Slip Sliding Away

But not if you can avoid it

Most scientific research starts with an observation. And when in Antarctica I watched penguins, how they walk, hop, swim and (very occasionally) slip on the ice. I, on the other hand, would have slipped and fallen all the time, hadn’t I worn special footwear with spikes, termed in-step crampons, fastened under the soles of my boots. Now penguins don’t wear boots with crampons but hardly ever slip when marching along on slippery, icy surfaces. How come? I examined the underside of the feet of a dead penguin by microscopy and found numerous tiny bumps and protuberances that could have been there to improve the feet’s steadfastness on the ice. But how about the African or Galapagos species of penguins? They never see any ice and their only risk of slipping comes from climbing onto wet and slippery rocks along the seashore. Their feet still await to be examined. And so do the feet of the Adelie penguins when in the ocean for many months and not breeding on land.

Sliding along on a slippery surface can, on rare occasions, be advantageous: for example, when penguins go into a mode of locomotion called ‘tobogganing’ or when pond skaters slide along the water surface. But more usually slipping is to be avoided as it entails the risk of getting injured. The problem is to find a compromise between reducing speed and increasing grip, i.e. stability  – and that often depends not only on structures involved in permitting or avoiding sliding, but environmental conditions. L. Heepe et al. found in 2016 that the feet of ladybird beetles (Coccinelllidae) achieve the highest attachment forces (i.e. the least amount of slipping) at a humidity of 60%, but lower and higher humidities would lead to a decrease in attachment ability. Not getting stuck too tightly is, of course, another problem that animals that possess adaptations to prevent slipping, need to control.

A pioneer in connection with adhesiveness and slipping on wet surfaces was Prof Jon Barnes, whom I had invited to give a lecture on his research with frogs when I had been in charge of our department’s weekly seminar series. Jon gave an incredibly memorable and exciting lecture, in which he described his and his colleagues’ observations on frogs of different sizes and their adhesive abilities on wet and dry surfaces. Another pioneer in the field of adhesive mechanisms in animals is Prof S. Gorb, who explains that the wet adhesive system depends mainly on capillarity (think of a wet paper plastered onto the window glass) while dry adhesiveness involves molecular interactions, known as van der Waals forces. The latter have been identified in gekkos, who are known to scuttle along upside down on the ceilings of houses. However, in frogs sitting on slanted and wet surfaces of leaves, a mechanism akin to peeling sticky tape off a surface comes into play. Their attachment forces are significantly enhanced by close contacts and boundary friction between the frog’s toe pad epidermis and the substrate. These toe pads are wet with watery mucus, assisting attachment due to the fluid-filled close coupling between the pad and the substrate, but wet adhesion alone would not hold a frog on a slippery and vertical surface. Incidentally, frogs always choose to rest on a slippery surface head-up; if the surface is turned, the frog readjusts its position as the turn reaches approximately 55 degrees. The underside of the frog’s toes features “peg-studded hexagonal cells separated by deep channels into which mucus glands open” and some vague similarities to the structures I found on the underside of the penguin’s feet, seemingly to prevent slipping, are apparent.

Frogs, of course, unlike penguins, have four legs on the ground and spiders have even eight. I had always wanted to study if each leg contributes equally to the total adhesiveness of an animal, but this question was answered by S. Gorb’s group in an interesting publication of 2014, which showed that the whole was more than the sum of all its parts. What apparently still has not been repeated was a little study of mine jn 1993 in which I reported that flies adhere to surfaces significantly longer when it is dark than under illumination. That makes sense, of course. But it needs a scientific explanation and I have so far not been able to follow up that result (but I do hope someone will).

© Dr V.B. Meyer-Rochow and, 2021. Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and with appropriate and specific direction to the original content.

Most Insects have Five Eyes

Really, is that so?

Most people know that insects have compound eyes with often hundreds and even thousands of hexagonal facets. But what most people do not know is that many insects in addition to the two large compound eyes possess also an additional three much smaller single lens eyes (known as ocelli) on their forehead. These extra eyes are usually arranged in a triangular position on the insect’s forehead with the two lateral ones placed a little higher alongside the single, median ocellus.

Despite the huge amount of research that over the years has been conducted in connection with the compound eyes’  structure and function and has led to a considerable amount of understanding how the visual signals are received, analysed, transmitted to the insect’s brain and the elicit behavioural responses, the role (or roles) of the ocelli are still not fully clear. Although there is some evidence that they help a flying insect to maintain a balanced course and that damage to the ocelli, at least in some insects, interferes with their orientation mechanism, it is puzzling why members of some insect orders possess ocelli and others do not. If the ocelli, as another functional suggestion has it, work in concert with the compound eyes and analogously to a photometer prime the compound eyes by setting up their overall sensitivity to the ambient light level, then one could have perhaps expected to find ocelli in all insect species, but that is clearly not the case. Ocelli are almost always present in those insect orders with aquatic larvae like dragonflies, mayflies, stoneflies and some caddisflies. But they are absent in virtually all 400,000 or so species of beetles and in butterflies, bugs (Hemiptera), lacewings,  scorpionflies and true flies (Diptera) some have them and some do not. Ants, wasps, bees, etc, almost always have them, but so do the unrelated winged termite castes.

