2 - Arthropod Characteristics

• Bilateral symmetrical elongated body subdivided into segments or somites is characteristic of arthropods.
• Arthropods can have two or more somites, depending on their class.
• Insects have jointed paired appendages and a chitinous exoskeleton.
• The exoskeleton is primarily made of chitin, similar to plastic.
• Chitin is naturally self-lubricating to prevent the insect's surface from drying out.
• Arthropods have a greatly reduced general body cavity or coelom called haemocoele.
• A well-developed head, usually with compound eyes, is a distinguishing characteristic of arthropods.
• Nearest allies of an arthropod are the segmented worms or annelids (i.e earthworms, leeches).
• Insect segments can be classified as 3 distinct, 2 distinct, or no distinct segments.

Common Characteristics

• Insects share common characteristics with humans
• The insect has three parts: head, thorax, and abdomen.
• Human torso corresponds to the thorax and abdomen of an insect.
• Spiders have a combination of head and thorax called the cephalothorax, and an abdomen.

Antenna

• Antennae are primary non-visual sensory organs for smell, taste, touch, and hearing.
• Antenna are used for sizing prey, objects, or holding during mating.
• Protura are insects which lack antennae.
• Most insects have a mechanosensory organ on the pedicel.
• Only the basal antenna segments contain intrinsics muscle.
• Male insects have more elaborate antennae than females to find them.
• The greater the surface area of the antennae, the more dilute the sense they can detect.
• Male insects with feathery antennae are more sensitive than those with filamentous antennae.
• The segments of the antenna: scape, pedicel, and flagellum.
• The scape is the base
• The pedicel is aka Johnston's organ

Eyes

• Insects have two different sorts of eyes
• Compound eyes are large and visible with individual facets or lenses
• They are made up of several smaller eyes functioning in unison.
• Ocelli are simple eyes that are dorsal or lateral.
• Dorsal ocelli: Found in most insects
• Lateral ocelli: Found in larvae of some insect orders
• The shape and location of eyes and antennae vary from insect to insect

Mouth

• Mouthparts are a fusion of different segments
• The labrum is the upper lip structure covers the entire mouthpart and is generally movable.
• Mandibles are a pair found below the labrum
• Most robust mouthparts of a chewing insect
• Maxillae are a pair located beneath or caudal to the mandible
• Used to partly masticate food in chewing insects.
• Important in manipulating soft, liquid, or particulate food.
• Labium is the lower lip of the insect.
• Assists in the manipulation of food.
• The hypopharynx is a globular structure located medially to the mandibles and maxillae
• Assists in the swallowing of food and sucking of liquid

Types of Mouthparts

• Non-piercing biting and chewing mouthparts are a primitive form exhibited by cockroaches, grasshoppers, and beetles
• The insect mouth moves horizontally rather than vertically like in humans.
• Piercing type haustellate mouthparts are highly modified
• Sucking type mouthparts seen in mosquitoes

Modified Mouthparts

• Orthopteran type is a type of biting and chewing mouthparts exhibited by cockroaches, beetles, chewing lice
• Hemipteran type is piercing-sucking mouthparts exhibited by bedbugs and kissing bugs
• Anopluran type is piercing-sucking mouthparts exhibited by sucking louse
• Dipteran type is piercing-sucking mouthparts with subtypes: mosquito, horsefly, stable fly and housefly
• Siphonaptera type is piercing-sucking type exhibited by fleas
• Hymenopteran type is exhibited by honeybees which are mandibulate and suctorial
• Lapping wasps and ants have biting mandibles
• Lepidopteran is suctorial coiled tube exhibited by butterflies and moths

Feeding Apparatus

• Explains the mechanism of acquiring and transmitting pathogens
• The structure of the mouthparts may indicate certain feeding behaviors and means to penetrate host tissues
• Pharyngeal or esophageal pumping apparatus allows sucking up of blood or other body fluids
• Relationship between salivary glands and mouthparts dictate how the pathogens are stored and transmitted
• Regurgitation of infectious fluid from the digestive tract into vertebrate tissue or on to the food or vertebrates

Thorax

• Thorax is the midsection, second segment of the insect.
• It is the wing- and leg-bearing section of the insect
• Usually has two pairs of wings, totaling four wings, and three pairs of legs, totaling six legs
• Wings have the support structure of thickened veins
• The tips of an insect's feet usually contain a sticky pad, hook, or sucker, allowing it to stick to various surfaces or clasp onto its prey
• The central location of the more powerful muscles used in operating the wings and legs in insects

