Arthropoda III

Arthropoda III

Subphylum Hexapoda

Class Insecta



Biology/Importance



  • Species within Hexapoda intersect more with humanity than any other group of animals

  • Insects are responsible for billions of dollars in agricultural losses annually and transmit diseases that kill millions of people each year

  • On the other hand, through pollination, insects provide humanity with a tremendous ecological service. A service valued at 24 billion dollars in the US alone



General Characteristics

  • Six legs, all uniramous

  • Three tagmata:

    • Head

    • Thorax

    • Abdomen

  • Appendages are primarily located on the head and thorax



Diversity and Classification

  • Two classes:

    • Entognatha: hexapods with mouthparts enclosed within the head capsule

      • Ex: collembola, springtails

    • Insecta: base of mouthparts attaches to the outside of the head

      • Ex: typical insects (roach, beetle, dragonfly, fly, etc). 



Hexapoda: Class Insecta



Diversity and Distribution

  • Most abundant and diverse group of arthropods

  • 1.1 million species named by science

    • Estimates of true number may be between 5 and 10 million

  • Wide distribution - almost all freshwater and terrestrial environments

    • Marine forms are noticeably lacking (1 genus of water strider)




General Characteristics

  • Most insects are small (<2.5 cm)\

  • Differ from other arthropods by 2 primary ways

    • Mouthparts attached outside the head

    • Two pairs of wings on the thorax

  • External form and function:

    • Body is composed of sclerites connected by flexible joints

      • Like plate armor of knight

    • Head with compound eyes, single pair of antennae, and (generally) three ocelli

      • Antennae vary greatly in form and function: simple to branching 

    • Mouthparts typically consist of seven elements 

      • Labrum 

      • Pair of mandibles

      • Pair of maxillae

      • Labium

      • Hypopharynx

    • Thorax (a tagma) is composed of 3 segments

      • Prothorax (anteriormost)

      • Mesothorax (median)

      • Metathorax (posteriormost) 

      • Each of these segments bears a pair of legs

        • Legs are highly modified for a species’ ecology

    • Wings are cuticular extensions of the epidermis

      • Unlike in mammals and birds, insect wings are not modified forelimbs 

      • Most insects have two pairs

        • Exception is the flies (Diptera)



Locomotion

  • Walking/running

    • Use tripod arrangement: on one side two legs are in contact (front and rear) and other side only the middle. Alternates between. 

  • Flying:

    • Some are slow flappers (butterflies) others fast (mosquitoes) 

    • Difference is related to:

      • Direct and indirect flight muscles

      • Synchronous and asynchronous flapping



Homeostatic functions and structures

  • Nutrition

    • digestive system in three sections:

      • Foregut: food acquisition

      • Midgut: digestion and absorption

        • Gastric ceaca increase the surface area for these function

      • Hindgut: consolidation

    • Mouthparts - highly specialized for feeding

  • Feeding:

    • Most are herbivorous, some are carnivores, some are detritivores, some are parasitic

    • Parasitoids are parasites that kill its host in the process of completing its lifecycle

  • Circulation

    • Open circulatory system 

    • Heart pumps blood into dorsal vessel which empties into hemocoel

    • ‘Blood’  is hemolymph

  • Respiration:

    • Tracheal system:

      • Spiracles on abdomen are site of inspiration

      • Tracheal tubules run throughout the body

  • Excretion and water balance:

    • Malpighian tubules:

      • Network of narrow tubules running throughout the body

      • Collects metabolic waste and empties into intestine

      • Fluid feeders must get rid of excess water



Nervous system

  • Similar to that of crustaceans

  • Ventral nerve cord with ganglia spaced along its length

  • Sense organs are well developed

    • Mechanoreception

    • Auditory reception

    • Chemoreception

    • Visual system



Reproduction

  • Sexual reproduction is the norm for insects

  • Parthenogenesis is common in hemipterans (true bugs) and hymenoptera (wasps, bees, ants).

  • Fertilization is internal

  • Generally, many eggs laid

  • Parental care is lacking in some groups, very intensive in others

  • Many insects will undergo a metamorphosis:

    • Ametabolous: direct development, no larval stage (young similar to adults)

    • Hemimetabolous: incomplete metamorphosis; gradual change into adult form

    • Holometabolous: complete metamorphosis; drastic change from distinct larvval stage to adult (most insects)



Sociality

  • A few insects have evolved complex societies 

  • Many bees, wasps, ants, termites

    • Note, not all are social (bumblebees are solitary).

  • Societies have rigid, genetically determined caste system:

    • Sexually mature female = queen

    • Sexually mature males = drone

    • Sexually inactive females = workers

  • Castes are determined by fertilization and foods that are fed:

    • Drones develop parthenogenetically (without fertilization, all have the same mother, but no father)

    • Queens and workers develop from fertilized eggs (all have same mother and father).

    • Female larvae that are fed royal jelly will have gene expression modified and will develop into queens.