More brain mysteries - miniature brains with great behavioral repertoire

 By Design: Brain Miniaturization in Some Very Small Insects

Eric Cassell

January 14, 2025, 6:14 AM

https://evolutionnews.org/2025/01/by-design-brain-miniaturization-in-some-very-small-insects/https://evolutionnews.org/2025/01/by-design-brain-miniaturization-in-some-very-small-insects/

In the last several years there has been increased interest in and research into so-called “microinsects.” These are insects that are miniaturized compared to the majority of other insect species. They include species of wasps, beetles, and ants. An example is the wasp Megaphragma mymaripenne (pictured above), which is only 200 micrometers long (¹⁄₁₂₅ inch) and is the third smallest known insect. Such microinsects are comparable in size to some single-celled organisms. M. mymaripenne has only 4,600 neurons, several orders of magnitude fewer than in larger insects.¹ This is the smallest number of neurons in all insects. For example, honey bee brains have about one million neurons. General characteristics of miniaturized insect brains are a decrease in neuron size, decrease in neuron spacing, increase in neuron density, and increased density of synapses.²

One of the more interesting aspects of insect miniaturization is the reduction in the size of neurons. The cell bodies of neurons in human brains typically are about 20 micrometers in diameter, while the nucleus is about 5-10 micrometers.³ The smallest diameters of neuron cell bodies previously documented in insects were “2 to 3 micrometers, possibly because neuron cell body diameter is restricted by the size of the nucleus.”⁴ As explained by one group of researchers, one potential explanation for the reduction of neuron size is, “A benefit of smaller neurons over larger ones is that they are energetically less expensive both at rest and whilst signaling and may be packed more densely.”⁵

Another microinsect example is the parasitic wasp Trichogramma evanescens, whose entire brains “are only marginally larger than a single motor neuron in the human brain.”⁶ The size of their brains range between 160-330 micrometers, while the largest motor neurons in human brains are called Betz cells, which can be as large as 50-100 micrometers.⁷

The smallest free-living insect is a beetle called Scydosella musawasens, which illustrates another example of miniaturization. The body size of S. musawasens is only 325 micrometers. Their brains contain approximately 9,500 cells with an average diameter of approximately 1.25 micrometers.⁸

Who Needs a Nucleus?

Another characteristic in some miniaturized species associated with the reduction in the size of neurons is the elimination of the nucleus. As explained in one research paper, “Of 4,600 neurons in the brain of M. mymaripenne, approximately 95% were anucleate neurons, of which the somata (neuron cells) were almost twice as small as those of the nucleated neurons in the adult brain.”⁹

Recent research by a group of Russian scientists has found a second parasitic wasp species (Megaphragma polilovi) whose brain neurons lack a nucleus. The paper’s authors write, “Our finding shows that a similar variant of saving space due to the lysis of the cell bodies and nuclei of neurons in the brain and other parts of the central nervous system evolved at least twice in the course of insect evolution.”¹⁰

Small Animals and Relatively Larger Brains

A common characteristic of small animals, including not just insects, is that they generally have larger brains relative to body size. According to so-called Haller’s Rule, smaller animals have proportionally larger brains than larger-bodied forms.¹¹  A study that was performed on 23 various insect species from 11 families and five orders found that, “The rule of brain size changing allometrically with body size, known as Haller’s rule or brain–body allometry, has been confirmed for many vertebrates, insects, spiders, or other invertebrates; it is fully realized in the smallest insects.”¹² In other words, the brains are not literally larger, but the brain to body size ratio is bigger.

Cerebral index is the term that relates the weight of the brain relative to the entire body. In humans it is 2.5 percent.¹³ It was long believed that cerebral index among animals was the highest in hummingbirds at approximately 8 percent. However, research has determined that it, “Reaches 8.36% in the miniature hymenopteran Trichogramma, and 15 percent in Brachymyrmex, some of the smallest ants.”¹⁴ It is a similar situation with brain volume, where, “Extremely large relative brain volumes are found in miniature insects, the brain of which can occupy more than 10 percent of the body volume, for example, in the first instar nymph of the booklouse Liposcelis bostrychophila in which the volume of the entire central nervous system can exceed 16 percent of the body volume.”¹⁵

Relatively larger brains do come at a cost as, “A disproportionately large brain not only imposes geometric restrictions on the body size but also leads to huge energy expenditures. As a result, the size of the brain is one of the main factors that limits the miniaturization of animals.”¹⁶ The basic theory that is intended to explain Haller’s Rule is that, “In very small animals, brain-body size allometry implies that brain size becomes a limiting factor of body miniaturization because costs for development and maintenance of energetically expensive brain tissue.”¹⁷ As mentioned previously, smaller neurons require less energy, therefore that can offset the expense of brains that are relatively larger. The implication is that the physical limits may have been reached for how much brains can be compressed.

