Is there a limit to how small an adult arthropod can be? If so, what is the reason for the limit?
David Bortin
Whittier, California, US
Advertisement
Arthropods, like everything else, must have a theoretical lower size limit. Consider the following points.
Quantum physicists know that no physical object can have a dimension smaller than a Planck length, or about 1.6 × 10-35 metres.
Organic chemists know that living tissue must contain carbon-based macromolecules, such as DNA, which are typically around 0.001 micrometres in diameter.
Semanticists don’t all agree whether a virus particle is living or just a complicated complex of macromolecules, but because of the covid-19 pandemic, we all know what they look like: typically, they are around 0.5 micrometres in diameter, and .
Microbiologists know that the smallest incontrovertibly living organisms are bacteria – typically around 1 micrometre in diameter.
Zoologists know that some arthropods are among the tiniest multicellular animals. They include insects, spiders and crustaceans; all have segmented bodies, jointed limbs and a shell of chitin that is shed periodically.
There are marine arthropods just 85 to 100 micrometres long and terrestrial insects (fairyflies) that are only slightly longer. In terms of body mass or volume, the fairyfly might actually be the smallest.
Considering the complexity required to qualify as a true arthropod, this is surely close to the theoretical limit, but no one can know for sure if the smallest – either extant or theoretical – has actually been found. These organisms are near the limit of visibility to the unaided human eye, although under ideal conditions, objects down to about 35 micrometres can be resolved.
Mark Thompson
Tewkesbury, Gloucestershire, UK
There isn’t a strict limit to how small an adult arthropod can be, but there are practical constraints imposed by their biology and physiology.
The smallest adult arthropods are often microscopic, and can be less than 0.1 millimetres in length. These organisms have evolved specialised adaptations that allow them to function at such small scales. For example, they have simple respiratory systems that rely on passive diffusion and may have reduced or simplified organ systems.
However, there are limits to how small arthropods can become due to various factors. These include cell size and function. Cells have a minimum size required for basic functions, such as DNA replication and protein synthesis. This is a constraint on the organism’s size.
There are also respiratory limitations. As arthropods get smaller, their surface area-to-volume ratio increases, making it harder for them to exchange gases like oxygen and carbon dioxide with their environment. This can limit how small they can be while still obtaining enough oxygen to survive.
The smallest known adult arthropods may be Stygotantulus stocki, which are parasitic on Harpacticoid planktonic copepods. You could fit 50,000 of them on a teaspoon.
To answer this question – or ask a new one – email lastword@newscientist.com.
Questions should be scientific enquiries about everyday phenomena, and both questions and answers should be concise. We reserve the right to edit items for clarity and style. Please include a postal address, daytime telephone number and email address.
Â鶹´«Ã½ retains total editorial control over the published content and reserves all rights to reuse question and answer material that has been submitted by readers in any medium or in any format.
Ìý