Eyes and Vision Flashcards
What are 3 different light sensing organs in insects?
- compound eye
- ocellus
- stemma
What are compound eyes used for?
perceive light, movement, complex image formation
- provide wide field of view
What are ocelli used for?
very sensitive to changes in light intensity
- probably not used in image formation
What are stemmata used for?
minimal image formation
What insects use compound eyes?
most adult insects except lice and female scale insects
what insects use ocelli?
many adult insects, larvae of hemimetabolous insects
what insects use stemma?
holometabolous larvae ( they are the only visual organs used by them)
What are ommatidia?
- an aggregate of subunits in the compound eye
- external surface of eye is faceted (hexagonal shaped in insects with large #s of ommatidia)
what are the two morphological elements of a compound eye? what do they do?
- optical part - light gathering part
2. sensory part - transforms light energy into an electrical signal
What are the parts of the optical part of the compound eye?
- cuticular lens
2. crystalline core
What is the cuticular lens of the compound eye? what does it do?
clear, colorless lens which is secreted by modified epidermal cells (corneagen cells).
- optimizes transmittance and limits reflectance of incipient light
What is the crystalline core of the compound eye?
consists of 4 “semper” cells and made of hard, clear intracellular substance.
- surrounded by primary pigment cells (corneagen cells)
What are retinula cells? where are they found and are they part of the optical or sensory part of the compound eye?
a group of elongate primary neurons that transduce stimuli and transmit signals onward
- found behind the cone of the compound eye
- part of the sensory part of the compound eye
What are rhabdomeres?
a margin running the length of each retinula cell that contains light sensitive pigments
How many retinula cells does each ommatidium have?
6-8 in close contact
What is a rhabdom?
a fusion of rhabdomeres into a central light sensitive rod
What are rhabdoms surrounded by? how many? why?
12-18 secondary pigment cells that serve to isolate each ommatidium
What are rhadopsins?
visual pigments similar to those in vertebrates contained in the rhabdom
- conjugated proteins
What is a retinaldehyde?
the chromophore in the rhodopsin proteins
- aldehyde derivative of vitamin A
what happens to the chromophore in the presence of light?
it isomerizes from the cis to the trans configuration, producing depolarization of the neural membrane, generating a nerve impulse (Action potential)
what kind of insects have a photopic eye?
diurnal insect species
what kind of insects have a scotopic eye?
nocturnal/crepuscular insect species
What is the structure of the scotopic eye?
there is a long translucent filament connecting the cone to the rhabdom. the filaments are isolated by secondary pigment cells whose pigments can migrate
What happens in scotopic eyes in the light?
the pigments occur throughout the cells to isolate the filaments such that the eye functions much like a photopic eye
What happens in the scotopic eye in the dark?
the pigments migrate upwards, allowing the filament to receive not only from its own cone (light parallel to the longitudinal axis of the ommatidium). but from the adjacent ommatidium as well.
The scotopic eye allows what? (in terms of visualization)
low visual acuity but high light sensitivity
What is the apposition type of eye?
the eye received a large number of point images (corresponding to each rhabdom)
- sharpness proportional to the number of ommatidia responding
What is the superposition type of eye?
point images at each rhabdom were formed from light rays collected by several adjacent ommatidium
What type of eye is and photopic eye and scotopic eye?
photopic: apposition
scotopic: superposition
What is the current concept for image formation?
image formed at the rhabdom has no physiological significance per se. there is a large amount of anatomical convergence of rhabdomere neurons in the optic lobes of the brain - cross-fibre patterning may form the image
What is the real function of the compound eye?
may be the perception of movement rather than image formation
How is movement seen in the compound eye?
as rapid changes in light intensity at any given ommatidium
what is the flicker effect?
the number of stimuli per unit time to which the eye is sensitive
What does the flicker effect depend on?
the speed of neural recovery from a previous stimulus ( the flicker fusion frequency)
What is a ‘fast’ eye? what insects have it?
faster succession of light stimuli produces better image formation via rapid scanning to produce the shape of the object.
- diurnal, fast flying insects have it
what insects have ‘slow’ eyes?
nocturnal, slow flying insects
What pattern do honeybees perceive form on?
the basis of the brokenness of a pattern
- broken or interrupted patterns may produce a flickering visual impression as a bee passes during flight
What are contained within ocelli?
500-1000 retinular cells sharing a common lens; each 2-5 adjacent cells forms a rhabdom
What transfers code to the brain in ocelli?
secondary neurons
What wavelengths do ocelli respond to?
same wavelengths as compound eyes but much more sensitive to low light levels
What depend on ocelli?
- diurnal rhythms
- may influence the insects response to information received from the compound eyes as painting over them inhibits light directed behavior even when compound eyes are functional
What do stemma resemble?
single ommatidium
What do stemma consist of?
a lens, cornea and rhabdom
What are stemma capable of?
most minimal image perception (large areas of dark vs. light)
