Insect Sensory System

Insect Sensory System Lecture 15

How insects perceive the world chemical, tactile visual visual chemical chemical chemical, tactile acoustic tactile

How insects communicate with other organisms

Vision • Compound eyes • Ocelli • Stemmata • Other visual receptors

Compound Eyes • Composed of ommatidia • ~10,000 in dragonflies, 5,500 in honeybees, 800 in Drosophila • Ommatidium (sing.) consists of • an optical, light-gathering part: corneal lens and crystalline cone • a sensory part: rhabdom (retinula cells [usually 8], microvilli [forming rhabdomere]), pigment cells

Compound Eyes • Apposition Eyes • Each lens forms an inverted image at the tip of rhabdom • Because rhabdomeres function as a single unit, the impression of the image not retained • Image is a series of apposed points of light of different intensities • Light sensitivity limited by small diameter of facet lenses • Many diurnal insects

Compound Eyes • Superposition Eyes • Ommatidia not isolated optically from each other by pigment cells • Retina separated by a wide clear zone from lenses • Light rays are refracted internally within the lens • A single upright image collectively formed at the tips of rhabdoms • Great light sensitivity, but poor resolution • Nocturnal Insects (moths, beetles)

Image Formation This is a misconception.

Image Formation Original Through an insect eye

Wavelength discrimination • All insects have a visual pigment with maximum absorption in the green range of the spectrum and range extends below 400nm (UV) and up to 600nm (orange) • Most insects have two additional pigments, one with maximum sensitivity in the UV, another in the blue region • Most insets can’t see red, but dragonflies and some butterflies have additional pigments sensitive to 600-700nm range.

Polarization sensitivity • Light waves vibrate in planes at right angles to the direction in which they are traveling – polarization • Light coming from a blue sky is polarized • Some insects can perceive polarized light to maintain a constant and steady orientation relative to the sun

Ocelli • Single thickened cuticular lens, retinal cells, rhabdoms • Integrate light over a large visual field • Highly sensitive to low light intensities and subtle changes in light • Not designed for high-resolution vision

Stemmata • Only visual organs of larval holometabolous insects

Other visual receptors • Dermal light sense • Some epidermal cells are sensitive to light • Some butterflies have photoreceptors on genitalia • In some species, daylength, regulating diapause or polyphenism, is registered directly by the brain

Insect color production • Physical color • Scattering: light scattered by irregularities of a surface • Interference: reflection of light from a series of superimposed surfaces separated by distances comparable with the wavelengths of light • Diffraction: light strikes a surface with groove or ridge, different wavelengths are bent to varying degrees, leading to spectral colors

Insect color production • Pigmentary colors: reflect certain wavelengths of light • Black (aphins, melanin) • Red (ommochromes, pterins, carotenoids, etc) • Brown (ommochromes) • Orange (pterins) • Yellow (caroteinoids, flavonoids) • Green (insectoverdin) • Blue (caroteinoids) • White (uric acids, flavonoids)

Bioluminescence • Light production • Many beetles,a few dipterans (e.g. Mycetophilidae), one cockroach • In fireflies, enzyme luciferase oxidizes a substrate luciferin, in presence of ATP and oxygen, to produce oxyluciferin, CO2 and light • Light is produced by photocytes, in light organs

Mechanoreception • Perception of any mechanical distortion of the body • Touching objects • Vibration • Distortion of body due to gravity • Three broad structural categories • Cuticular structures • Subcuticular structures (chordotonal organs) • Internal structures (stretch receptors)

Hair-like tactile mechanoreceptor • Trichoidsensilla consists of: • Trichogen cell (grows hair) • Tormogen cell (grows socket) • Sensory neuron (grows dendrite and axon)

Position mechanoreceptor • Proprioceptors (self-perception receptors) • Sense relative position of body parts • Hair plate • A bed of grouped hairs at joints or at the neck, in contact with the cuticle of adjacent body part • Campaniformsensilla • A central cap surrounded by a raised circle of cuticle, located on joints

Chordotonal organs • Subcuticular sensory receptors sensitive to vibration • Consist of scolopidia • Scolopidium consists of neuron, scolopale cell and attachment cell • Vibration displaces scolopale cap producing sliding scolopale, leads to bending at the base of the cilium

Chordotonal organs • Subgenual organs • Proximal parts of tibia • Responds to vibrations of the substrate

Chordotonal organs • Johnston’s organ • Found in all adult insects and many larvae • In the pedicel, the second antennal segment • Sense movements of the antennal flagellum relative to the rest of the body • Dual as hearing organ in male mosquitoes and midges

Chordotonal organs • Tympanum • Sound reception • Tympanal membrane responds to distant sounds transmitted by airborne vibration • Often associated with air-filled sacs • Lots of variations in where the structure is found

Stretch Receptors • Multipolar neurons with free nerve endings • Associated with muscles, connective fibers or epithelia • Monitor the rate of stretch

Sound production • Percussion • Vibration produced by impact of part of the body against the substrate (death watch beetle, stonefly, lacewing) • Stridulation • Production of vibrations by moving a cuticular ridge on part part of the body over a toothed ridge (many orthopterans and hemipterans) • Tymbal • An area of thin cuticle, surrounded by a ridge frame; a strong tymbal muscle distorts the membrane and ribs to produce a sound (video)

Stridulation • Adult grasshoppers, katydids, and crickets often have specialized structures for producing sounds that they use to communicate with conspecifics

Cricket Katydid Mole cricket

Chemoreception • Taste – detection of aqueous chemicals • Smell – detection of airborne chemicals • Chemoreceptors are sensilla with pores • Uniporous (one pore): mainly for contact • Multiporous (many pores): mainly for olfactory cues

Semiochemicals • Pheromones: chemicals used in communication between individuals of the same species, that release a specific behavior or development in the receiver • Aggregation, alarm, courtship, sex, sex attraction, spacing, trail-making, etc. • Produced by exocrine glands derived from epidermal cells • Kairomones: benefit the receiver, but disadvantage the producer • Allomone: benefit the producer, by modifying the behavior of the receiver (neutral effect on receiver) • Synomones: benefit both the producer and the receiver

Sex pheromones • Sex attraction pheromone – often produced by females; volatile, odorous plume

Sex pheromones • Courtship pheromone – usually males produce courtship pheromone

Alarm pheromone • Chemical releaser of alarm behavior • Most social insects • Volatile, non-persistent compounds • Provoked by predators, threat to nest

Insect Sensory System