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.

The Plague of Global Plastics

It seems an unstoppable ecological nightmare

We hear and read almost daily about the likely consequences of global warming and that various measures are being taken to stop it or at least to slow it down. However, it could be that global warming does not only produce ‘losers’, but that some regions could actually benefit from it,  be it by much reduced heating costs during winter, longer growth periods in the summer, prolonged open waterways and ports, fewer ice-related accidents, etc. But the ever-increasing amounts of plastics and their residues in our environment affect us all, from the tiniest organisms to the biggest in the sea, on land and in the soil.  And nothing appears to stop the ever-rising accumulation of plastics like polyvinylchlorides (PVC), polyethylenes (PE),  polypropylenes (PP), teflon, nylon and their additives.

Not only is it disturbing to see plastic bags caught in the branches of a tree or discarded plastic boxes along the wayside or plastic debris on our beaches, the problem is much more serious as what we see is just the “tip of the iceberg”. What clutters up the landscape are macro-plastics, the ones we notice, but so-called micro and nanoplastics as well as the chemical residues (that remain even after physical abrasion has led to the fragmentation of a plastic item) will still be around.  It is worrying enough that most plastics, having half-lives of hundreds of years, will not become degraded in one lifetime; but even more alarming should be that compounds, often added as hardeners like bisphenol A (BPA) and/or flame retardants like polybromides (BRFs), could make it up aquatic as well as terrestrial food pyramids and affect the health of individuals at the end of the food chain. The debate as to what are safe limits of the questionable compounds goes on, but there is evidence from a variety of species that plastic additives which get into the food chain can cause endocrine (= hormonal) and epigenetic disruptions, lead to fertility problems and possibly tumours, increase susceptibility to disease, affect longevity and adaptations to stressors. These invisible substances accumulate in mussels, fish, even whales, and in the form of fish meal, fed to poultry and pigs, may then be passed on to  humans.

However, not just thinking of humans, it has become abundantly clear that macro as well as micro and nanoplastics affect numerous species adversely and that the lower the molecular weight of a plastic, the greater its chance is that the polymer can be intracellularly digested by bacteria.  Floating plastic debris in the sea can be mistaken as jellyfish by especially turtles; it can accidentally snare individuals and impair their growth or kill them or it may unwittingly be ingested by fish and fill their stomach or clog up the digestive tract. Shocking photos have been published on the consequences of plastic debris-animal interactions in the sea as well as on land.  The pollution with plastics and their residues has no bounds, affects the world from pole to pole and has not even spared the deep sea or the remotest places on Earth. There are so called gyres in all major oceans that concentrate the floating plastics (there are more that would have already sunk) and the most infamous is the Great Pacific Garbage Patch twice the size of France and estimated to contain at least 100 million tonnes of plastics.

The tragedy is that we are all too hooked on plastics: they don’t break easily, are lighter than glass, wood and metal, are relatively easy to make, shape, mold, and transport and their production will not decrease. Will biodegradable plastics with less controversial additives ever be available or is that wishful thinking? The irony is that after the Englishman Alexander Parkes had invented the first plastic in 1862, the American John Hyatt patented ‘celluloid’ in 1869 to produce billard balls (thereby hoping to save elephants whose ivory used to be the source for billard balls until then). And what are the plastics saving now? Mainly money! And if it’s not already true, we are getting closer and closer to the pop artist Andy Warhol’s famous wish “I love plastic. I want to be plastic”.

© 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.