Miniaturization and Behavior

One question that can be asked is: Does reduced brain size affect the behavior of animals? The answer appears to be that there is no effect. The authors of a paper on the miniaturized wasp Trichogramma addressed this issue and concluded that, “The extremely small brain of this species does not seem to affect their behavioural performance. Female wasps, even the small phenotypes, display a rich behavioural and cognitive repertoire similar to much larger insects, including flight, walking, courtship, deciding over the size and sex of their progeny, vision, olfaction, learning and long- and short-term memory formation…These complex behavioural traits are essential to find and parasitize suitable host eggs in nature and might require a certain, minimal brain size.”¹⁸ Similarly, regarding another wasp species (Megaphragma polilovi), whose brain neurons do not have a nucleus, researchers conclude, “Miniature insects retain complex forms of behavior and locomotion, which shows that the anucleate neurons remain functional.”¹⁹

Given that basic functionality and behavior is not negatively affected by miniaturized brains, including those with neurons that do not have a nucleus, the next question is: How is function maintained? That is still unknown. Regarding the function of neurons that lack a nucleus, “A number of other important issues also remain unresolved. First, the mechanisms and control of the process of lysis of the cell bodies and nuclei of neurons remain unknown. And, second, the most important question is how efficiently the dendrites, axons, and remaining nuclei of the neurons function, and what cellular molecular mechanisms provide for their functioning.”²⁰ So there is still much to be learned about miniaturized brains. 

An Analogy with Electronic Circuits

An analogy can be drawn between the miniaturization of brains and the miniaturization of electronic circuits. Ever since the invention of the transistor there has been a consistent reduction in the size of electronic semiconductor circuits. This transformation was even given a name “Moore’s Law” by Gordon Moore, the co-founder of Fairchild Semiconductor and Intel. According to Moore’s Law the number of transistors in a given area of an electronic circuit doubles every two years.²¹ And just as electronic circuits have been reduced in size, while also maintaining and even improving performance, miniaturized insect brains also achieve performance similar to nominally sized insect brains.

Another similarity between brains and electronic circuits is their complexity. While there has been a long history of neuroscientists and engineers in making the comparison between biological neural networks and artificial neural networks, the fact is that biological neural networks (brains) exhibit more complexity.²²

One observation that can be made is that another thing in common between the miniaturization of brains and electronic circuits is that they are the product of engineering design. It has required a significant amount of engineering, as well as developments in physics and manufacturing, to reduce the size of electronic circuits. The same can be said of the process of miniaturization of brains. It is difficult to see how that could have occurred through a random evolutionary process.

In recent years experts in the semiconductor industry have been proclaiming that Moore’s Law was no longer applicable due to limitations imposed by basic physics that have restricted further reduction in the size of electronic circuits.²³ It appears the same is true for miniaturization of brains due to limitations in the size of neurons, and other factors in maintaining the function of neural networks.

[[But this comparison with electronic circuits says nothing about explaining behavior since the electronic circuits are packed with many many more elements when they are miniaturized.]]

