The respiratory systems of insects and their functions differ markedly from vertebrate respiratory systems, and have been a source of scientific curiosity for decades. Early investigations between the 1930s and 1960s inspired the development of modern sophisticated techniques that opened the field to other biologists. A brief overview of this rapidly growing field was recently published as a special edition in the Journal of Insect Physiology. This issue, edited by S. K. Hetz, contains contributions that reflect a dazzling diversity in insect respiration.
Lazaro Centanin and colleagues review the mechanisms of oxygen sensing that modulate and control the morphology of the Drosophila tracheal system, specifically the oxygen sensing mechanisms of tracheal cells via factors that regulate transcription factors from the HIF gene family that establish a strong developmental genetic basis for physiological responses to variable oxygen availability. In a discussion of a parallel adaptation to tracheal modulation, Christian Pick and co-authors suggest that hemocyanin (Hc) is an oxygen delivery pigment in late embryos and first-instar nymphs of an ovoviviparous cockroach.
In water boatmen (diving insects) Philip Matthews and Roger Seymour compare measured changes in gas gill volumes and their PO2 during dives in water containing different oxygen concentrations with predictions from current models, and describe compressible gas gills as mechanisms to maximise gas exchange in air-breathing aquatic insects. The potentially problematic trade-offs of sufficient oxygen uptake vs water loss in insect eggs are reviewed by Art Woods. Using Manduca eggs as a model system he discusses the eggshell's role in these trade-offs during the changing metabolic demands of developing eggs.
Several papers deal with discontinuous gas exchange cycles (DGC). Thomas Förster reinvestigates the reactions of moth pupae's spiracles to gas mixtures of varying oxygen and carbon dioxide content by measuring pressure changes to detect thresholds of intra-tracheal gas concentrations which could result in spiracular fluttering. Alex Kaiser and co-authors cover the changes in intermittent gas exchange during mealworm pupal development. They investigated changes in carbon dioxide release patterns of single pupae during development and found that interburst durations changed while burst volumes remained constant as pupal development modulated metabolic rate. By contrast, John Terblanche and Steven Chown found that interburst durations remained constant while burst volumes changed as temperature changes (another factor modulating metabolic rate) in tsetse flies Glossina morsitans.
Heidy Contreras and Tim Bradley also used temperature as a modulator for spiracle activity via metabolic rate in a cockroach and a bug. Higher temperatures increased metabolic rates, and their animals changed gas exchange patterns from discontinuous to cyclic to continuous. Reet Karise and co-authors found similar temperature effects in bumblebees, showing DGCs mostly at lower temperatures or in inactive states. Christian Mörbitz also confirmed that low metabolic rates coupled with sufficient spiracular conductance are required for insects to maintain DGC. However, during periods of high metabolic demand, such as insect flight, the insect tracheal system must be capable of extreme oxygen delivery capacities. Fritz-Olaf Lehmann reviews recent studies in flying Drosophila and describes how convection by different, sometimes strange, mechanisms increases gas exchange capacity tremendously.
The above are only brief summaries of a few of the contributions made to this special edition of JIP that exemplify the wide range of topics currently under investigation. While we do acknowledge that technological improvements have bolstered the recent boom in insect respiration studies, it is the contributions made by researchers benefiting from these new technologies that have expanded the field of insect respiratory biology far beyond the ‘breathe in, breathe out’ approach that initiated the first investigations of these fascinating biological systems.
