(1) to make up in part for the lack of sensory coverage in lecture
(2) to give you a hands-on appreciation of optics and vision
(3) using Drosophila, give you invertebrate animal lab demonstrations
(4) show you some properties of insect vision (notably ultraviolet sensitivity)
(5) demonstrate microscope imaging relevant to rhodopsin's spectral properties
(6) to use a clever iWorx application to measure visual and auditory reaction times


Drosophila phototaxis

A few years ago

Let me attempt to summarize the issue, related to the portion of my research addressed to this topic:

Drosophila have 2 compound eyes that mediate positive attraction to light. Each of about 800 ommatidia (facets) have 8 receptor cells of 3 anatomical types, R1-6, R7 and R8. R7 is an ultraviolet (UV) light receptor that dominates phototaxis at photopic light levels (levels above very dim) even in the rdgB mutant which has selective degeneration of the more sensitive R1-6 cells. It seemed interesting that UV phototaxis was easily demonstrated in rdgB despite the flies' disability. Another mutant, ora= outer rhabdomeres absent, was later shown to be a nonsense allele of the gene for R1-6 rhodopsin. Theoretically, ora should have the same challenge as rdgB, lack of R1-6 function while R7/8 work fine. However, my colleagues and I, testing ora, found several times that it did not have any attraction to light. These days, a mutant called ninaEoI17, a big deletion of the R1-6 rhodopsin gene, is used in many studies.

Does ninaEoI17 also lack phototaxis? The 2004 physiology lab class collected pilot data suggesting that they do not. Then, an undergraduate student, Michael Haskins, working in my lab, followed by obtaining the data to prove this finding. His work earned him second place in biology's 2006 undergraduate research award, a monetary award, and recognition in the precommencement program and the publication of a research note (Drosophila Information Service volume 89 pp 17-19, 2006, on line). This year, I will show you this apparatus. (optics, a close up of the arena)


A Mercury arc lamp feeds into a monochromator to make monochromatic lights of the spectrum. You will also see a spectrum in next week's slide show. We have a monochromator which generates a spectrum using a grating (the obvious alternative being a prism). On the front of the monochromator is a slit that picks off a small section of the spectrum. Using another monochromator, where I could look inside, I obtained this picture to show how the slit selects a portion of the spectrum. The slit picks off 6.4 nm/mm, i.e. if it is 1 mm open, it lets through 6.4 nm of the spectrum.

Projecting the beam onto a screen and cranking the monochromator, we note:

(A) it is brighter at some wavelengths than others*

I scanned this graph of the spectral output of various light sources to demonstrate that there are "lines" in the mercury arc (HBO) spectrum at 580, 550, 438, 405 and 365 nm

(B) a setting of about 579 nm looks uniquely yellow

(Just 5 nm higher looks orangish, while just 5 nm lower looks greenish)

(C) at 365 nm, an index card looks blue, but that is fluorescence

(D) While calibrating the light intensity for the phototaxis research mentioned above, we noted that the light flickered. Many lights probably flicker, but not incandescent lights since the filament stays white hot during alternating current. With different flicker-fusion frequencies, our rods and cones can detect flicker, but not for frequencies as high as alternating current. We can trick our eyse into seeing this flicker by "converting time into space." Jiggling a lens in front of the eye spreads this flicker across the retina, and then the light looks like a dotted line.

*A second demonstration (of A)

With a continuous interference filter, note that (1) outdoors, and (2) the directional incandescent lamps give an even spectrum while the fluorescent lights give several bright lines.

Demonstration of rhodopsin - metarhodopsin conversions and green fluorescent protein (GFP)

Here is a new (December, 2013) text of this demo plus the confocal



Frequency discrimination and beats

A three year sample of quiz questions

What region of the spectrum are insects famous for being able to see but people cannot?


What fraction of the light should get through a 1.2 log unit neutral density filter?

one half to the 4th power = 1/16

Suppose a certain amount of light goes through a test tube with water while 1/10 as much light goes through it when a pigment is in the water. What is the absorbance difference?

the log to the base 10 of 10 = 1

You can clearly see a blue appearance by shining 365 nm light onto an index card because of what process?


Why is the wavelength of emitted light longer than that for the excitation light in fluorescence?

energy lost before photon emitted

I said that a 0.3 neutral density filter attenuates the light to half. To see if you understand this, you get your calculator and put 10 to the 0.3 power and the answer is 2. Resolve.

attenuation: -0.3 (negative)

A light is flickering above the flicker-fusion-frequency for your rods and cones. What trick can you use to verify that it is flickering?

look at it through a lens or binocular, jiggle that, change time to space on retina

What is the phenomenon called where Drosophila are attracted to the light?


Blue (480 nm) light was used to do two different things in the
two respective microscope demonstrations. Tell me one of them.

excite fluorescence in GFP (green fluorescent protein
convert visual pigment (rhodopsin to metarhodopsin

This page was last updated 12/4/13

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