As Tears Go By

A look at animal and human tears

Children cry easily and even after a minor bump or hurt will shed tears. Adults may feel pain, cry and scream when hurt, but unlike children will not shed tears. An adult’s tears are associated with emotions or may be caused by some disorder, an eye infection or irritation, but not pain. And animals? They, too, have lacrimal (= tear) glands and can have watery eyes as the result of an infection or as part of a physiological control to remove excess salt from the body, but apparently not in connection with an injury. Sea turtles and a few other reptiles remove excess salt not only via their kidneys but with the help of their orbital eye glands as “white tears of saline” that drip out of their eyes.

Although human tears are not white, but watery, transparent and very slightly sticky because of mucins in them, they too contain salt  – as do, in fact, the tears of all land vertebrates that may not ‘shed tears’ but use the lacrimal fluid to lubricate their eyes and keep the cornea moist. Chemically tears are mostly water (ca. 98%); and apart from salts the lacrimal fluid contains a cocktail of amino acids and proteins, antibacterial enzymes and minute quantities of stress hormones. A tear’s chemical composition depends on the cause of its shedding and varies on whether the tear’s function is to wash out dust from the eye, to fight off irritants such as fumes (smoke or onions come to mind), to lubricate the eye’s surface, and as a response to physical pain and emotional upheaval. The autonomic nervous system through its parasympathetic branch governs the production and release of tears from the lacrimal glands, which are located in the upper region of the eye’s orbit. The tears are stored in the lacrimal sac near the nasal corner of the eye; from there the fluid via lacrimal canaliculi is released into the eye upon a signal from the parasympathetic nerve’s acetylcholine transmitter. In healthy individuals, there is a constant release of minute quantities that are distributed with each eye blink across the cornea, but of greater amounts if required. Excessive fluid is drained through the nasolacrimal duct and causes the ‘sniffle’ during weeping.

Basal tears are continually-produced via the 5th cranial nerve’s innervation to keep the eye’s cornea moist and to prevent bacterial infections. In humans, about 0.75-1.1 ml of the liquid is produced each day. Reflex tears are produced when the eye is irritated, and through their copious amount and high water content function to remove the irritation from the eye. Psychic, also known as ‘emotional’  tears, occur in response to strong feelings, which could be sadness, but also joy, stress and  physical pain. Because these tears contain such natural painkillers like leucine-enkephalin and prolactin, it may explain the role of the parasympathetic nervous system and that “a good cry can feel relieving”.  But it does not explain why men shed tears less often than women, a fact that is often explained with the traditional roles men and women are expected to play in life (the advice “boys don’t cry” is a case in point).

The fourth reason for tears is related to diseases and the release of tears accompanying other activities (e.g. yawning). Although elephants have been described as shedding emotional tears, crocodile tears are not an expression of emotional distress, but the result of compression of a nerve that controls the jaw muscles during feeding. In humans suffering from Bogorad syndrome “crocodile tears” also accompany swallowing. Reference to tears can generate resolve (Churchill’s famous “Blood, Sweat and Tears” comes to mind); tears evoke empathy: children know that (and actors train to shed tears at will) and tears appear in poems and songs (the record “Tears on my Pillow” is in my collection) and who wouldn’t remember Marianne Faithful’s beautiful song “As Tears Go By” or Eric Clapton’s touching “Tears in Heaven” (which I heard it for the first time in Chile in 1993). I actually heard of people who shed tears when listening to it.

© Dr V.B. Meyer-Rochow and, 2022. 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.

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Dawn of vision

Dawn of vision – And why blue light might be so special

Virtually all mammalian species exhibit some kind of circadian rhythm that affects their activities. These rhythms parallel the cycle of day and night and are, thus, in most cases linked to the perception of light. However, major surprises were in store for scientists, when they investigated whether mutant mice, lacking the well known image-forming rod and cone cells in their retinas, still possessed a circadian rhythm. —>—>