Sections of the Thorax

• Prothorax (Pronotum): Origin of forelegs (1st pair of legs); Bears no wings
• Mesothorax (Mesonotum): Origin of midleg (2nd pair of legs) and forewings (1st pair of wings)
• Metathorax (Metanotum): Origin of hindleg (3rd pair of legs) and hindwings (2nd pair of wings)

Legs

• The segments of an insects' leg: coxa, trochanter, femur, tibia, and tarsus
• The coxa is Section of the leg attached to the body
• The trochanter is second segment of the leg between the coxa and femur
• It is similar to a lever
• The femur is the third segment between the trochanter and tibia
• In jumping insects (e.g. grasshoppers), the femur is enlarged with powerful muscles
• The tibia is the fourth segment located between the femur and tarsus
• The tarsus is the last segment which can subdivided into several sections
• Insects can have knees, and spurs on the tibia depending on the type of insect

Leg modifications

• Cursorial legs are for running seen in Cockroaches
• Saltatorial legs are for jumping seen in Grasshoppers
• Raptorial legs are for seizing prey seen in Praying Mantis
• Fossorial legs are for digging seen in Mole Cricket
• Clasping legs are for climbing seen in Head Louse

Wings

• Wings are outgrowths of the body wall for flying
• The first pair of wings is sometimes modified into a protective covering for the third or hindwings
• Insects and vertebrates are the only groups of animals that successfully evolved flight
• Wings are composed of two membranes of the cuticle that are pressed together and supported by a series of veins or venation
• The pattern of veins or venation is not haphazard and is regular
• Venation is useful in species identification
• Triangular in form
• Regions of venation: costa, subcosta, radius, media, cubitus, anal, and cross veins
• Margin of the wings: costal, apical, and anal
• Angles of the wings: apical, anal, an humeral

Abdomen

• The abdomen makes up the last somite
• It is the most recognizable part of the insect
• Location for digestion, respiration, excretion, and reproduction
• Contains the digestive system, reproductive organs, and sting organs (if applicable)
• Reactive part of an insects body, which means it can expand when the insect feeds
• Has 11 segments, only 10 are conspicuous or visible
• Each segment consists of:
• Dorsal sclerite or tergites (tergum)
• Vental sclerite or sternites (sternum)
• Pleural membrane - joins tergum and sternum laterally
• The front margin of each segment often telescope inside the sclerites of the preceding segments, allowing the abdomen to expand and contract
• Appendages such as cercus may arise from the dorsal part of the last abdominal segment

Insect Integument

• The Insect Integument is composed of the cuticle and the underlying epidermal cell that secretes the cuticle
• Functions in locomotion, breathing, respiration, feeding, excretion, protection from desiccation, behavior, osmoregulation (water control) and food reserve
• The many roles played by the integumentary covering are, in part, reflected in the complexity of its structure and chemistry
• It is adapted to function in the ecology of its owner

Composition

• The cuticle/cuticula serves as the exoskeleton of the insect and site for muscle attachment.
• First line of defense against pathogens, predators, parasites and even environmental chemicals including pesticides.
• Characteristically sclerotized or hardened in certain areas, forming body plates or sclerites, which makes the exoskeleton hard.
• Epidermal cells always a single layer of epidermal cells lying immediately beneath the cuticle secreting the new cuticle at molting

Layers of the Insect Integument

• The epicuticle is the outer layer/ exterior layer of the cuticula with very thin surface that is non-chitinous
• The exocuticle is the primary cuticle/cuticula with darker pigmentation with chitin
• Layer of body wall containing hardening substances that form the sclerites
• The endocuticle is next to the exocuticle
• Inner secondary cuticula and chitinous
• The epidermis is the cell layer of the body wall

Chitinous characteristics

• Most fluids (even acids) cannot penetrate haemocoel due to the chitin
• It contributes to the survivability of insects and even radiation cannot penetrate.
• Newer chemicals penetrate the membrane of wings and legs (tarsus) and mixed with active agents that can melt the integument
• Chemicals are put into the food of insect pests as to make it a systemic poison