Notes

1. Van der Woude, et al., “Breaking Haller’s Rule: Brain-Body Size Isometry in a Minute Parasitic Wasp,” Brain Behav Evol2013; 81: 86-92.
2. A. Makarova, et al., “Small brains for big science,” Current Opinion in Neurobiology 2021, 71:77-83.
3. Mark F. Bear, Barry W. Connors, and Michael A. Paradiso, Neuroscience: Exploring the Brain (Baltimore: Lippincott Williams & Wilkins, 2007) 28, 30.
4. Van der Woude, et al., “Breaking Haller’s Rule.”
5. Van der Woude, et al., “Breaking Haller’s Rule.”
6. Van der Woude, et al., “Breaking Haller’s Rule.”
7. Nolan, et al., “Betz cells of the primary motor cortex,” J Comp Neurol. 2024; 532.
8. A.A. Makarova and A.A. Polilov, “Structure of the Brain of the Smallest Coleoptera,” Doklady Biochemistry and Biophysics, 2022, Vol. 505, 166-169.
9. Van de Woude, et al., “Breaking Haller’s Rule.”
10. Polilov, et al., “Extremely small wasps independently lost the nuclei in the brain neurons of at least two lineages,” Scientific Reports (2023) 13:4320. 
11. Van de Woude, et al., “Breaking Haller’s Rule.”
12. Polilov, A.A and & Makarova, A.A, “The scaling and allometry of organ size associated with miniaturization in insects: A case study for Coleoptera and Hymenoptera,” Sci. Rep. 7:43095 (2017).
13. Polilov, A.A and & Makarova, A.A, “The scaling and allometry of organ size associated with miniaturization in insects.”
14. Polilov, A.A and & Makarova, A.A, “The scaling and allometry of organ size associated with miniaturization in insects.”
15. A. Makarova, et al., “Small brains for big science.”
16. Makarova, et al., “Small brains for big science.”
17. Van de Woude, et al., “Breaking Haller’s Rule.”
18. Van de Woude, et al., “Breaking Haller’s Rule.”
19. Polilov, et al., “Extremely small wasps independently lost the nuclei in the brain neurons of at least two lineages.” 
20. Polilov, et al., “Extremely small wasps independently lost the nuclei in the brain neurons of at least two lineages.”
21. John Shalf, “The future of computing beyond Moore’s Law,” Phil. Trans. R. Soc. A, 2020, 378.
22. Eric Cassell, “Design, Engineering, Specified Complexity: Appreciating the Fruit Fly Brain,” Evolution News, November 14, 2014.
23. Tom Simonite, “Moore’s Law is Dead. Now What?” MIT Technology Review, May 13, 2016.

 

Scientific status of Intelligent design

and an excellent example of the work done at Dicovery.org.

 https://evolutionnews.org/2025/01/new-long-story-ervs-pseudogenes-onions/

Ohr Somayach campaign

 Ohr Somayach is desperately trying to cover a 3 million dollar deficit. This of course means that the educational staff has not been paid. I urge you with all my heart to look at the material on the link and please try to help.

https://causematch.com/ohr-somayach-international-24/474345 

Discovering again what we don't know

 

The Ocean Teems With Networks of Interconnected Bacteria

Nanotube bridge networks grow between the most abundant photosynthetic bacteria in the oceans, suggesting that the world is far more interconnected than anyone realized.

https://www.quantamagazine.org/the-ocean-teems-with-networks-of-interconnected-bacteria-20250106/?mc_cid=fd0b65eee0&mc_eid=61275b7d81

Prochlorococcus bacteria are so small that you’d have to line up around a thousand of them to match the thickness of a human thumbnail. The ocean seethes with them: The microbes are likely the most abundant(opens a new tab) photosynthetic organism on the planet, and they create a significant portion — 10% to 20% — of the atmosphere’s oxygen. That means that life on Earth depends on the roughly 3 octillion (or 3 × 10²⁷) tiny individual cells toiling away.

Biologists once thought of these organisms as isolated wanderers, adrift in an unfathomable vastness. But the Prochlorococcus population may be more connected than anyone could have imagined. They may be holding conversations across wide distances, not only filling the ocean with envelopes of information and nutrients, but also linking what we thought were their private, inner spaces with the interiors of other cells.

At the University of Córdoba in Spain, not long ago, biologists snapping images of the cyanobacteria under a microscope saw a cell that had grown a long, thin tube and grabbed hold of its neighbor. The image made them sit up. It dawned on them that this was not a fluke.

“We realized the cyanobacteria were connected to each other,” said María del Carmen Muñoz-Marín(opens a new tab), a microbiologist there. There were links between Prochlorococcus cells, and also with another bacterium, called Synechococcus, which often lives nearby. In the images, silvery bridges linked three, four, and sometimes 10 or more cells.

Muñoz-Marín had a hunch about the identity of these mysterious structures. After a battery of tests, she and her colleagues recently reported(opens a new tab) that these bridges are bacterial nanotubes. First observed in a common lab bacterium only 14 years ago, bacterial nanotubes are structures made of cell membrane that allow nutrients and resources to flow between two or more cells.

The structures have been a source of fascination and controversy(opens a new tab) over the last decade, as microbiologists have worked to understand what causes them to form and what, exactly, travels among these networked cells. The images from Muñoz-Marín’s lab marked the first time these structures have been seen in the cyanobacteria responsible for so much of the Earth’s photosynthesis.

They challenge fundamental ideas about bacteria, raising questions such as: How much does Prochlorococcus share with the cells around it? And does it really make sense to think of it, and other bacteria, as single-celled?

