Basic functions of insect compound eyes:
6. spatial vision, perception of distances
Sensory organs of insects
Sensory organs of insects
Image formation in (A) photopic eye; and (B) scotopic eye (a–f) Paths of light rays
Sensory organs of insects
Example of a dichoptic compound eye of a fly
Sensory organs of insects
Example of a holoptic compound eye
(larger facests provide higher visual acuity)
Sensory organs of insects
A. The lens of vertebrates projects a reversed image on the concave receptor surface.
B. The lens system of the insect compound eye projects an unreversed image on a convex receptor surface.
Basic functions of compound eyes I.
Image formation
- mosaic like picture
- resolution - elaboration of the picture:
• in contrast to humans (1’ = centesimal minute) in case of insects 1-3°
(dragon flies 0.24°);
• depends on the number and size of ommatidia, on the interommatidial angle and on the bending of the eye which can change on different surface areas of the eye
• resolution is often non-uniform over the whole eye, there are regions with high resolution (acute zones or „foveas”) as well as regions with lower resolution
Sensory organs of insects
Sensory organs of insects
Mechanism of vision, visual cascade:
- Starts with activation of rhodopsin and the resulting membrane depolarization.
- The unstable cis-retinal is converted to its trans form when it absorbs a photon of light.
- Its transformation to the stable form of metarhodopsin activates G-proteins that ultimately causes the membrane depolarization.
- The metarhodopsin loses its ability to activate the G-proteins after it is phosphorylated and binds with arrestin proteins to assume its unstable form.
- Light converts this metarhodopsin to an inactive rhodopsin that releases its arrestin and again becomes capable of responding to light.
- Regeneration of the photopigments is a rapid process (0.7 ms) trans-retinal - 11-cis-trans-retinal conversion
Sensory organs of insects
The two forms of retinal that combine with opsins to create rhodopsin.
The biochemical pathway starts with the activation of rhodopsin and the resulting membrane depolarization. The unstable cis-retinal is converted to its trans form when it absorbs a photon of light. Its transformation to the stable form of metarhodopsin activates G-proteins that ultimately causes the membrane depolarization. The metarhodopsin loses its ability to activate the G-proteins after it is phosphorylated and binds with arrestin proteins to assume its unstable form. Light converts this metarhodopsin to an inactive rhodopsin that releases its arrestin and again becomes capable of responding to light.
Basic functions of compound eyes II.
Colour vision
Evidences confirming colour vision:
- attractive property of coloured baits, and artificial flowers..
- spectral sensitivity:
- wave length range: generally from 350 nm (UV) to 550 nm (yellow)
exceptionally over 610 nm (orange and red) (some species of Odonata, Lepidoptera, Hymenoptera); compound eyes are most sensitive to 350 nm (UV) and 490-500 nm (blue-green); because UV perceptibility special mixed colours: „bee purple” (yellow+UV); „bee violet” (violet+UV)
- wave length discrimination with the help of:
different types of photosensitive pigments: different rhodopsin variants receptor cells with different sensibility of colours
Sensory organs of insects
Theories of insects’ colour vision:
1) „trichromatic” colour vision
in case of honey bee (Apis mellifera) 3 basic colour; 9
photoreceptors: 3 UV-receptors - 335 nm; 2 blue receptors - 435 nm 4 yellowish-green receptors - 540 nm
2) „bichromatic” colour vision
– 6+2 yellow-UV: Formica polyctena
– 5+3 green-UV: Periplaneta americana – 6+2 blue-UV: Drosophila melanogaster
3) „tetrachromatic” colour vision: yellow-green-bluish green-UV:
Manduca sexta
Sensory organs of insects
Adaptation to different light conditions
- discrimination of different light intensities
insects are able to perceive smallest changes e.g.: Musca domestica 0.5-7.5%
- their temporal resolution is unique
- FFF = Flicker Fusion Frequency: the frequency at which an intermittent light stimulus appears to be completely steady to the observer
- FFF is important in all technologies for presenting moving images, nearly all of which depend on presenting a rapid succession of static images (e.g. the frames in a cinema film, TV show, or a digital video file). If frame rate falls below the flicker fusion threshold, flicker will be apparent to the observer, and movements of objects on the film will appear jerky. For the purposes of presenting moving images, the human flicker fusion threshold is usually taken as 16 Hz. In actual practice, movies are recorded at 24 frames per second, and TV cameras operate at 25 or 30 frames per second, depending on the TV system used.
Sensory organs of insects
FFF depends on the lifestyle of certain insect species:
a.) „fast-eyed” species: diurnal, fast flying (honey bee, dragon fly, blowfly..): 180-300/sec
(that is the evidence why is so difficult to flick a fly)
b.) „slow-eyed” species: nocturnal or diurnal (cockroach 25-60, locusts, fruit flies 60-90): 25-100/sec
• - The eye translates forms in space into a sequences of events in time. It follows that fast eyed insects with have the best form perception: e.g.: bees can distinguish solid shapes from striped patterns, though they cannot distinguish between two solid shapes or between two patterns of stripes.
Sensory organs of insects
Types of photoadaptation of compound eyes by:
1) change of the light quantity reaching the receptors 2) change of the sensitivity of receptor cells
Adaptation to dark and bright conditions
- up and down movement of pigment grains within the accessory pigment cells of superposition eyes
move up = adaptation to dim or dark conditions
move down (optical isolation) – adaptation to bright conditions e.g. Cydia pomonella
- expansion and contraction of primary pigment cells:
by the pupillary opening of apposition eyes e.g. Camponotus spp. Ant species
Adaptation by reduction of sensitivity of photoreceptors - accumulation of inactive metarhodopsin in retinule cells
- „synaptic adaptation”: decreased quantity of neurotransmitters at the axonal terminals of retinule cells
Sensory organs of insects
Perception of polarized light
• Although the light originating from the Sun is unpolarized and vibrates in all directions, the particles it encounters as it travels through the earth’s atmosphere cause it to become polarized and vibrate in a specific direction.
The degree of polarization changes with regard to the position of the sun and the orientation of the observer, making it possible to determine the sun’s position even when clouds obscure it.
• Some of the elongated rhabdomeres contain uniformly oriented rhodopsin within their microvilli and the absorption of light is maximal when the light is polarized in the same direction as the pigment is oriented.
• In bees and ants there is also a group of specialized ommatidia at the dorsal margins of the compound eyes believed to the rhabdoms within this dorsal rim area are shorter with a larger cross-sectional area, and their microvilli, containing oriented pigments, are also oriented 90° to each other.
• Many other insects bear these dorsal rim area ommatidia but have not yet been studied for the ability to detect polarized light.
Sensory organs of insects
Sensory organs of insects
(Left) An elongated rhabdomere. (Right) The uniform orientation of rhodopsin within the microvilli of the rhabdomere that allows the reception of polarized light.