Nervous System

• The nervous system is a series of ganglia connected by a pair of ventral nerve cords that run along the length of the abdomen
• Located on the ventral portion of the insect
• Consists of ventral diaphragm, Brain, ventral nerve cord, segmental ganglia, nerves
• Similar to humans but located ventrally and controlled by ganglia

Parts of the Nervous System

• Consists of brain, subesophageal ganglion, nerve cord
• Considered as the large ganglia found in the head area/region, also known as the supraesophageal ganglion
• Anatomically divided into 3 regions
• Many arthropods have well-developed sensory organs which include compound eyes for visions and antenna for olfaction and pheromone sensation which is controlled by the brain
• Sensory information is processed by the brain
• Subesophageal Ganglion is behind the brain and is composed of 3 pairs of fused ganglia
• Controls the mouth parts, salivary glands and certain muscles

Nerve Cord

• Knot-like enlargements aka Segmental Ganglia for movement
• Thoracic Ganglia
• In a generalized insects, for every 3 thoracic segments there is 2 thoracic ganglia
• Locomotion of the legs and the wings
• Abdominal Ganglia Responsible in movement of abdominal parts and reproduction
• Segmental nerves are fine strands

Differences

• Most body segments have one ganglion on each side but some are fused to form the brain and other large ganglia
• Common Housefly has a combined thoracic and abdominal ganglia
• Bristletail Each segment has its own ganglion, forming a series of ganglia

Circulatory system

• It is located in the dorsal side/region of the insect
• Insects have an open circulatory system

Parts of the Circulatory System

• Haemocoele which is the cavity exposed when the body of an insect or cockroach is cut open
• Provides the internal environment the cells and tissues
• It consists of the dorsal diaphragm which partially separates the dorsal part of the haemocoele, within which the heart is located
• Dorsal vessel is where all the hearts are located
• Alary Muscles fan-shaped groups of muscles of the heart which supports the pumping and integrity of the heart

Haemolymph

• It is the "blood" of the insect that flows through these blood vessels and also throughout the insect's body cavity
• The movement of the haemolymph is an open type system

Respiratory System

• All Insects are aerobic organisms that obtain their oxygen from their environment in order to survive
• In insects, the respiratory system is responsible for delivering sufficient oxygen to all cells in the body and for removing carbon dioxide that is produced as a waste product of cellular respiration
• It is separate from the circulatory system
• Main structures: spiracles, trachea, tracheoles

Tracheal System

• Operates by the air entering the body, though the spiracles, trachea, and tracheoles
• The oxygen eventually diffuses to the cells in the body.
• The carbon dioxide also leaves through the spiracles.
• Insect respiratory systems are either open or closed

Open System

• Air breathers in which air enters the insect body through a bulb-like opening in the exoskeleton called spiracles
• Located laterally along the thorax and abdomen, usually 1 pair per body segment
• Example: Cockroach

Closed System

• Respire using gills example are Naiads
• Young dragonflies have 3 feather-like appendages at the end of their abdomen
• Used for breathing and movement as it whips back-and-forth

Plastron

• A plastron is a series of hairs or bumps on the surface of an aquatic insect used to trap a thin layer of air against the body
• Thin layer of air is prevented from shrinking due to the actions of the hairs and bumps
• The diffusions of the oxygen means the thin layer of air acts as a physical gill that supplies Insect with oxygen without returning to the water surface

Air Tubes

• Siphons exhibited by the mosquito, attaches itself on the surface of the water using the siphon as a breathing tube

Digestive System

• Also called alimentary canal
• Divided into 3 divisions: fore gut, mid gut, hind gut

Fore Gut

• First section for food
• Contains a pharynx that contains dilator muscles and connected to the esophagus
• Aids pumping of food
• Crop that serves as temporary storage for undigested food which begins digestion of complex sugars
• Gizzard that regulates the passage of food between the crop and the ventriculus or stomach mixes digestive fluids thoroughly into the food mass

Mid Gut

• Second digestive section
• Primary area for digestion and absorption of nutrients including uterus
• Gastric Caeca Elongated pouches at the beginning of the midgut
• Contain the bacteria which aid in digestion
• Contents from the mid gut and malpighian tubules empty into the hind gut

Hind Gut

• Last section of the digestion tract
• Section where the feces and urine exits through the pylorus aka the gatekeeper
• Consists of the ileum and the rectum

Excretory System

• Part of the digestive system with malpighian tubules remove the nitrogen-containing wastes and regulate water and salts in an insect hemolymph