Structurally these little eyes, where they are present, are rather similar. There is, as could be expected, some variation with regard to the diameters of the ocelli, the curvatures of their corneal lenses, their precise location on the forehead and to what extent hairs on the insect head surrounding them affect their visual field. However, it has convincingly been shown that these eyes are incapable of forming an image on their respective retinas, because the images are always underfocused and would, at best, produce a very blurry representation of the real world. The retinas of the various ocelli in the different insect orders all contain typical insect photoreceptive cells with ultrastructurally similar membrane tubes that house the photopigments in them. The orientation and arrangement of the photoreceptive membranes, however, can vary between species, suggesting that some ocelli may be capable of perceiving linearly polarized light that could help them navigating. Yet again, this would not explain why not all flying insects share this ability and, in fact, why flying beetles do not even possess ocelli at all.

Can they perceive colours? I was perhaps one of the first in the world to test the spectral sensitivity of a dorsal ocellus of a bumble bee electrophysiologically and determined that it had two sensitivity peaks: one in the ultraviolet to light of around 350 nm wavelength and one in the green range of the spectrum around 520 nm wavelength. In terms of their visual field, I found that it covered an approximately 60 degree wide diameter. What I did not examine was the overlap between the visual field of the three ocelli with each other and the compound eyes. This was recently investigated by a group of researchers headed by Emily Baird in Sweden, who were interested why only in bumble bees but not in honey bees the three ocelli are placed in a horizontal row rather than being triangularly positioned and bumble bee males and females had similar eyes while in honey bees they were dissimilar.  The researchers found that the occluding hairs around the ocelli played an important role to reduce visual overlaps and that male bumble bees appeared to be foraging more like female bumble bees , while honey bee drones and female honey bees differed much more from each other. The data presented by the Swedish group allowed me to calculate an F-number that shows that the bumble bee’s dorsal ocelli could function under much dimmer light than humans could see in. And yet, as to the precise function of the little insect eyes, well, we still don’t know.

© Dr V.B. Meyer-Rochow and, 2021. Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and with appropriate and specific direction to the original content.

Silent Helpers to Treat Parkinson & Alzheimer Diseases:  Fish and Fruit Fly

One fascinating (and very useful) aspect of the nervous systems and its units, the neuronal cells (known as neurons), is that structurally and functionally there is virtually no significant difference between those operating in worms, insects, fish or humans. In fact throughout the animal kingdom the nervous system basically functions on identical principles. And that explains why research on diseases like Parkinson’s and Alzheimer’s can resort to using fish and fruit flies as models. As the global human population ages, we can expect to have more and more cases of people suffering from these diseases, which are classified as “neuro-degenerative ”. This means that they lead to a gradual loss of neuronal function, to the degeneration and ultimate death of nerve cells in the brain.

In Parkinson’s Disease the most visible symptom is the tremor and that was also the diagnostic feature when the English surgeon James Parkinson in 1817 described the disease as “shaking palsy”. It was the renamed “Parkinson’s Disease” by the Scottish physician William Sanders in 1865. It is known that the movement disturbances are caused by the loss of the neurotransmitter “dopamine”, a substance vital for signal transfer from one neuron to another via contacts between nerve cells known as “synapses”. For Alzheimer’s Disease, named after the German psychiatrist Alois Alzheimer, who published his observations in 1906, neuronal dysfunction is also characteristic, but here a build-up of toxic protein deposits known as amyloids cause the neurons to malfunction and slowly die, which then leads to cognitive problems like loss of memory, delusions, hallucinations, etc.

What the diseases have in common apart from being neurodegenerative is that there is certainly a genetic component, but that environmental triggers are also important. Especially in connection with Parkinson’s Disease a link to metabolic disorders like diabetes mellitus, cardiovascular problems, high blood pressure, a fat-rich diet, obesity, etc. have been established and a sufficient amount of insulin available to the brain to maintain essential glucose levels for meeting the brain’s energy requirements, has been identified to be critically important. Insulin resistance in the brain affects turnover processes of dopamine in the synapses and causes the characteristic movement disorders in sufferers of Parkinson’s Disease. But how can fruit flies help? Since fruit flies can be bred in large numbers, have short life spans, possess neurons that function like those in humans and exhibit motor behaviours like crawling, climbing, grooming, flying, etc. they can serve as models for the disease and its underlying genetics. One distinguishes between the familial Parkinson’s Disease and an expression of the disease that’s caused by environmental stimuli like toxic compounds. To identify  the underlying susceptible genes is one goal in which fruit flies help. After all they and humans share 61% of their genes including those that control the molecular mechanism of neurotransmitters. Paraquat (a pesticide) and rotenone (a poisonous plant substance) have been identified to disrupt the fruit flies’ metabolism in ways that resemble Parkinson’s Disease. There is, thus, hope that the fruit fly results can lead to treatments not just of the symptoms of the disease but the genetic causes as well.

Treating sufferers from Alzheimer’s Disease may one day benefit from research on the brain of the zebra fish, a small tropical aquarium fish that just like the fruit fly has become a “work horse” for genetic research of all sorts. In the past, the main approach to treat Alzheimer’s Disease was to try to prevent or slow down the degeneration of the affected neurons. But the research on the zebra fish has shown that there exist in this species’ brain some cells that can be induced to replace lost neurons. Hope is that such neurons in the human brain can be identified and induced to restore or replace neurons lost to Alzheimer’s. Progress often comes from unconventional approaches and as David Horrobin wrote “If a hypothesis which most people think is probably true does turn out to be true (or rather is not falsified by crucial and valid experimental tests) then little progress has been made. If a hypothesis which most think is improbable turns out to be true, then a scientific revolution occurs and progress is dramatic”. I love this comment on research!

© Dr V.B. Meyer-Rochow and, 2021. Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and with appropriate and specific direction to the original content.