Totally Tubular

Many bacteria have active social lives. Some make pili, hairlike growths of protein that link two cells to allow them to exchange DNA. Some form dense plaques together, known as biofilms. And many emit tiny bubbles known as vesicles that contain DNA, RNA or other chemicals, like messages in a bottle for whatever cell happens to intercept them.

It was vesicles that Muñoz-Marín and her colleagues, including José Manuel García-Fernández, a microbiologist at the University of Córdoba, and graduate student Elisa Angulo-Cánovas(opens a new tab), were looking for as they zoomed in on Prochlorococcus and Synechococcus in a dish. When they saw what they suspected were nanotubes, it was a surprise.

 

Growing between these bacteria (left: Prochlorococcus; right: Bacillus subtilis) are nanotube bridges, through which cells transport substances such as amino acids and enzymes. Although these nanotubes were first observed only in 2011, biologists now think that bacteria have been making these structures all along unnoticed.

Nanotubes are a recent addition to scientists’ understanding of bacterial communication. In 2011, Sigal Ben-Yehuda and her postdoc Gyanendra Dubey at the Hebrew University of Jerusalem first published images(opens a new tab) of tiny bridges, made of membrane, between the bacteria Bacillus subtilis. These tubes were actively transporting material: The researchers showed that green fluorescent proteins produced in one cell of the network quickly percolated through the others. They found the same result with calcein, a small molecule that is not able to cross bacterial membranes on its own. These cells were not existing placidly side by side; their inner spaces were linked, more like rooms in a house than detached dwellings.  

It was a startling revelation. The news compelled other biologists to reexamine their own images of cells. It soon became clear that B. subtilis was not the only species producing nanotubes. In populations of Escherichia coli and numerous other bacteria, small but consistent fractions of cells were spotted with nanotubes. In experiments, scientists watched cells sprout the tubes and then investigated what they carried. Moving across these bridges from cell to cell were substances such as amino acids(opens a new tab), the basic building blocks of proteins, as well as enzymes and toxins(opens a new tab). Bacteria, biologists now think, have probably been making these structures all along. Scientists simply hadn’t noticed them or realized their significance.

Not everyone has found it straightforward to get bacteria to make nanotubes. Notably, a group at the Czech Academy of Sciences could see nanotubes only when cells were dying(opens a new tab). Their suggestion that the tubes are a “manifestation of cell death” cast doubt on whether the structures were truly an important part of the cells’ normal biology. Since then, however, additional work has carefully documented that healthy cells do grow the structures. All this suggests that certain conditions must be met for bacteria to take this step. Still, “I think they are everywhere,” Ben-Yehuda said.

The latest findings are particularly eye-opening because Prochlorococcus and Synechococcus are not your average dish-dwelling bacteria. They live in a singularly turbulent environment: the open ocean, where water movement might reasonably be expected to break the fragile tubes. What’s more, they are photosynthetic, meaning that they get most of what they need to survive from the sun. What need could they have for trading through tube networks? There has been another sighting(opens a new tab) of nanotubes in marine bacteria, but those microbes are not photosynthetic — they gobble up nutrients from their immediate environment, a lifestyle in which swapping substances with neighbors might have a more obvious benefit.

So, when Muñoz-Marín and Angulo-Cánovas saw their nanotubes, they were initially skeptical. They wanted to make sure that they weren’t mistaking some accident of how the cells were prepared or how the images had been taken for a natural structure.

“We spent a lot of time to ensure that what we were finding in the images was actually something physiological and not any kind of an artifact,” García-Fernández said. “The results were so shocking in the field of marine cyanobacteria that we were, on the one hand, amazed, and on the other hand, we wanted to be completely sure.”

They put the cells under four radically different kinds of imaging devices — not only a transmission electron microscope, which they had been using when they first spotted the structures, but also a fluorescence microscope, a scanning electron microscope, and an imaging flow cytometer, which images live cells as they zip by. They looked at Prochlorococcus and Synechococcus on their own and at cultures where they lived together. They looked at dead cells and living ones. They even looked at fresh samples of seawater fished out of the Bay of Cádiz. In all the samples they spotted bridges, which connected about 5% of the cells. The nanotubes did not seem to be artifacts.

From left: José Antonio González-Reyes, Jesús Díez, María del Carmen Muñoz-Marín, Elisa Angulo-Cánovas and José Manuel García-Fernández, all based at the University of Córdoba. The researchers were part of an interdisciplinary group that discovered and studied the bacterial nanotubes that grow between photosynthetic ocean bacteria.

