Author Archive

Insect Morphology Seminar – Trachea and Spiracles

Tuesday, March 20th, 2012

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.

Insect Morphology Seminar- Midgut & Peritrophic Matrix

Wednesday, February 29th, 2012

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.

Insect of the week – number 47

Friday, November 26th, 2010

Chelonarium lecontei from Texas photo taken by Mike Quinn on Bugguide.

Insect of the week – number 17

Friday, April 30th, 2010

Diptera: Tabanidae: Microtabanus pygmaeus (Williston, 1887)

Horse flies are a common annoyance in North Carolina, particularly in the warmest summer months. Tabanus, or greenheads, attack pool and beach goers, and Chrysops, the deer flies, tend to bother the heads of hikers. These brief encounters don’t allow most people to experience the true diversity of horse flies in this state. North Carolina is also home to some of the rarest, most enigmatic and unusual horse flies.

The fly I’m covering today has all those attributes along with the added bonus of, according to me at least, cuteness. Microtabanus (Fairchild, 1937), a monotypic genus, and its species M. pygmaeus, as both parts of the name suggests, is very small. Along with its less than 1 cm stature, M. pygmaeus is characterized by reduction of several other characters, including a total lack of a frontal callus and fewer antennal flagellomeres than most other horse flies. The frontal callus is a raised, glabrous area between the eyes of female horse flies with a species-specific shape. What appears to be a callus on the photographed specimen is just where the pile rubbed off. Only three genera out of the ~60 genera in the Diachlorini have species with fewer than 4 free antennal flagellomeres; Microtabanus has 3.

Microtabanus is only found in the Eastern US has is collected extremely rarely. This distribution is particularly intriguing because the tribe in which it is placed, and which I’m revising for my dissertation, Diachlorini, is almost entirely Southern Hemisphere in distribution. Diachlorini is the most generically and morphologically diverse tribe in Tabanidae, and is not monophyletic. Another rare diachlorine genus, Anacimas, has a similar distribution as Microtabanus and is also morphologically reduced and bizarre. Microtabanus is so derived and unusual that the two attempts to classify the Diachlorini, by Fairchild (1969) and later by Trojan (1996, 1998), left it unplaced to genus group. Having freshly collected material for molecular and morphological study would help elucidate whether Microtabanus is the result of a northern expansion of Diachlorini or is part of an earlier radiation. So, I’m keen on collecting it.

I’ve only seen specimens from WV, TN, NC, and FL. There are probably fewer than 20 specimens in collections other than the Florida State Collection of Arthropods. Curiously, according to C. B. Philip et al (1973), one day hundreds were collected huddling on dune grass on an island off the gulf coast of Florida on a windy day. Few other specimens appear to be associated with beaches. The FSCA has more specimens from malaise traps run for years in Austin Carey forest in Gainesville. Based on specimen data, it isn’t clear what habitat Microtabanus prefers. It has never been collected in baited traps. It appears to be widespread but rare, apart from that one mass emergence on Navarre Beach. One specimen was found in a parked pickup truck in a populated area, so any reader of this blog might see one before I do. I would really appreciate fresh material, preferably killed in alcohol or frozen.

During a survey of the horse flies of NC run by former NCSU professor Robert Axtell, a series of specimens (well, one or two per year) was collected in malaise traps in the late summer in the early 1980s in Duplin Co., NC, in the coastal plain. The locality is labelled simply as ‘Mccoy Farm.’ The owners of the only McCoy Farm in Duplin County I could find would not let me collect on their property. I contacted the local extension agent, and he informed me that there are several other McCoy Farms in Duplin but none of their proprietors remembered entomologists erecting malaise traps on their land in the early 80s. Another NC specimen was collected in Killdevil Hills on the 26th of May, though this specimen is at Cornell, not NCSU.

