biology zoology blog benno meyer rochow heart flies

Hearts of Flies and Humans

Not so terribly different after all

One of the standard laboratory exercises for students in physiology I had to supervise in New Zealand, but never enjoyed much ‘cause I like frogs, involved live hearts of frogs. Sure, it’s interesting for students to see how the isolated heart keeps beating all by itself and to prove that the heart muscles of the atria and ventricles operate independently and have their own intrinsic rhythms. The students had to apply the so-called Stannius ligatures between sinus venosus and the atria and the atria and the ventricles. These ligatures disrupt the coordinated contractions from sinus venosus via atria to the ventricles and slow down the heart’s beat of the latter, but do not eliminate it. The experiment served to demonstrate the ‘myogenic origin’ of the vertebrate and therefore of course also the human heartbeat: to switch on the heartbeat, a nerve input was not required (although the vagus nerve can slow it down but not stop it while sympathetic nerves contacting the sino-atrial node can increase the heartbeat by adrenaline).

In the lectures I would then say that insects don’t operate with myogenic, i.e. self-beating hearts but have neurogenic hearts, in other words hearts that according to text book wisdom, beat only when a nerve impulse causes them to contract. It’s all wrong according to careful studies by the jovial and imposing, famous Czech academician Karel Sláma. Insects possess an open blood system without arteries, veins and capillaries and their blood does not carry oxygen around to the various tissue, because insects “breathe” with air-filled tracheae and tracheoles. But insects do have a tubular heart on the dorsal side of their body. It beats and propels the colourless blood mostly forward towards the head via systolic contractions of 4-7 Hz in the fruitfly, but up to 10 Hz in the hoverfly Episyrphus balteatus, in which systolic contractions reach propagations of 32.2 mm/s. Occasional switchovers from a forward-directed heartbeat to a retrograde beat, in which the heart reverses the direction that it propels the blood is common in insects. Young fruitfly larvae, however, only exhibit a unidirectional forward systolic contraction. But what is the evidence that Dr Sláma advances to show that the hearts of insects and those of humans aren’t all that different?

First of all, the primordial formation of insect and human heart is orchestrated by similar sets of genes. There is also an electrophysiological analogy with regard to the onset of depolarization of the systolic contraction at the apex of the heart. There is the conical compact muscular chamber of the insect heart at the abdominal base that is almost like the ventricle in the human heart. Most convincing is the demonstration of the purely myogenic nature of the insect heart, when the neuromuscular system of waxmoth larvae was paralysed by a venom obtained from a parasitic wasp and injected into the larvae. The larvae then remained perfectly motionless, unable to move any body muscle for 3-4 weeks. Despite their immobility, their heart continued to beat like clockwork and the heart muscle contractions were fully preserved without any nerve input. The contractions in the insect heart muscle were determined by a terminal pacemaker nodus in insects, analogous to the atrioventricular nodi in humans. Is that the end of the assumed categoric dichotomy between vertebrate myogenic and insect neurogenic hearts? Maybe not quite, as there are some insect species in which heartbeats are under considerable neural control. But the similarity goes even further, for when Dr Sláma tested the actions of drugs like digitoxin and nitrates on the insect hearts he examined, he found responses that resembled those that could also be observed in human hearts. Might the results serve one day as a convenient and inexpensive way, avoiding the use of dogs and other large animals, for testing cardiologically active chemicals? Dr Slama (and I, too) hope so.

© 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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Chicken Eggs

Is there anything you did not yet know about them?

We used to keep chicken in New Zealand and it was my job every morning to collect their eggs and turn them into fried or poached eggs, boil them hard or soft, or serve them as scrambled eggs with some chives added to them. At that time, I frequently came across some abnormalities which excited my children (but not my Indian Brahmin wife, who never consumed eggs, fish or meat). Most interesting to them were the occasional eggs with two egg yellows, but even though all this happened many years ago, I remember all of the exceptional eggs I came across at that time. This morning, however, I had an egg with an abnormality I’d never seen before. The egg (like all fresh eggs do) sank nicely to the bottom of the pot, was boiled for 5 min, held briefly under cold water and then carefully de-shelled. You’d normally find an “air space” on the egg’s blunt end, but in this egg the air space was under the egg’s “equator”!

It was this incident this morning which made me look up some of my old notes on egg abnormalities that I (and others like a 19th century character with the beautiful name Wilhelm v. Nathusius-Königsborn) had encountered when studying eggs and their shells. First of all there is the egg’s shape, which can vary from almost spherical to torpedo-shape, but usually is -well, ‘egg-shaped’. I loved the occasional egg with a wrinkled appearance as if it had gone through the washing machine, but my children felt it was too odd and avoided it. I, on the other hand, felt these eggs were particularly tasty. Once I had a longish egg with no bilateral symmetry that looked as if bent. And then there is, of course, the occasional egg with little sand-like protrusions, i.e. calcium carbonate pimples on the shell’s surface. After the war (WWII that is) and not from our chickens, I had once received an egg with no hard shell at all, a case that suggests the hen had received insufficient (or too much) calcium or was stressed. However, if fresh and there’s nothing wrong with them, such eggs can be eaten just like eggs with white, brown or even greenish shell colours. A lot of such shell, shape and colour abnormalities reflect an unbalanced diet,  stress, hen’s age or even sickness, but some  -especially the shell colours- are genetically fixed.

