Insect Nervous System and Vision: Insights into Behavior and Navigation

1. Behaviour etc • nervous system • sensory apparatus • integration • behaviour - processes and function

2. Nervous system • insect nerve cells are complex • insect nerve cells have highest metabolic rate of any known tissue • insect nerve cells respond faster than ours (small size of ‘brain’) • insect nerve cells can’t send long range pulses quickly • insect nerve cells don’t have redundancy

4. organisation • 2 ventral nerve chords + segmental ganglia • CNS primitively ‘ladder’ • CNS has tendency to become fused • ‘brain’ = supra- + suboesophageal ganglia

7. sense organs • mainly setae … many kinds of sensory setae • mechanosensory, chemosensory etc • campaniform - cuticular stress • placoid - chemoreception • chordotonal organs - limb position • equilibrium sensors (e.g. Johnson’s organ

9. vision • ocelli • stemmata • compound eyes

10. ocelli - late instar hemimetabolous insects, adult insects … fast response, some may detect images. Used in horizon detection, flight control • stemmata - larvae of holometabolous insects. Largely light/dark detectors, limited image formation • compound eyes - larvae of hemimetabolous insects, adult insects. Used to detect images

13. ommatidia organisation • corneal lens • crystaline cone - feeds light into rhabdomeres • rhodopsins oriented in villi of rhabdomeres • 3 colour (sometimes 4 colour) vision • UV/blue/green, sometimes red • detector cells twist, short (UV) cell detects polarization

14. apposition compound eyes • commonest form in insects operating in daylight • each ommatidium provides information from a narrow solid angle about its axis • axes not oriented radially, some areas densely sampled by ommatidia arranged almost parallel (fovea) • complex neural circuitry combines information from adjacent ommatidia

15. superposition compound eyes • mainly nocturnal insects (& (modified) in butterflies) • lens systems of many ommatidia act as little telescopes and generate an erect image on the ‘retina’ (made up of the packed detector elements of many ommatidia) • eyes have a ‘clear space’ and produce ‘eye-shine’ • resolution not quite as good as apposition eye, light collection ~10-100x better

16. muscid eyes • only found in muscoid flies (houseflies, blowflies, tachinids etc) • apposition eyes BUT detector elements don’t twist AND detector elements from adjacent ommatidia that are ‘looking’ in the same direction are hooked up through a complex nerve mesh • good light detection capability, good resolution • associated with need to collect photons to compensate for effects of rapid turning flight

17. vision • extensive neural processing in optic lobe,feature detection circuits similar to ours • motion detection • image detection • speed of processing (flies have flicker-fusion thresholds > 5 x ours)

18. insect vision is a field of very active research – and ANU is a world leader • we now know insects are MUCH more capable than was thought the case even 10 years ago • emulation of insect vision is proving a fertile field in robotic vision • other insect senses are likely to prove equally ‘impressive’

19. behaviour • navigation • behaviour/ecology • development • maintenance • mating systems

20. navigation • use of vision • landmarks … wasp, bee first flights • use of sun compass • use of polarization pattern if sun not visible • time clock to compensate for sun’s apparent movement • other senses - chemical, remembering steps

21. use of landmarks • originally investigated in sphecid wasps • Philanthus work - Tinbergen • use of landmarks • availability • kinds preferred • hierarchy of landmarks used at different scales • hierarchy of ‘backups’ remembered

23. sun compass • use position of sun in sky to navigate • time clock to compensate for sun’s apparent movement (even overnight!) • enables flight over long ranges or uniform habitat (ranges of kms) • use of polarization pattern in small patches of clear sky if sun not visible

24. other senses • chemical gradients • magnetic sense • remembering steps • REAL navigation almost always involves a hierarchy of different senses, with backups

25. behaviour and ecology • behaviour is a key process underlying ecology • example we will take: ‘dragonfly life history’(will bounce around a range of species)

26. larval stages • Diphlebia is the concrete example • females lay eggs in rotten wood floating in pools • micro-habitat of earliest larval stages unknown • later stages occur under rocks in riffles • emerge at night to hunt prey on rocks

27. adult maintenance • thermoregulation • conformers • heliothermy • myothermy • feeding • prey detection • interception

28. mating systems • e.g. dragonflies (very well studied) • ‘rendezvous’, operational sex ratio • male behaviour • sperm competition • female responses to limit interference • mating in dragonflies requires female action … males can hold on to encourage - but may lose opportunities

29. sperm competition • Insects preadapted for strong sperm competition - sperm stored, only a few used per egg, eggs fertilized at laying • displacement or extraction of previous sperm • mate guarding to prevent take over by another male (with consequent loss of stored sperm)

30. exercise • examination of dragonfly mating systems

31. References • Physiology: Imms ‘Outlines of entomology’ as revised …CSIRO ‘Insects of Australia’ • Behaviour: navigation, mating systems/sperm competitionAlcock ‘Animal behavior: an evolutionary approach’ • dragonflies: Corbet ‘Dragonflies: behavior and ecology of Odonata’