Several Wiegmann lab members and I maintained malaise traps in a Cabin Lake county park in Duplin last summer as a fallback. The park was very pretty, though I’d recommend you visit earlier in the season as the horse flies were belligerent and numerous, like Morbo’s family. The heads of the traps were filled with tabanids, but no Microtabanus showed up. Figuring out how to reliably collect this diminutive diachlorine, and even better, finding out how its biology influences its unusual morphology and rarity, truly are fascinating challenges in the study of North American horse flies.

Find out More

Fairchild, G. B. 1937. A preliminary list of the Tabanidae of Florida. Florida Entomologist 19: 10-11.

Fairchild, G. B. 1969. Notes on Neotropical Tabanidae XII. Classification and distribution, with keys to genera and subgenera. Arq Zool S Paulo 187: 199-255.

Philip, C. B., H. V. Weems, Jr. and G. B. Fairchild. 1973. Notes on Eastern Nearctic Haematopota, Merycomyia, and Chrysops, and Description of Male of C. zinzalus (Diptera: Tabanidae). Florida Entomologist. 56: 339-346.

Trojan, P. 1996. The tribe Lepidoselagini and its taxonomic division. Annals of the Upper Silesian Museum, Entomology 5:97-172.

Trojan, P. 1998. Supraspecific taxa of Tabaninae (Diptera: Tabanidae). III. The tribe Diachlorini and its taxonomic division. Annals of the Upper Silesian Museum (Entomology), 8-9:5–91.

Williston, Samuel W.  1887.  Notes and descriptions of North American Tabanidae. Transactions of Kansas Academy of Science. 10:129-142.

D. or S.? melanogaster

Thursday, February 12th, 2009

One interesting high impact dipterological discussion popping up on science news sites is the nomenclatural snafu that is Drosophila melanogaster. In a sentence, if melanogaster Meigen was not a model organism but still was part of a modern systematic revision, it would not be in the genus Drosophila. The problem is the genus. Drosophila contains about 1500-2000 described species and the earliest divergences in the genus occurred more than 60 Mya. More than half of the species in the family Drosophilidae are currently placed in Drosophila. Drosophila as currently conscribed is in the top five largest fly genera in term of described species (along with Tipula, Tabanus, Simulium, and Megaselia- future blog post topic, for sure). I would be surprised if any of those five are monophyletic. Drosophila is an ecologically diverse genus, with some generalist saprophages, some fungivores, and some phytophages, including some that are host species specific. The genus is morphologically diverse in terms of size and patterning, from classic tiny brown flies to ornate, house fly-sized Hawaiian picture winged Drosophila.

These flies are vastly popular in experimental biology. Summarizing all of the contributions to modern science from studies of these flies would be quite a task. Nobel prizes have been awarded to scientists who study Drosophila for such discoveries as homeobox genes. Most research is done on Drosophila melanogaster but a few other dozen other drosophilid species are cultured for research. Elements of the life history of Drosophila melanogaster from its behavior to its genetics are vigorously researched. Scientists have sequenced the genomes of D. melanogaster and eleven other Drosophila species in largest comparative genomics project currently available. The phylogenetic relationships of those twelve species represent one of the most certain areas on the entire tree of life. Rigorous phylogenetic hypotheses of Drosophilidae have followed suit.

Many lucid, well-written studies have concurred that Drosophila is paraphyletic with respect to at least seven other genera. If these were tiny unimportant genera, maybe they could be sunk into Drosophila and problem solved. Unfortunately, these genera, such as the speciose, cosmopolitan Scaptomyza have hundreds of species, are morphologically diverse and are easily distinguished from Drosophila. Furthermore, the type species of Drosophila, D. funebris Fabricius, is nowhere near melanogaster phylogenetically. According to common taxonomic practice, Drosophila melanogaster's subgenus Sophophora would be elevated to genus, creating Sophophora melanogaster. Indeed, only 3 of the 12 Drosophila species with sequenced genomes are Drosophila sensu stricto. The other nine are in subgenus Sophophora and the other six or seven smaller subgenera of Drosophila sensu lato are not represented by sequenced genomes. Understandably, there is a lot of backlash towards Sophophora melanogaster.

