Subphylum Hexapoda – Characteristics and Taxonomy

1. Subphylum Hexapoda – Characteristics and Taxonomy • Characteristics of Class Insecta: body divided into head, thorax, and abdomen; uniramous appendages; one pair of antennae; three pairs of walking legs; most with wings; waterproof cuticle; base of mouthparts external (vs. Class Entognatha) Major Orders of Insects Other Orders include: Thysanura (silverfish), Plecoptera (stoneflies), Isoptera (termites), Phthiraptera (lice), and Thysanoptera (thrips)

2. CO 21 The dominant group of animals on Earth today

3. Fig. 21.1

4. External and Internal Structures • External Structure • Exoskeleton: more scleroproteins, less chitin and mineralization than in crustaceans ( lighter); waterproofing allows terrestrial lifestyle • Head with compound eyes, one pair of antennae, and mouthparts; thorax with wings and three pairs of legs; abdomen lacks appendages in adults (excl. genitalia, ovipositor, and stinger) • Mouthparts: labrum, pair of mandibles and maxillae, labium, and hypo- pharynx (all modified for either chewing, sucking, or lapping) • Internal Structure • Circulation via heart and dorsal aorta; aided by body movements • Gas exchange via tracheal system: thin-walled tubes stretch throughout body (open to outside via spiracles); aquatic stages with gills or snorkels • Excretion and water balance via Malpighian tubules: excretions range from solid uric acid to liquid honeydew, depending on water requirements • Excellent neuromuscular coordination, cross-striated muscles, and resilin allows amazing feats of strength (jumps of fleas, lifting by ants)

5. Fig. 21.2

6. Fig. 24.17

7. Fig. 24.17 (cont.)

8. Fig. 21.12

9. Fig. 21.18

10. Fig. 21.19

11. Locomotion • Walking: “tripod arrangement” of legs provides stability; hindlegs in grasshoppers, crickets, and fleas adapted for jumping; adaptations for walking upside down include claws and adhesive pads • Flight: most with two pairs of thoracic wings • Wings not homologous to vertebrate wings; wing follows figure- eight path during upstroke and downstroke motions • Direct flight muscles attached to base of wings • Indirect flight muscles alter shape of thorax • Synchronous neural control: each wing beat a response to a nerve impulse (ex. butterfly ~ 4 beats/sec) • Asynchronous neural control: faster wing beats possible (ex. fruit fly ~ 300 beats/sec)

12. Fig. 21.4

13. Fig. 21.11

14. Reproduction and Development • Reproduction(most with copulation) • Spermatophores, sperm plugs, and nuptial gifts common • Sperm stored in seminal receptacle many eggs fertilized • Eggs often laid in (or on) specific food source for larvae (ovipositors in certain wasps used to deposit eggs; ex., in figs or caterpillars) • Development (direct development in few species) • Molting under hormonal control (ex. ecdysone, juvenile hormone) • Hemimetabolous (incomplete) metamorphosis: winged adult develops from successive molts of nymphs (ex., grasshoppers, mantids, mayflies, stoneflies, dragonflies) • Holometabolous (complete) development: adult stage develops from larvae via pupae, a non-feeding stage enveloped in a case (88% of insects; larvae include caterpillars, maggots, and grubs) • Diapause: an arrest in development timed to period of suitable conditions (ex., abundance of food)

15. Fig. 21.8

16. Fig. 21.25

17. Fig. 21.22

18. Senses and Communication • Senses • Photoreception • Simple eyes: sense light (non image-forming); most insects with three ocelli on top of head (monitor light intensity, day length) • Compound eyes: consist of multiple ommatidia (up to 25,000 in dragon- flies) containing rhodopsin; form non-focused, mosaic-like image; excellent for sensing movement; most can detect polarized light and see in the ultraviolet range • Chemoreception: sensory pits full of chemosensory cells distributed over mouthparts and antennae; many behaviors mediated via chemical signals • Mechanoreception: seta (hairlike process attached to nerve cell) and scolophorous organs distributed over antennae, legs, and body; vibrations detected via legs or tympanal organs • Communication: via chemicals (pheromones), sound (incl. courtship songs), visual signals (incl. bioluminescence)

19. Fig. 21.20

20. Fig. 21.3

21. Evolutionary Ecology of Insects • Occupy Specific Niches: specialization of mouthparts related to diverse food sources (competition reduced) • Pollination and Coevolution: insects diversified as flowering plants diversified; in bees, scouts use waggle dance to guide workers to flower patches • Crypsis, Warning Coloration, and Mimicry • Batesian mimicry: mimicry of a poisonous model by a harmless mimic • Müllerian mimicry: common warning coloration evolved by multiple species • Social Systems (bees, ants, termites) • Castes in eusocial bees include queen (1), drones (sexually mature males), and workers (sexually inactive females); drones die after mating • Human Relationships: harmful insects spread diseases (flies, fleas, and mosquitoes) and reduce yield of crops; helpful insects pollinate food crops, eat other insects, and recycle nutrients • Control of insects: pesticides, use of sterile males, pheromone traps, natural predators used (integrated pest management reduces use of pesticides)

22. Fig. 21.16

23. Fig. 36.23

24. Fig. 21.6

25. Figs. 21.27 and 21.28

26. Figs. 21.34 and 21.35

27. Other Harmful Insects