University of Córdoba

Next, to see whether the links were in fact nanotubes, they performed versions of the now-canonical experiments with green fluorescent protein and calcein described by Ben-Yehuda and Dubey. The networked cells lit up. The team also confirmed that the links were indeed made of membrane lipids and not protein, which would instead suggest pili. They were convinced, finally, that they were looking at bacterial nanotubes.

These tubes connect some of the most abundant organisms on the planet, they realized. And that immediately made something very clear, something the researchers are still turning over in their minds.

“At the beginning of this century, when you were speaking about phytoplankton in the ocean, you were thinking about independent cells that are isolated,” García-Fernandez said. “But now — and not only from these results, but also from results from other people — I think we have to consider that these guys are not working alone.”

A Cellular Network

There might be a good reason why cyanobacteria, floating in the vast expanse of the ocean, might want to join forces. They have curiously small genomes, said Christian Kost(opens a new tab), a microbial ecologist at the University of Osnabrück in Germany who was not involved in this study. Prochlorococcus has the smallest genome(opens a new tab) of any known free-living photosynthetic cell, with only around 1,700 genes. Synechococcus is not far behind.

Among bacteria, small genomes relieve organisms of the pressure of maintaining bulky DNA, but this state also requires them to scavenge many basic nutrients and metabolites from their neighbors. Bacteria with streamlined genomes sometimes form interdependent communities with organisms that produce what they need and need what they produce.

“This can be much more efficient than a bacterium that attempts to produce all metabolites at the same time,” Kost said. “Now, the problem, when you’re living in a liquid, is: How do you exchange these metabolites with other bacteria?”

 

Nanotubes may be a solution. Nutrients transferred this way will not be swept away by currents, lost to dilution or consumed by a freeloader. In computer simulations, Kost and his colleagues have found that nanotubes can support the development of cooperation among groups of bacteria.

What’s more, “this [new] paper shows that this transfer is both happening within and between species,” he said. “This is super interesting.” In a previous paper(opens a new tab), he and colleagues also noticed different species of bacteria connected by nanotubes.

This kind of cooperation is probably more common than people realize, said Conrad Mullineaux(opens a new tab), a microbiologist at Queen Mary University of London — even in environments like the open ocean, where bacteria may not always be close enough to form nanotubes.

We often speak of bacteria as being simple and single-celled. But bacterial colonies, biofilms and consortiums of different microorganisms can perform complicated feats of engineering and behavior together, sometimes rivaling what multicellular life can achieve. “I like to try to persuade people sometimes, when I’m feeling feisty: You’re a biofilm and I’m a biofilm,” Mullineaux said. If the sea is full of cyanobacteria communicating by nanotube and vesicle, then perhaps this exchange of resources could affect something as fundamental as the amount of oxygen in the atmosphere or the amount of carbon sequestered in the ocean.

Kost, Ben-Yehuda and Mullineaux agree that the new paper’s findings are intriguing. The authors have done all the right tests to ensure that the structures they are seeing are in fact nanotubes, they said. But more work is needed to explain the significance of the finding. In particular, a big open question is what, exactly, Prochlorococcus and Synechococcus are sharing with each other in the wild. Photosynthesis allows these bacteria to draw energy from the sun, but they must pick up nutrients such as nitrogen and phosphorus from the environment. The researchers are embarking on a series of experiments with Rachel Ann Foster(opens a new tab) of Stockholm University, a specialist in nutrient flow in the ocean, to trace these substances in networked cells.question is how bacteria form these tubes, and under what conditions. The tubes are not much longer than an individual cell, and Prochlorococcus, in particular, is thought to spread out in the water column. Muñoz-Marín and her team are curious about the concentrations of bacteria required for a network to form. “How often would it be possible for these independent cells to get close enough to each other in order to develop these nanotubes?” García-Fernandez asked. The current study shows that nanotubes do form among wild-caught cells, but the precise requirements are unclear.

Looking back at what people thought about bacterial communication when he began to study marine cyanobacteria 25 years ago, García-Fernandez is conscious that the field has undergone a sea change. Scientists once thought they saw myriad individuals floating alongside each other in immense space, competing with neighboring species in a race for resources. “The fact that there can be physical communication between different kind of organisms — I think that changes many, many previous ideas on how the cells work in the ocean,” he said. It’s a far more interconnected world than anyone realized.

The Quanta Newsletter

Bottom of Form