These findings leave the scientific community with three main options:

  1. leave Drosophila sprawling and paraphyletic.
  2. suck it up and accept Sophophora melanogaster.
  3. change the type species of Drosophila to melanogaster.

Here is a Nature News article about this predicament. I'll go over some of the pros and cons of each.

Drosophila melanogaster
The lowly pomace fly, Drosophila melanogaster… or is it Sophophora melanogaster. Thanks to gurkeeeee for capturing this image.

To be honest, option 1 has been the modus operandi for many years. The fact that Drosophila melanogaster belongs in a different genus than Drosophila funebris has been some combination of a migraine headache, open secret, and skeleton in the closet for fly taxonomists for at least 30 years. Leaving the genus non-monophyletic opposes a practical convention in modern systematic that every name should ideally describe a monophyletic taxon. It is obviously problematic if one wants to compare a species of Drosophila to its closest relatives, but it closest relative is in a different genus. It is possible to argue that this convention should be ignored as long it is made clear to all workers on drosophilids that Drosophila is not monophyletic. Upending this scientific convention for the sake of convenience (though some might argue that paraphyletic taxa are A-OK) is not likely to happen, at least not for long. Such phylogenetic focus on a single fly family from so many workers is rare, and we should take advantage of their findings.

The reasons for or against accepting Sophophora melanogaster are simple but coercive. This name change is the proper procedure under the ICZN. The zoological community created and should accept the decisions of the ICZN. However, this will lead to a headache-inducing upheaval in many other fields. D. melanogaster and the other Sophophora species pervade modern biological literature. “Chaos” according to Patrick O'Grady, “Genetic work could be lost. It would be hard to find things.” Andrew Polaszek for instance, claims that D. melanogaster is the most studied organism other than Homo sapiens. That seems hyperbolic to me; I would say that is the most studied organism that we don't eat, that doesn't make us sick, and that isn't us. Certainly it is the most important organism that might potentially change its name. It is often pointed out that many references to 'Drosophila' really mean 'Drosophila melanogaster.' The commonly used abbreviation 'Dmel' will also be outdated. Changing all those names will cause instability in the short term, and the ICZN was created to maintain stability.

The last option has already been submitted as a case to the ICZN by van der Linde et al. (a community ecology postdoctoral student, curiously.) The proposal will:

  1. raise all subgenera other than Drosophila and Sophophora to generic level
  2. switch the type genus of Drosophila to melanogaster, making Sophophora a junior synonym (melanogaster is conveniently the type species of Sophophora).
  3. split the old Drosophila subgenus into three genera, resurrecting already existing names.
  4. move the 78 species unplaced in Drosophila senso latu to Drosophila sensu stricto/novo.

So Sophophora goes bye-bye, and nine of the species with genomes sequenced stay Drosophila. Two other sequenced Drosophila species will be in Siphlodora and grimshawi, the sequenced Hawaiian species, will be in Idiomyia. One thing this proposal does is remove the Hawaiian Drosophila radiation from the genus Drosophila. A lot of ecological and systematic literature concerns that radiation (and calls it Drosophila). More species will change their names than would if Sophophora was accepted. The worst part of this proposal to me is what happens to the unplaced species. Sophophora is a well-defined monophyletic clade, and most of those 78 unplaced species probably have nothing to do with Sophophora, but they will all be in the same genus according to this proposal. I think that moving those species to incertae sedis within the tribe or subfamily is a better plan. Switching the type species of a genus has been done very rarely, if at all, in the history of the ICZN. Clearly, no matter which option is chosen, well-studied organisms will change genera. Synonymizing Sophophora is not a silver bullet. This proposal might create more nomenclatural stability problems than would exist in the Sophophora melanogaster scenario.

