Lecture 27: Arthropoda (V: Hexapoda)

Hexapoda

• Phylum Arthropoda → Subphylum Hexapoda

• mostly insects and kin

  • only terrestrial inverts that fly/develop indirectly

• most diverse phylum of animals on earth

• evolved on land, co-evolved with flowering plants

Hexapoda Body Plan

• body composed of 20 segments organized in 3 tagmata:

  • head (6 somites)

  • thorax (3 somites)

  • abdomen (11 somites)

• large fat body is mainly concentrated in abdomen

• ectodermally derived Malpighian tubules

Hexapoda Head

• bears:

  • compound eyes and ocelli

  • antennae

  • clypeolabrum

  • mandibles

  • maxillae

  • labium

• fused exoskeleton of head forms a unique internal tentorium

Hexapoda Legs

• uniramous

• present on the 3 thoracic segments of adults

• composed of 6 articles:

  • coxa

  • trochanter

  • femus

  • tibia

  • tarsus

  • pretarsus

Hexapoda Respiration

• gas exchange by spiracles and traceae

Hexapoda Digestion

• gut with gastric (digestive) ceca

Hexapoda Reproduction

• gonopores open on the last abdominal segment

• gonochoristic

  • development indirect or direct

Entognathous Hexapods

• non-insect Hexapods

• mouth parts with bases hidden within the head capsule

  • mouth inside

• most or all antennal articles with musculature

• wingless

Class Insecta

• “in sections”

• external mouth parts

Insecta Body Plan

• 3 tagmata

  • head

  • thorax

  • abdomen

• 3 pairs of legs and 2 pairs of wings on thorax

• 1 pair of compound eyes

  • 3 ocelli

• 1 pair antennae

• mandible and other mouthparts

Insecta Feeding and Digestion

• mouthparts specialized for feeding behaviour

  • chewing, piercing, sucking, lapping, sponging

• salivary secretions

  • soften, lubricate, digestion enzymes in some, silk in some

• specialized digestive structures

  • pharynx, esophagus, crop

• unique fat body

  • cluster of fat cells in hemocoel which works as a ‘liver’

Insecta Respiration and Circulation

• tubular heart pumps hemolymph

  • which does not transport oxygen

• pulsatile organs at the base of wings/long legs

• tracheal system

  • tubular invaginations of body wall supported by spiral taenidia

  • bring air directly to cells

  • spiracle → trachea → tracheole

  • cell no more than a few micrometres from a tracheole

  • shed with molt (except tracheole)

• most aquatic insects have gills

Insecta Excretion/Osmoregulation

• water conservation challenge for terrestrial/flying insects

• Malpighian Tubules

• waterproofing of exoskeleton via sclerotized/waxy cuticle

• water collection via dew drops/condensation

  • evaporate cooling

• many nocturnal

• dormancy periods

  • ex. aestivation

Malpighian Tubules (Insecta)

• unbranched outgrowths of gut at midgut/hindgut junction

• blind end extends in hemocoel

• 100s of tubules

• via osmotic pressure

  • because insufficient blood pressure

• uric acid crystals passed along with feces

Insecta Nervous System

• basic arthropod plan

• brain + 2 ventral nerve cords + segmental ganglia

Insecta Sensory Organs

• simple ocelli in larval, juvenile, and sometimes adult

• well-developed compound eye

  • resembling that of crustaceans

  • reduced, or absent in parasites/cave dwellers

• microscopic tactile/chemosensory setae

  • sensilla and peg organs

• phonoreceptors (aka ears)

  • modified setae

  • modified antennae

  • tympanic organs

Insecta Legs

• walking legs end in terminal pads and claws

• hindlegs of grasshoppers and crickets are enlarged for jumpinh

• swimming insects have modified paddles

• examples:

  • mole crickets have front legs adapted for burrowing in ground

  • forelegs of praying mantis, water scorpion allow them to grasp prey

  • honeybees have leg adaptations for collecting pollen

Insecta Wings

• first flying animals, and only flying invert

• cuticular extensions on thorax: wings

  • most have 2 pairs

  • some have no wings

Types of Wings

• Dipteran

  • 1 pair of wings with modified hindwings

• thin and membranous

  • common

• thick

  • beetles

• scaley

  • butterflies

  • moths

• parchment-like

  • grasshoppers

Wings/Gills

• new evidence (2023) has come out that wings may have evolved from gills

Insect Flight

• figure 8 movement moves insect forward

• direct or indirect

Direct Flight

• flight muscles attach to wing

• dragonflies

Indirect Flight

• flight muscles alter the shape of the thorax to cause wing movement

• bees, flies