Reply to Dorrington: Oxygen concentration and predator escape abilities are important controls on insect size

Dorrington (1) argues that oxygen is not an important control on insect size and that selective pressure for maneuverability and predator escape is not inversely related to body size. Because these claims are contradicted by many experimental studies, we stand by our hypothesis (2) that controls on maximum insect size shifted from environmental (primarily oxygen concentration) to biotic (primarily predation) following the evolution of flying predators like birds.

First, Clathrotitan is actually not unusually large for the modeled oxygen concentration in the Middle Triassic; in fact, it is Middle-Late Permian insects that are anomalously small. The correlation between maximum size and oxygen will not be perfect because of the vagaries of insect preservation and sampling, and because of slight differences in the absolute ages in the time scales used (we use the current Anisian boundaries of 247.2–242 Ma, whereas GEOCARBSULF used a previous time scale).

Second, changes in density likely would also have affected the size of flying insects, but would have exactly paralleled and amplified oxygen effects. Most workers have focused on the role of oxygen, however, because late Paleozoic gigantism is also observed in terrestrial arthropods and because there is abundant experimental evidence indicating that tracheal scaling is a key constraint on insect size (ref. 3 and references therein).

Third, although Meganisoptera may have been weaker fliers, gigantism is observed in a variety of groups. For example, the Carboniferous stem-group mayfly Bojophlebia had a forewing length of 215 mm. Similarly, Triassic Titanoptera were much larger than younger orthopteran relatives. Post-Jurassic size decreases are observed even among Anisoptera (the Late Jurassic Cymatophlebia suevica and Aeschnogomphus kuempeli had forewing lengths of 110 mm and 106 mm, respectively) and Orthoptera.

Fourth, we agree that oxygen is now a less important control on maximum size, at least for insects in natural settings, because of other factors (including predation). However, the claim that tracheal surface area would permit an order of magnitude size increase is inconsistent with experimental results (3). Tracheal volume scales hypermetrically, not linearly, with body size (3), and constraints are imposed by specific body parts before whole-body scaling limits are reached (4).

Fifth, although larger Anisoptera may be favored in intraspecific competition, Dorrington (1) notes that good maneuverability and higher speed play a role; thus, the relative importance of maneuverability in this instance is not clear. Most predator evasion instead involves liftoff from a surface, and, because smaller insects have intrinsically higher acceleration (5), they will more easily be able to escape those attacks. Greater acceleration of smaller insects will also lead to greater aerial maneuverability (although aerodynamic factors are also important).

Finally, comparison of living and fossil insect faunas is not valid because of inherent differences in the method of preservation and sampling effort. Because fossil species are a small subset of the living fauna, the fossil record will always underestimate the largest taxon extant during any interval. We demonstrated (ref. 2, SI Text) that the observed overall trend was not driven by variations in sampling of fossil assemblages, so the conclusions remain valid.