Insects are a highly successful clade of organisms, as evidenced by the myriad forms one can daily observe. This success is due to the layering and interaction of many different advantageous evolutionary and ecological factors over the eons, allowing for such massive speciation. One such advantage may be that of size. Insects are reasonably small in comparison with the wide variation of scales other clades of living organisms have achieved. Why? The correct answer is a combination of several hypotheses, but what combination?
Among the most prevalent hypotheses are those pointing the finger of size-limitation blame towards tracheal respiratory system limitations, an unfavorable surface area to volume ratio causing biomechanical exoskeleton non-scalability, and the evolution of more potent flying life forms such as birds and bats posing potential biotic size-limiting factors including predation and competition. At the opposite end of the scale, evidence shows that insect miniaturization is limited more simply by egg size large enough to produce viable larvae (for females), brain size, and the inability to fly, a highly advantageous evolved behavior (Grebennikov 2008, Niven & Farris 2012, Polilov 2012).
Insects’ breathing system consists of pairs of holes called spiracles along their bodies connecting to tubes called tracheae which transport oxygen to cells and remove carbon dioxide. As insects increase in size, tracheae take up a proportionally greater amount of space in their bodies because they need to increase in length and width to deliver oxygen as a larger body size increases oxygen needs. This inhibits growth because at a certain point the tracheae become impractical and crowd other organs (Kirkton 2007).
Unlike an endoskeleton, which allows continuous growth, an exoskeleton limits an organism to discrete or incremental growth through molting. During molting, an organism is highly vulnerable to predation and the forces of gravity. A very large insect would take too long to molt, put too much energy into the production of a new exoskeleton, and collapse under its own weight. Also, an exoskeleton must be scaled to the dimensions of the organism to prevent it from collapsing. For example, if an ant’s exoskeleton must become thicker as body size increases (strength is proportional to length squared whereas mass is proportional to length cubed), at a certain point this thickness would cause the ant’s legs to crumple under the strain of holding up its own body weight.
One hypothesis points to biotic factors constraining size. If flying insects were large, they would be easier to prey upon because they would be more conspicuous and less acrobatically maneuverable. Also, insects would be outcompeted by other flying animals like birds and bats. This hypothesis is outlined and explored further below.
One way to logically determine the possible evolutionary pressures limiting insect size is to trace the different evolutionary paths a primitive insect may have taken on its journey of terrestrial invasion and ultimate colossal diversification. Imagine a primitive insectoid form crawling out of the ocean. It pioneered the terrestrial space for its slimy kin. This creature does fairly well for a while, but then its squishy body becomes a hindrance to its fitness to reproduce-it is too physically vulnerable and too costly in terms of moisture conservation. Therefore, the organism evolves an exoskeleton to help conserve moisture and protect itself from predators. However, this primitive exoskeleton reduces the insect’s diffusion, and it must evolve a respiratory system-either tracheal or some intermediary. If the insect possesses an exoskeleton upon colonization, but has a diffusive respiratory system causing it to lose too much moisture, it may also evolve a tracheal system.
In either scenario, the primitive land-bound insect is limited by size because of its exoskeleton and tracheal system. If for some reason the insect first has no reason to evolve an exoskeleton-perhaps it is small enough or evasive enough to avoid prying predators – then it would be limited by its tracheal system. The opposite is true if the insect has a different respiratory system than a tracheal one – it would be limited by its exoskeleton. Interestingly enough, a lack of exoskeleton could also have limited the pioneering insect’s size. If it got too large but remained squishy and vulnerable, it would have been much easier to see and capture as well as to rip apart and ingest. Fast forward to the Carboniferous period. During this time period, oxygen levels were higher, meaning insects needed smaller quantities of air to meet their oxygen demands and could thus grow much larger (Clapham & Karr 2012). In fact, one study suggests insects may have needed to grow larger to avoid hyperoxia. Aquatic larvae which respire through diffusion are more sensitive to oxygen fluctuations and primitive forms which may have been unable to regulate their oxygen intake could solve this issue by growing bigger (Verberk & Bilton 2011). Only those organisms unconstrained by the exoskeleton could have grown large during the Carboniferous period, such as odonates, which overcome the surface area to volume problem by changing their shape. Bloated boxy beetles would have been size-limited, but organisms with long skinny body parts could overcome this obstacle.
When insect size did decrease, this event coincided first with the evolution of birds, and then again with that of bats. With predatory birds in the air, the need for maneuverability and evasiveness trumped the evolutionary drive to be big, and insects decreased in size. Additionally, large insects were outcompeted by birds and bats for air space. At this point in time, biotic interactions superseded oxygen as the most important constraint on insect maximum body size (Clapham & Karr 2012).
The scenarios outlined above and their various implications on insect size-limitation are simple and consider few variables. In reality, the issue is much more complex, including many different factors (genetic, environmental, abiotic, biotic, etc.) that complicate the issue of picking apart the importance of various factors on insect size limitation and evolution in general. Currently, insects are limited by all of the above. In order of priority, basic math-related issues (exoskeleton and tracheae) are the most crucially size limiting because without any other constraints they would still limit body size. As outlined earlier, competition and predation with birds and bats supersedes oxygen availability constraints. Perhaps one could make the claim that insects are ultimately constrained by gravity – in a low-gravity world exoskeleton and body weight would be irrelevant and they could just swim through the air gulping down oxygen like fish rather than bothering with tracheae! Only by further experimentation and research can scientists piece together the highly complex puzzle depicting the size limitations on and evolutionary path of insect forms.
Beutel, R.G., H. Pohl and F. Hünefeld. 2005. Strepsipteran brains and effects of miniaturization (Insecta). Arthropod Structure & Development 34:301-313.
Clapham, M.E. & J.A. Karr. 2012. Environmental and biotic controls on the evolutionary history of insect body size. Proceedings of the National Academy of Science USA 109:10927-10930.
Grebennikov, V.V. 2008. How small you can go: factors limiting body miniaturization in winged insects with a review of the pantropical genus Discheramocephalus and description of six new species of the smallest beetles (Pterygota: Ptiliidae). European Journal of entomology 105:313-328.
Kirkton, S.D. 2007. Effects of insect body size on tracheal structure and function. Hypoxia and the Circulation 618:221-228.
Niven, J.E. & S.M. Farris. 2012. Miniaturization of nervous systems and neurons. Current Biology 22:R322-R329.
Polilov, A.A. 2012. The smallest insects evolve anucleate neurons. Arthropod Structure and Development 41:29-34.
Verberk W.C.E.P. & Bilton D.T. 2011. Can oxygen set thermal limits in an insect and drive gigantism? PLoS ONE 6(7): e22610. doi:10.1371/journal.pone.0022610.