I think I agree most with the position that Chris Thompson & the other editors of the BDWD have adopted: let the name changes go the way they would with any other lucid taxonomic paper, e.g. go ahead with Sophophora melanogaster. Then, see whether the broader scientific community accepts the name or not. To further editorialize, I think that if the change happens and you call Sophophora melanogaster 'Drosophila melanogaster,' no one will slap you in the face; it will be clear what you are talking about. I don't really understand how any information will be lost. S. and D. melanogaster won't refer to different species, one name will just be more correct than the other. I think one could go as far as to consider 'Drosophila melanogaster' a common name for the fly, just as Boa constrictor is the scientific and common name for that snake. To get even more snarky, there are no rules set up by a respected international committee of geneticists and developmental biologists that have formally decided that model organisms can never have their names changed. The ICZN, on the other hand, has established proper nomenclatural protocol. Of course there are some solutions that I have not discussed (PhyloCode anyone?).

This is a difficult problem, and deserves thought and input from everyone from entomologists to medical researchers to high school teachers. I am not decided for sure, and will continue to follow the situation. It would be great for you to read the proposal and the comments on the ICZN site and voice your opinion here or elsewhere.

Also, Happy Darwin Day!

2005: Oreoleptidae

Friday, October 24th, 2008

For those of you who have not heard of Oreoleptidae, particularly the American entomologists among you, this should come as a shock. The other new families of flies are from places like New Zealand and Chile, so perhaps some of you might say 'oh, these are super-rare flies from exotic locations, I would never see, hear, or collect these new families.' Oreoleptidae, however, is found in the United States of America. Specifically Idaho. It is also the only one of these new families for which the larvae are known. In fact, no adult has ever been collected in the field; imagines have only been reared from larvae.

The larvae of this singular family were collected in the Rocky Mountains from Montana and Idaho north to Yukon. They were tentatively assigned to Athericidae or Tabanidae by several experts but their formal description was held off until adults were reared. Zloty, Sinclair, and Pritchard described Oreoleptis torrenticola from material collected in Alberta, Canada, and gave reasons why it deserves a separate family level designation in Systematic Entomology (2005), 30, 248-266. The time it took entomologists to find and describe Oreoleptidae is fully attributable to its elusive, specialized biology.

Oreoleptis larvae are found in deep wells and very fast torrential streams. The larvae have durable cuticle but are flexible. They have 2 pairs of prolegs per abdominal segment and move quickly in their specialized habitat, preying upon mayflies nymphs and larval caddisflies. There are no thoracic prolegs, so the larval head is to the right in the photo below. The pupae have been found in sandy soil, gravel, and on foliage surrounding streams above the spring runoff high water mark. A new population was recently found in the Cascade Range in British Columbia. Nothing is known about the adult biology. They might be extremely short-lived as adults similar to other mountain stream specialists flies such as mountain midges (Deuterophlebiidae), but the presence of a proboscis (absent in deuterophlebiids) suggests otherwise. Why adults have never been found in nature is a mystery.

Oreoleptis torrenticola is dear to me because it is an outgroup in the molecular phylogenetic study of Tabanidae on which I am working with Shelah Morita and Brian Wiegmann. Elucidating the relationships within the Tabanomorpha is not the purpose of the study so I can't say much for certain, but Oreoleptis is decidedly not a member of Athericidae nor Tabanidae. This agrees with Zloty, Sinclair & Pritchard who placed it as sister to Athericidae + Tabanidae. For more details, please read Zloty, J., Sinclair, B & G. Pritchard (2005) Systematic Entomology 30:248-266. Thanks to Brad Sinclair for looking over this post. Check in for the 5 more new fly families described since 2002!

Adult Oreoleptis torrenticola
Adult Oreoleptis torrenticola . Note the odd humpbacked shape, wing venation, and antennae. From Zloty, Sinclair & Pritchard 2005. Thanks Brad Sinclair for letting me use this image.

Larval Oreoleptis torrenticola
Larval Oreoleptis torrenticola . The head is to the right and the prolegs are to the left. From Zloty, Sinclair & Pritchard 2005. Thanks Brad Sinclair for letting me use this image.