The basic structure of the eye

The visual receptors of a fly are much more difficult to understand than a human's. For one, an eye on a fly has many facets and lenses. Each of these facets has to work together to compound the image it sees. How can hundreds of individual eyes work together to produce a single image? It is, in fact, a highly ordered process of organization of neurons that identify specific regions of space in the visual field.

Each facet is connected to a long chamber called an armitidium. In each armitidium, there are eight neurons that gather information from the light received through the facet. These neurons gather information in a interesting way. Instead of getting the light waves that are directly in front of the neuron, it receives information from an angle made by the bottom of the facet, creating a binocular effect. Since these neurons look at different regions, they need to be separated into more specific regions so the information can be translated easier. The eight neurons then travel down the armitidium, until they are separated at the end into specific columns. These columns organize the neurons from different facets into the regions of space that they are receiving information from. That is, each column represents a small area of visual space. The columns keep traveling down and come to a motor response region when some neurons will trigger an immediate bodily response. After going through the motor response region, the columns are then organized into columns again called v-shapes cells. Each v-shape cell receives information from nine columns and travel through the lobula to the brain and other parts of the body.

The male specific neurons

When male and female flies' heads are compared to each other, there is an area of the male fly's head where the setup of the facets are much different than those on the female fly (assigned by the dotted line). The corner of the eye next to the mouth is where the facets for the male specific neurons are housed. When a object moves laterally in line of sight of this area, the male will quickly attempt to intercept the object. So what are the mechanisms behind this reaction?

There are basically two types of the male specific neurons, the MLG (male lobula giant) and the MCol. The MLG neurons are very dense and thick. They generally split when they are close to the outer part of the eye and branch out in many directions after the main column has been split. There have been 9 MLGs that have been isolated. The MCol neurons are slender columns that branch in unique and fairly symmetrical patterns that begin to branch after the MLGs have begun to split. An interesting point about the MCols structure is that the neuron will accumulate information on one side of the eye and carries the signals to the opposite side of the lobula. There have been six of these MCols that have been isolated.

When these neurons are organized in their relative positioning to each other, there is a remarkable similarity between the organization of the neurons and the Land-Collett circuit proposed 25 years ago. The MLG 1, 2, and 4 are positioned near the front of the eye where the velocity receptors are on the Land-Collett circuit would be. MLG 2 and 4 act as the most exterior branches where the information is received and sent through the same eye. Where as the MLG 1 is slightly deeper in the eye and receives the information in one eye and transmits it to the other eye. MLG 3 is located along the lower part of the arc where the proposed positional receptors on the Land-Collett circuit are. The only real difference between the actual placement of the neurons and the Land-Collett diagram is the absence of the MCol neurons. The MCol neurons are positioned deep in the eye near the bridge between the two eyes. They have an interesting structure in which they transmit information through both eyes, inverting their position when going to the other eye. Even though Land and Collett where not exactly correct in their circuit hypothesis, it is extremely amazing that they got so very close by just studying the behaviors of the flies.

The knowledge of the existence of the MLG and the MCol neurons is good to know for trivia, but what do these neurons translate into? Each of the neurons code for specific regions of space. So when an object crosses this region, the neuron is excited and a signal is sent down these neurons to stimulate activity. These regions of space can be plotted on a sphere so that the effect of the male specific neurons can be visualized. Through the visualization of the male specific areas, behavioral speculation of the male can be made.