World’s Smallest Insect Pays a Hefty Price for its Size

October 20, 2015 | Sarah Tse

The world's smallest free-living insect, Scydosella musawasensis.
Photo credit: Dr. Alexey Polilov

This tiny beetle is as small as unicellular organisms! How does it manage to fit all the structures it needs to survive into such a miniscule body?

Entomologists have declared Scydosella musawasensis to be the world’s smallest free-living insect. S. musawasensis, a featherwing beetle, can be as small as 0.325 millimeters (as tiny as certain unicellular organisms). Alexey Polilov of Lomonosov Moscow State University collected 85 of the beetles from Colombia, and his measurements of the tiny specimens confirm their ranking as the world’s tiniest free-living insects. In order to even measure such a tiny organism, Polilov had to use a scanning electron microscope with specialized software.

Why are insects in general so small? Evolution favors small insects for two reasons. The first is their unique respiratory systems; insects use tubes called trachea that must extend throughout their bodies to deliver oxygen and remove carbon dioxide. The larger the insect, the more space these trachea have to take up to satisfy their increased oxygen demand.

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The second constraint on giant insects comes from their chitinous exoskeletons. While this tough outer shell grants insects protection and structure, it also keeps them from growing too large. Molting and growing an entirely new coat is an incredibly exhaustive process. As the insect grows larger, it also needs a thicker exoskeleton that would eventually become too heavy to support.

Now that we’ve covered why insects have to be small, the next reasonable question is what keeps them from shrinking even smaller than Scydosella? The problem is that multicellular organisms must make a lot of sacrifices to fit all those structures and organs into such a tiny body. Miniaturization in metazoans is actually Polilov’s main research interest, and he discussed the unique effects of shrinking on structure and physiology in a paper published earlier this year.

The principle of organs scaling down as the body shrinks is called “allometry.” Microinsects achieve their tiny size through allometry as well as simplifying or entirely eliminating structures. For example, their chitinous exoskeleton constrains their morphology at both ends of the size spectrum. In exchange for smaller size, microinsects give up the thicker protection enjoyed by larger insects. The exoskeleton also becomes less complex and articulate, and the fusion of elements and segments makes the insect less flexible than its larger relatives.

This trend towards fusion into fewer, separate parts affects many organ systems. Microinsects like Scydosella lack the crops and gizzards present in larger insects, and may also lose the muscles that typically line the esophagus and intestines. The circulatory system in microinsects is basically reduced to nothing more than a simple heart and a short aorta, with Scydosella losing those organs entirely. Those microinsects with wings suffer from weakly developed veins, which poses a huge problem for flight performance.

Perhaps the most dramatic consequences of miniaturization occur in the nervous system. When forced to fit into such a tiny volume, neurons are reduced in both size and number and must pack together more tightly. The result? Greater noise and interference with the transmission of signals between cells. This also creates an exceptionally dense demand for energy, while leaving less room in each nerve cell for mitochondria, the intracellular structures that provide that energy. Microinsects try to get around these handicaps by expanding and contorting their nerves to fit every nook and cranny, but this comes at the expense of other organ systems.

Of course, the reproductive system gets top priority. While everything else dwindles, gonads must remain disproportionately large to maintain their function. In particular, females devote a significant portion of their body size to producing a large enough egg for viable offspring. Some species of microinsects even produce sperm that are longer than the male’s entire body.

Based on these observations, the miniaturization of insects is likely limited by the space required for the most crucial structures in the nervous and reproductive systems. While smaller size certainly gives these insects great advantages in conserving energy and avoiding predators, it also comes with a host of complications. Like many things in life, it looks like moderation is key.


Based on materials from provided by Pensoft Publishers.

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