How Small Can Small Be ?

A study of the eyes of miniature insects

In the year 2010, I published with my PhD-students S. Fischer and C. Müller in the journal “Visual Neuroscience” an article with the provocative title “How small can small be”. Although the article dealt with the compound eye of the parasitoid wasp Trichogramma evanescens, an insect of 0.4-mm total body length, the question of how small an animal or a part of it can be before it stops to function is of course an intriguing one. The smallest mammals are the Etruscan shrew and the bumblebee bat Craseonycteris thonglongyai; the smallest fish of just 1 cm total body size is Paedocypris progenetica, and the smallest frog and reptiles are Paedophryne sp. and dwarf geckos of 1 and 2 cm respective sizes.

Back to insect compound eyes. In 1975 I had examined the tiny eye of a beetle known as Corylophodes, whose total body size was below 1 mm. Compound eyes consist of a multitude of facets, each equipped with a small lens and a retinula, which contains a rod-like light-perceiving structure known as the rhabdom. It is obvious that for an eye of a given size sensitivity (related to the amount of light that can enter the facet) and spatial resolution (the amount of detail the eye can resolve) are in conflict with each other. Photography and camera-buffs know the relationship between aperture setting and exposure time: for high resolution you want a small aperture, but need more light (e.g. longer exposure).

If we take a look at the hemispherical eye of an insect and assume a constant facet diameter, increase of the eye would lead to an increase in sensitivity and resolution, because more facets would allow more light to be perceived and the angle between neighbouring facets would decrease, thereby improving resolution). But what about the situation in very small insects? They would have a problem, because a small head offers less space for big compound eyes. So, the facet size should be reduced, but there is a limit and a researcher named Barlow as long ago as 1952 calculated that smaller facets would be increasingly limited by diffraction. He concluded facet sizes below 10 µm would probably not occur. If facet sizes weren’t allowed to become reduced as the eye itself got smaller and saw its curvature increased, then the angles between neighbouring facets would become wider and resolving power would suffer. How tiny insects find a ‘compromise’ to solve this impasse is what interests us.

Another problem is that the rod-like rhabdom mentioned above acts like a light-guide in the bigger insect eyes, but as Alan Snyder 1979 worked out, effectiveness of a light-guide based on total inner reflection drops off as the diameter of the light guide gets smaller. With diameters below 1.5 µm light in the form of modes travels partly on the outside of the rhabdom, making it unavailable for the photo pigment in the rhabdom’s constituent membranes. Doekele Stavenga in the Netherlands showed that the fraction of the light conducted inside the rhabdom decreases with increasing wavelength for all modes. Thus, shorter wavelengths like blue, violet and UV would be the most useful for the tiny eyes of small insects. And yet, surprises still await the researcher: on the island of Hachijojima I caught a fly of 0.65 mm body length and examined its eyes with Dr Yumi Yamahama. Not only did this fly have three compound eyes (two lateral ones with 35 facets each and one dorsal eye with 90 facets on the top of its head), it also had an extremely shallow retinula and surprisingly thick lenses. We published the results in 2019 in the journal Entomologie Heute, but are still at a loss to explain how this miniature insect could be such a swift flyer, repeatedly being attracted to the light of my computer screen at midnight.

Finally, here’s a challenge: there are mini-snails with body lengths of 1mm or even less. I noticed they have black eyespots. How would their eyes be constructed and what could they possibly see with them?

© Dr V.B. Meyer-Rochow and, 2021.
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