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.

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.