NC State University Insect Museum

This week we discussed insect respiration and tracheae, and several papers ended up intertwining in curious ways.

This figure, from Snelling et al 2011, based on Wigglesworth, shows the typical morphology of a trachea at the cellular level

This figure shows how the intima of the trachea detaches during molting in the leg of an orthopteran

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.

After Mauren Turcatel’s presentation on the midgut, and Andy’s pep talk, we came to class last Wednesday inspired and motivated to share fascinating research about insect insides.

Steve Turner first discussed an annual review paper about the peritrophic matrix. This is a curious structure which surrounds the interior of the midgut in many insects. We debated the optimal term to describe this structure, and agreed that calling it a membrane (often seen in older literature) is not suitable because membranes in biology imply a phospholipid bilayer, which this is not. Matrix isn’t perfect because it isn’t particularly three-dimensional, and envelope is misleading because it is permeable. We settled on peritrophic matrix because this is the term the paper used. It is expressed and developed in various ways in different insects, and several orders lack a peritrophic matrix entirely. The two main functional types are peritrophic matrices that are produced only in response to food, and those that are continuously produced as a ‘sock’, which is found in Dermaptera and Diptera, for instance. The distribution of these characters suggests that this matrix has been gained and lost multiple times throughout the evolutionary history of insects. The matrix is composed of bundles of chitin microfibrils and other proteins and is elastic. Though the peritrophic matrix does function to aid digestion, most research on it focuses on its protective properties, and the matrix can protect the midgut lumen from abrasion, and the insect itself from pathogens, parasitoids, and toxic compounds. There is interest in using pesticides or RNAi to prevent the proper development of the peritrophic matric in  pest species.

Trish Mullins then discussed a paper about viruses that attack the midgut, entomopox viruses (EPV) and nucleo polyhedral viruses (NPV). The insect consumes these viruses in the environment, and they quickly get to work using fusilin spindle proteins to disrupt midgut function. Although these two virus groups are not found in nature, Mitsuhashi was interested in observing their combined efficacy in biological control. The researcher fed the viruses to 3rd instar caterpillars, and while the peritrophic matrix was quickly destroyed, if the insect didn’t die, it could restore midgut function.

I introduced the group to a paper which used microtomography and TEM to investigate  the process of regenerating the protective midgut epithelium in Collembola, which was more or less an unknown topic prior to this work. The epithelium is rebuilt from a few specific regenerative cells, which don’t multiply themselves, suggesting that the adult springtail has a very real internal clock counting down to its demise which was set at the last molt, or perhaps at birth. This mechanism of regeneration is different than that found in other wingless hexapods such as firebrats and silverfish. Also found in the midgut epithelial tissue are mysterious urospherite-like ‘crystals’ or protein conglomerations, which are present in new epithelium but decline in old epithelium that will be shedded. They are thought to have an excretory function as springtails lack malpighian tubules, but what they really do is unclear.

Heather Campbell discussed a review paper about the process and genetics of midgut epithelium development in larval Aedes. I couldn’t explain it better than she (or the ladybug teacher) does in this delightful video she prepared for us. Check it out, and give her lots of likes with your own sockpuppets.

Andrew Ernst also told us about mosquitoes, but focused on and odd morphological structure in adults, the basal labyrinth. When the adult ecloses, the midgut is very organized, with a posterior bulb. Here the midgut lining is neatly folded, awaiting a blood meal. When that goal is met, the midgut expands so much that the cells lining the midgut are 90% thinner. When the blood meal is digested, the labyrinth is gone and the cells that composed it are left ragged and disorganized.

Ann Carr came with a paper about another type of hematophage, this time triatomine kissing bugs and the chagas causing trypanosomes they vector. Gonzales et al found is that the triatomine midgut is generally hostile to trypanosomes, so to overcome this, the pathogens use glycosaminoglycans to affect the morphology of the membranes of the perimicrovilli in the midgut, which allows them to attach themselves. The bugs were fed charged carbon ions to interfere with the trypanosomes’ ability to attach themselves. The midgut cells had a 75% drop in pathogen load. To illustrate the morphology of the system, Anne baked us a cake, with gummy worms as stand-ins for T. cruzi.

Ann showing us her delicious trypanosome cake

Trypanosome gummi worms complete with glycosaminoglycans to attach to the midgut

Colin Funaro recalled that in previous insect physiology classes, it was explained that insects make up for a short area for nutrient absorption with a counter current flow that allows the nutrients to repeatedly cycle back over the midgut cells. He was curious how this could have been discovered, and found these two papers by Espinoza-Fuentes and Terra on the topic. The story was more nuanced than how it was presented in the class, and for his dogged determination to decipher the truth, Colin’s was the paper of the week for me. The countercurrent flow has only been investigated in calyptrate Diptera, which are atypical because they lack a peritrophic matrix and because part of their midgut has a pH of 3.1, one of the only acidic digestive systems outside of vertebrates, which may serve to allow flies to digest bacteria more efficiently. By following dyes, the researchers found that the midgut is functionally divided into three sections, anterior to posterior. Water is secreted in the fore- and hind-midgut, and absorbed in the mid-midgut, the only area with extremely low pH. The only part of the gut with a true countercurrent flow is the hind-midgut, which traps activated trypsin digestive enzymes.

Colin explains the mid-midgut

Then to cap off a stimulating learning session, we heard about the basics of Malpighian tubules and the hindgut from Trish.