insect exoskeleton is its adaptiveness to a myriad of forms and colours. This is the basis of mimicry in insects, which is a survival strategy in many insect species.

7. Smallness of size.

8. Efficient water conservation methods – possession of waxy cuticle greatly reduces the rate of transpiration from the surface of the body. Another device is the closing mechanism possessed by the tracheal system and the spiracles which control water loss while at the same time allowing enough oxygen to go through for respiration purposes.

9. Exhibition of metamorphosis.

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