Insect photoreception

Question 1

Compound eyes and why some insects have reduced eyes

Answer 1

• consist of groups of ommatidia
• parasitic insects have reduced eyes
• ants reduced eyes as always underground
• have fovea where eyesight best - often more than 1

Question 2

Dragonfly compound eyes

Answer 2

•10,000 ommatidia per eye
•variation in facet size
•larger facets at top and smaller at bottom
•larger more sensitive to movement, smaller better visual acuity
•evolved for how they hunt - come underneath prey can see if its moving like their prey would
-fly above it and catch in basket of their legs
-legs covered in spines - sieving their prey out of the air

Question 3

Ommatidium standard structure

Answer 3

Eucone eye - ‘ice cream cone’
•cuticle on outer surface = lens is sclerotised
•crystalline cone - 2 cells, normal cells have lost their nuclei and most other material
-cytoplasm modified to be solid and crystalline, acts like lens
•outside this are primary pigment cells
•light collecting unit beneath - connecting nerve to brain, consists of stacked microtubules like jenga
•surrounded by retinula cells, then outer again secondary pigment cells

Question 4

Modifications to ommatidium

Answer 4

1. Eucone eye = normal
2. Acone eye = normal cells instead of crystalline cone (apterygotes)
3. Exocone eye = crystalline C replaced by outgrowth of transparent cuticular material from lens - huge lens (fireflies adapted to low light)

Question 5

Modifications to how isolated ommatidia are

Answer 5

1. Apposition eye - eucones are optically isolated

2. Superstition eye - can change in different light sensitivities

Question 6

Apposition eyes

Answer 6

• field of view different to that of neighbour = panoramic view
• actual image formed by series of opposed points of light and different intensities
• light sensitivity limited by small diameter for facet lens

Question 7

Superstition eyes

Answer 7

•different structures for light and dark
•in dark, ommatidia not optically isolated from one another by pigment cells - separated by wide clear zone from cuticular facet lens
-many lenses focus light onto single rhabdome
•can cope with varying light intensities
•secondary pigment cell is mobile - if light will make ommatidia isolated (like apposition)

Question 8

Functions and capabilities of insect eye

Answer 8

• eye goes straight into optic lobe of brain
• capable of high resolution or good movement perception
• resolve images to 1 degree of angular separation (humans 0.05 - better)
• pigment in rhabdome called retinin
• 2 sensitivity peaks in blue and UV, can’t see red or yellow very well

Question 9

Ocelli

Answer 9

Light sensitive units…
•3 forming an inverted triangle on top of head
•large cuticular lens on top
•crystalline cells underneath have 2+ rhabdome-like structures underneath
-many from single lens
•not used for visual acuity but detecting light sensitivity in day - day lengths, to control circadian rhythms, and adjusting pigment cell movement in superposition
•detect ambient light levels

Question 10

Stemmata

Answer 10

Restricted to larval lepidopterans - holomebolous insects
•large head capsule on side are clusters of cells, where compound eyes would normally be
•not ommatidia but large crystalline lens structure with light insulation and rhabdome down middle
•detects moving objects in peripheral vision -e.g. predator