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Respiratory gas exchange in insects occurs via a branching tracheal system. The entrances to the air-filled tracheae are the spiracles, which are gate-like structures in the exoskeleton. The open or closed state of spiracles defines the three possible gas exchange patterns of insects. In resting insects, spiracles may open and close over time in a repeatable fashion that results in a discontinuous gas exchange (DGE) pattern characterized by periods of zero organism-to-environment gas exchange. Several adaptive hypotheses have been proposed to explain why insects engage in DGE, but none have attracted overwhelming support. We provide support for a previously untested hypothesis that posits that DGE minimizes the risk of infestation of the tracheal system by mites and other agents. Here, we analyze the respiratory patterns of 15 species of ground beetle (Carabidae), of which more than 40% of individuals harbored external mites. Compared with mite-free individuals, infested one's engaged significantly more often in DGE. Mite-free individuals predominantly employed a cyclic or continuous gas exchange pattern, which did not include complete spiracle closure. Complete spiracle closure may prevent parasites from invading, clogging, or transferring pathogens to the tracheal system or from foraging on tissue not protected by thick chitinous layers.

The size of the ommatidia that compose the insect compound eye is linked to visual capacity, physiological performance, and cell size. Therefore, rapid and reliable methods for measuring ommatidia can advance research on insect ecology and evolution. We developed an automated method to measure ommatidia in nail polish imprints of the eyes of three Carabidae beetle species using the widely available, free software ImageJ. Our automated method was equivalent to a traditional manual method in terms of accuracy but had the advantage of being 70 times faster. We provide access to our algorithm, which can be used to investigate biological phenomena ranging from the functional architecture of the compound eye to the cellular basis of the evolution of body size and metabolic rates.

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