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Flashcards in Eyes and Vision Deck (44)
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What are 3 different light sensing organs in insects?

1. compound eye
2. ocellus
3. 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?

1. 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?

1. 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