Cracking an egg open can give you the surprise of not only finding one, but two or even more yellow egg yolks, but finding no yellow at all may surprise you the most!  Such eggs are known as fart or wind eggs and were once thought to be those of roosters. In fact, it’s the very young or very old hens that are likely to produce yolkless eggs. On one occasion I found a complete little egg with its own shell inside another and, of course, much bigger egg with its own shell. Apparently, this can be caused by an egg reversing its course in the egg tube or oviduct and getting packaged into a newer egg. However, it’s not a big problem: when fresh, both are edible. Also edible, when fresh, are unsightly eggs that exhibit blood patches near or in the yolk. The blood stems from vessels that carry yolk-building material to a hen’s ovary.

There are in rare cases chicken egg abnormalities that I myself during the time we kept chickens had thankfully never been confronted with. A nematode worm known as Ascaridia galli can occur in eggs laid by hens that suffer from a high parasite load. The twisted whitish bands in the egg white that are attached to the yolk and hold it in place are a normal component of eggs and must not be confused with such worms. Certainly more worrying are microbes like Mycoplasma synoviae (which alter the shell surface, cause thinning and increased shell translucency) and Salmonella enteritidis. The latter can enter damaged eggs or may even occur in clean, intact shelled eggs as a result of infections of the reproductive tissue of laying hens. However, it doesn’t show up as an egg abnormality and you won’t notice it until you start vomiting, develop a fever, stomachache, and diarrhoea. But then it’s too late and you had better decide – if you feel like eating an egg again-  to purchase your next carton of eggs from a different supplier who only sells fresh eggs kept not above 20℃ (Salmonella loves temperatures around 30℃).

© 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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Flying in Winter

A problem for insects (but also not exactly fun for pilots)

Insect lovers living in temperate climes have a hard time in winter. The hums and buzzes of flying insects can’t be heard and to even spot an insect in winter is a lucky break. When I was a child my grandfather encouraged me to look for insects on our winter Sunday walks by promising me a candy for every insect I saw. Nowadays that the winters have become a little warmer than 70 years ago, he probably would have needed quite a handful of candies. However, to see insects flying in the cold winter air is still not easy, because flying in winter is a problem for insects as well as airplane pilots, but for different reasons.

On the web I came across the statement “at low temperatures insects are not able to fly”, attributed to Dr Soszynska-Maj. But this is simply not true. Let’s start with the champion winter fliers: winter moths of the genus Operophtera. I had given a PhD-project once to a student from Hong Kong to examine the eyes and retinal organizations of males and females of the winter moth Operophtera brumata. To see someone in mid-winter with a butterfly net must have amused people, but he managed to catch many individuals  –  all males (for the females are chubby and wingless, waiting on the stems of trees for a male to find them). The males have rather large wings and very slim and very light bodies. Since they do not feed as adults, they do not carry extra weight in their degenerated digestive tract. The amazing fact about these male moths is that they do not shiver to create body heat (like bees or other flying insects do). It’s been reported that their flight muscles work at temperatures close to 0⁰C and they have been seen to fly even at -3⁰C, although with wingbeat frequencies as low as 2 to 4 beats/sec to save energy.

Another group of moths flying in winter are some Cuculiinae, but to warm up their flight muscles they begin to shiver at temperatures as low as -2⁰C. In contrast to the winter moths that my Hong Kong student S.T.F. Lau had worked on, the winter Cuculiinae that the American entomologist Prof. Bernd Heinrich had studied, do replenish their energy reserves by licking up sap from injured trees. Moreover, they possess quite an elaborate circulatory system that ensures that heat from the 35⁰C warm thorax with its flight muscles is not lost to the abdomen with its temperature just above 0⁰C. Countercurrent heat exchangers in the abdomen and the thorax prevent undue heat losses. Unlike Operophtera brumata with its wingless females, the Cuculiinae moths do have winged males and also winged females.

Other species of flying insects at the beginning of or during winter are various species of some rather small crane flies, in particular those belonging to the family Trichoceriade. In these insects the long-legged males sometimes aggregate in swarms and in Norway, according to Sigmund Hågvar and Ewa Krzeminska, for example, they are present throughout the entire winter months with the highest numbers caught in December on windless days and at a temperature of 0⁰C. Loose snow that covers the ground helps these insects to shelter and to survive particularly cold phases. Insects commonly referred to as snow flies (like Chionea spp.) are also crane flies, but they are wingless and clamber around piles of snow. Cold-hardy flying species of insects, however, can be found amongst the stoneflies, of which I could always observe some as early as March and April in northern Finland, usually with ice and snow still around. Their black body coloration contrasted nicely with the whiteness of their environment, but although they were winged, they were usually not inclined to fly away and were just sitting on the surface of the ice that was covering the stream they had emerged from. Frozen individuals that had not left the ice before nightfall could be seen stuck to the ice the next morning. It seemed their blood, just like that of the aforementioned Cuculiinae moths did not contain any anti-freeze chemicals. You may feel chilly searching for insects in winter, but let me tell you, to see a live insect in the cold will certainly warm your heart  –  if you are an entomologist.

© 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.