Figure 1. Complexities and interactions in insect rearing systems. This diagram is my effort to show some of the complexities that underlie insect rearing systems, including the image in the lower left showing the “Fly Room” used by T. H. Morgan and his now famous assistants. I have also included a gut of a lacewing adult, and the microbes from the gut of a termite (centre left)

In all my courses, I try to emphasise how we can use critical thinking and basic science to better understand all the components in our rearing systems and how these components interact to make the systems function. I designed the diagram to be complex-looking to emphasise the many, many intricacies of rearing systems, especially the many kinds of interactions between the numerous “moving parts.”

In this diagram, I tried to represent how we can zoom-in on facets of the rearing system so that we can understand them in a piecemeal fashion, which leads to an overall understanding. Philosophers of science might call this going from the specific to the general, which is characteristic of inductive reasoning. My approach to rearing systems is to explore relationships inductively to arrive at generalisations, and then use the generalisations to help us better understand and predict the specifics (= deductive thinking). This kind of thinking is not new to me; it was approached by such famous scientists as Claude Bernard (1813-1878) and the person who suggested the concept milieu interieur, which is the basis of our concept of homeostasis.

In the diagram in Figure 1, a relationship between metabolic rate–oxygen–tracheal diameter is shown. This concept is discussed in reference to a paper that I cited from VandenBrooks et al. 2018 who showed that in response to lower than normal atmospheric oxygen levels, Drosophila melanogaster developed larger, more complex tracheal systems (tracheole branching, diameter, number) and more mitochondria while in higher than normal O2 atmospheres, the insects showed the opposite trends. In the post where I first raised this issue, I suggested that this phenomenon could be taking place in other insects, and I further suggested that under rearing conditions where the O2 levels were not as low as the experimental values used by VandenBrooks et al., the same types of changes in respiratory organs and organelles may follow the same trend, but possibly to a lesser extent. I further provided data from some of my rearing observations where I showed that indeed, under commonly found rearing conditions, insects displayed lowered O2 and elevated CO2, though my observations did not find such low levels as those imposed by the VandenBrooks team.

This is where my point about critical thinking comes in. A published study demonstrated that a series of very dramatic changes took place in one insect species’ respiratory system morphology and ultrastructure. Does this mean that such changes take place in other insects (such as my wax worms or somewhat crowded painted lady butterflies)? The finding with D. melanogaster does not establish that the types of changes discussed here take place in my subjects or in the other insects that are routinely reared. Also, the extent of the hypoxia (low oxygen tension) does not necessarily extend to lesser hypoxic conditions. An important part of the critical thinking that I am encouraging in my courses is that rearing personnel are well-served to take what we know about insect physiology, biochemistry, genetics, ecology, microbial relations, etc. and consider how such basic science knowledge MAY apply to their insects in their rearing systems.

This approach encourages inquiry into the possibility that our wax worm larvae, Medfly larvae, naval orange worm larvae, or black soldier fly larvae are crowded enough that 1) the O2 in their rearing container is chronically and substantially under the 21% (closer to 20.85%) of outside air? 2) the response of the respiratory structures is an increase in tracheole structures and mitochondria is to increase significantly over that found in “normal atmospheres”)? 3) if there are changes in the tracheoles and mitochondria, does this affect the fitness or quality of the insects that we are rearing? and 4) does the change in fitness or quality adversely affect the purpose of our rearing these insects? Other accessory questions could be 1) does the diversion of biomass to tracheoles and mitochondria come at the expense of other structures or fitness characteristics (wing size, cuticle resistance to microbial penetration, muscle activity potential)? 2) If the oxygen (and CO2) levels are abnormal due to the crowding and/or the gas exchange between the rearing containers and the rearing room air, is it possible that other density-related factors are askew (excess nitrogenous waste, enhanced microbial growth, deteriorated diet components)?

Of course, no rearing facility has the capacity to do all the inquiries that would be needed to answer all these (and MANY MORE) questions, but doing some critical thinking and data based inquiry can be derived from this type of thinking and respecting the complexity of the insects in our rearing systems!

More about this in posts and pages to come!