This week we discussed insect respiration and tracheae, and several papers ended up intertwining in curious ways.
These photos are both from Snelling et al paper Steve presented about tracheal molting in Orthoptera legs. The system appears to be more dynamic than expected, and specialized epithelial tissue pull trachea and tracheoles where they are needed as the system grows and expands.
Andy enumerated on a paper he mentioned last week, where a team including Mark Westneat observed a tenebrionid in a synchrotron, which allowed an x-ray effect at high resolution. They saw the tracheae, not just the air sacs, pulsing, which was unanticipated as taenidia imply that trachea are static
The next few papers all led into one another. Colin presented a paper about the thermal limits in insects to gigantism in fossil insects, particularly Plecoptera. The critical temperature for an insect to survive is related to oxygen levels. Experimentally, the authors raised plecopteran immatures in hypoxic, control, and hyperoxic environments. They found that if oxygen is plentiful, the nymphs can survive warmer temperatures, and can attain a larger body mass- perhaps to counteract oxygen toxicity.
Heather discussed a paper using Rhodnius kissing bugs and Gromphadorhina hissing cockroaches to observe how respiratory patterns and metabolic rates vary with temperature. The authors found three patterns of spiracle use, discontinuous, cyclical, and continuous, in other words always open. These patterns affect the capacity of the respiratory system to bring air to tissues. The authors performed experiments with 2 treatments- constant temperature and increasing temperature. At 15°C the spiracles closed longer, in a discontinuous pattern. At 25°C a cyclical pattern emerged, where the spiracles would open and close at regular intervals. At 35°C and above, there was continuous oxygen and CO2 exchange. Contreras and Bradley suggested that the metabolic rate determined the rate of respiration, not the phylogeny or ecology of the insects.
I discussed two papers that debate the function of discontinuous gas exchange. In 2005, a high profile paper in Nature posited that insects will close their spiracles for extended periods of time to limit oxidative damage. Since their tracheoles connect directly to cells, any reactive oxygen species (ROS) will have a greater chance of causing damage. You might have heard of ROS in human nutrition as free radicals. Only insects in a few orders can do discontinuous gas exchange, and those tend to be subterranean or live in arid environments. Matthews and White, instead, propose that there isn’t necessarily a physiological pressure for this, and instead it is akin to a sleep cycle in those insects. Boardman et al determine that the insects do react to ROS, but it is not due to damage, instead the concentration of ROS is a way for the insect to determine when to take its next breath.
Trish discussed a paper examining another type of respiration known as ﬂutter phase, and how different spiracles act during respiration, in a dung beetle Circellium bacchus. These wingless beetles have 8 pairs of spiracles. The abdominal spiracles are subelytral. During respiration, the spiracles open a little to let in carbon dioxide. The mesothoracic spiracles open at different times, which builds CO2 pressure builds in the abdomen. The beetle can move and contract their abdomen to push air into thorax. As the air escapes and pressure decreases in mesothoracic spiracle, oxygen goes in very quickly. As this occurs in the subelytral cavity, and water loss preventing microtrichia are present, very little water is lost during respiration. This is not the same as discontinuous respiration and instead is termed burst and interburst. We then discussed scarab and tenebrionid abdominal morphology, where are spiracles are located, and how strong of a seal the elytra can make with the abdomen. Andy and István both drew elaborate illustrations of how they hypothesized this system might function Andrew told us what he knew about flutter respiration in Cecropia moths. Istvan discussed how the different uses for the spiracles might explain disparate musculature. The ability of the insect to change pressure in its body explains why some spiracles are more sclerotized than other and have less musculature. In ceraphronoid wasps, for instance, the anterior thoracic spiracle shows many differences between genera. For inciting this exciting debate, Trish’s paper was the best this week for sure.
Stay tuned for our exploration of the circulatory system, led by a great presentation by Heather.