According to different studies, multicellular organisms tend to become smaller and smaller through time. This phenomenon is called miniaturization and is considered one of the most significative evolutionary trends among insects. Miniaturization is a driving force for diversity and evolutionary novelties, even though it must deal with some limitations.
Learn more about this phenomenon and met some of the most extreme cases of miniaturization among insects through this post.
Why are animals becoming smaller?
For some years now, multiple studies suggest there is a widely extended trend to miniaturization among multicellular animals (i. e. organisms composed by more than one cell).
Miniaturization is a remarkable natural phenomenon headed to the evolution of extremely small bodies. This process has been observed in different non-related groups of animals:
- Shrews (Soricomorpha: Soricidae), mammals.
- Hummingbirds (Apodiformes: Trochilidae), birds.
- Diverse groups of insects and arachnids.
To know more about giant insects, you can read “Size matters (for insects)!“
Diversification and speciation processes have given place to lots of new species through time, all of them constantly competing for limited space and food sources. This scenario turns even more drastic in tropical regions, where diversification rates are extremely high.
Learn about the ecological niche concept by reading “The living space of organisms“.
Facing the increasing demands of space and resources, evolution has given place to numerous curious phenomena such as miniaturization to solve these problems: by becoming smaller, organisms (either free-living or parasites) gain access to new ecological niches, get new food sources and avoid predation.
Despite many animals tend to miniaturization, this phenomenon is more frequently observed among arthropods, being one of their most remarkable evolutionary trends. Moreover, arthropods hold the record of the smallest multicellular organisms known to date, some of which are even smaller than an amoeba!
Guinness World Record of the smallest insects
The smallest arthropods are crustaceans belonging to the subclass Tantulocarida, which are ectoparasites of other groups of crustaceans, such as copepods or amphipodes. The species Tantulacus dieteri is still considered the smallest species of arthropods worldwide, which barely measures 85 micrometers (0,085 millimeters), thus being smaller than many unicellular life beings.
However, insects do not lag far behind.
Mymaridae
Mymaridae (or fairyflies) are a family of wasps inside the superfamily Chalcidoidea from temperate and tropical regions. Adults, ranging from 0.5 to 1 millimeter, develop as parasites of other insects’ eggs (e. g. bugs, Heteroptera). For this reason, fairyflies are very valuable as biological control agents of some harmful pests. Also, they are amongst the smallest insects worldwide.
Currently, the one holding the record as the smallest known adult insect is the apterous (wingless) male of the species Dicopomorpha echmepterygis from Costa Rica, with a registered minimum size of 0.139 millimeters. They neither have eyes nor mouthparts, and their legs endings are deeply modified to get attached to the females (somewhat bigger and winged) time enough to fertilize them. They are even smaller than a paramecium, a unicellular organism!
You can read “Basic microbiology (I): invisible world” to know more about unicellular organisms.
Fairyflies also include the smallest winged insects worldwide: the species Kikiki huna from Hawaii, with and approximate size of 0.15 millimeters.
Trichogrammatidae
Like fairyflies, trichogrammatids are tiny wasps of the superfamily Chalcidoidea that parasite eggs of other insects, especially lepidopterans (butterflies and moths). Adults of almost all the species measure less than 1 millimeter and are distributed worldwide. Adult males of some species are wingless and mate with their own sisters within the host egg, dying shortly after without even leaving it.
The genus Megaphragma contains two of the smallest insects worldwide after fairyflies: Megaphragma caribea (0.17 millimeters) and Megaphragma mymaripenne (0.2 millimeters), from Hawaii.
Trichogrammatids also have one of the smallest known nervous systems, and that of the species M. mymaripenne is one of the most reduced and specials worldwide, as it is composed by only 7400 neurons without nucleus. During the pupae stage, this insect develops neurons with functional nuclei which are able to synthetize enough proteins for the entire adulthood. Once adulthood is reached, neurons lose their nuclei and become smaller, thus saving space.
Ptiliidae
Ptiliidae is a cosmopolitan family of tiny beetles known for including the smallest non-parasitic insects worldwide: the genera Nanosella and Scydosella.
Ptiliidae eggs are very large in comparison with the adult female size, so they can develop a single egg at a time. Other species undergo parthenogenesis.
Learn some more about parthenogensis by reading “Immaculate Conception…in reptiles and insects“.
Currently, the smallest Ptiliidae species known and so the smallest non-parasitic (free living) insect worldwide is Scydosella musawasensis (0.3 millimeters), from Nicaragua and Colombia.
Consequences of miniaturization
Miniaturization gives rise to many anatomical and physiological changes, generally aimed at the simplification of structures. According to Gorodkov (1984), the limit size of miniaturization is 1 millimeter; under this critical value, the body would suffer from deep simplifications that would hinder multicellular life.
While this simplification process takes places within some groups of invertebrates, insects have demonstrated that they can overcome this limit without too many signs of simplification (conserving a large number of cells and having a greater anatomical complexity than other organisms with a similar size) and also giving rise to evolutionary novelties (e. g. neurons without nucleus as M. mymaripenne).
However, getting so small usually entails some consequences:
- Simplification or loss of certain physiological functions: loss of wings (and, consequently, flight capacity), legs (or extreme modifications), mouthparts, sensory organs.
- Considerable changes in the effects associated with certain physical forces or environmental parameters: capillary forces, air viscosity or diffusion rate, all of them associated with the extreme reduction of circulatory and tracheal (or respiratory) systems. That is, being smaller alters the internal movements of gases and liquids.
So, does miniaturization have a limit?
The answer is yes, although insects seem to resist to it.
There are several hypotheses about the organ that limits miniaturization. Both the nervous and the reproductive systems, as well as the sensory organs, are very intolerant to miniaturization: they must be large enough to be functional, since their functions would be endangered by a limited size; and so, the multicellular life.
. . .
Multicellular life reduction seems to have no limits. Will we find an even smaller insect? Time will tell.
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