In arthropods, the size, shape, color, ommatidium number and surface texture of the compound eye influence many features of the visual field including its dimensions, acuity and sensitivity (Rutowski 2000).
dorsalis 3751.80 776.25 1268.92 [+ or -] 37.84 a [+ or -] 12.55 b [+ or -] 11.20 a F 0.958 4.246 0.659 P 0.390 0.019 0.521 Species Individual square Number of ommatidium area ommatrichia ([micro][m.sup.2]) B.
This can be demonstrated by locating the unique "index" ommatidium
, which is the ommatidium
with its optic axis horizontal and normal to the body axis of the crab.
Figure 2 is a diagram of a typical ommatidium from the hemispherical regions of the eye outside the midband.
These types vary primarily in the construction of the rhabdom, so only this part of each ommatidium is diagrammed in Figure 3.
From the Hartline-Ratliff formulations of lateral inhibitory interactions in the retina (7), the firing rate of the pth ommatidium
([R.sub.p]) equals its pure excitatory response ([E.sub.P)] minus the sum of inhibitory influences from its neighbors; the inhibition delivered from the jth ommatidium
equals the strength of its inhibitory coupling to the pth ommatidium
([Ksub.pj]) multiplied by the response of the jth ommatidium
, These relationships can be expressed as:
We recorded the response of a single, optically-isolated ommatidium
to a steady 30-s light stimulus and every two seconds applied a brief (100 ms) train of four current pulses to the optic nerve trunk.
A second eccentric cell located in the same ommatidium
generated much smaller action potentials ([approximately]2.5 mV) with the same firing rate (see legend).
The distance between the proximal end of the rhabdom and the basement membrane of an ommatidium
is relatively short in Petrolisthes and other shallow-water decapods.
The former study, however, illuminated the entire retina, whereas the latter study limited the stimulus to a single ommatidium
. Thus, the difference between the TTFs in Figure 1A (solid vs.
The average area occupied by an individual ommatidium
was then measured and the number of ommatidia calculated from the total retinal area.
Illumination of a single ommatidium
in the retina generally evokes slow depolarizing potentials (0-5 mV) in association with a single train of action potentials (20-60 mV) in the compartment, whereas illumination of the entire retina elicits slow hyperpolarizing potentials (5-15 mV) and a spike train of reduced rate.