INSECT RESPIRATION

Last week near sunset, I saw a Great Heron and Snowy Egret patiently waiting for night to settle in.

INTRODUCTION

Because I take a holistic approach to my ecological research, I am always reading the latest literature on a wide variety of topics. Although I am familiar with a wide variety of animals, it is often a good to review the literature on their life history, anatomy and physiology. An example of this is the respiratory system of insects, particularly the giant green predacious diving beetle. Earlier thinking said that air flowed in and out of insect systems passively. Current research indicates that muscular movements have an active role in bringing oxygen in and expelling carbon dioxide.

Before I go further, let’s take a look at insect respiration. To understand this remarkable process, understand something about insect physiology, anatomy, and the process of diffusion.

Then I will move on to insect respiration and cover how the giant green predacious diving beetle (GPB) takes in oxygen and releases carbon dioxide.

INSECT ANATOMY OVERVIEW

First, although it is an over simplification, the job of the exoskeleton (the hard exterior) of insects and many others like them (i.e., crayfish and lobsters) is to protect a flexible membranous bag with organs, muscles, nerves, and body fluids in it. One thing all of these animals need to do is get oxygen in and release carbon dioxide to breathe.

Because of the exoskeleton cannot flex having lungs is not a solution. Some aquatic insects have gills when they are larvae and this solves the problem for them. However, adult predaceous diving beetles do not have this option. Instead, they accomplish this process through the process of diffusion.

DIFFUSION

Although our skin and other tissues appear impervious (solid as if nothing can pass through them) this is not true.  Membranes are layers of cells held together by different chemical bonds and it is possible for some molecules (i.e., carbon dioxide and oxygen) to get through.

You can visualize the mechanics of the process in this way. During parades, adults form a line along the route making it impossible for other adults to squeeze through in a civilized fashion (the membrane). It is common to see children (oxygen molecules) playing on the outside because they are disinterested in matching bands, fancy cars, or personalities. Hence, there are more on outside playing than along the parade route itself. However, as soon as fire engines or Smokey the Bear shows up, they scramble to get a closer look (their interest is the mysterious force causing diffusion). To do this, they slip through the crowd and end up front and then there are more along the parade route than behind the adults. This is because they are small and walk between the legs rather than forcing their way through the torsos; meaning they have diffused – sort of.

Something similar to this happens with the molecules I am discussing. To review, the wall of adults is the cell membrane and the children are molecules moving through a membrane.

Next, imagine a force around so that whenever there is the low number of the same kind of molecule on the inside of a membrane bag and a large number on the outside, molecules move across it to make the numbers equal.  It is the idea “moving down the concentration gradient”; moving from and area of higher concentration of molecules to one of a lower concentration of molecules; higher numbers to lower numbers molecules.

During insect respiration the number of carbon dioxide molecules continues to increase on the inside as muscles work. In fact, this is true for them as well as for us. At the same time, the number of oxygen molecules decreases as muscles use them. Therefore, on the inside of the number of carbon dioxide molecules remains high and the number of oxygen molecules remains low. In contrast, the number of oxygen molecules on the outside is high, and the number of carbon dioxide molecules is low.

INSECT RESPIRATORY SYSTEM

Now, that you understand diffusion all that is necessary is discussing and examining an example of insect respiration. This part is simple because there are spiracles (holes) on the outside of their body that allow carbon dioxide (CO₂) and oxygen (O₂) molecules to move in and out. On the inside are tracheas that are hard pipes of chitin for airflow and soft tissue ones, tracheoles, that for diffusion.

The system is effective, simple, and limits the size insects. Hence, the giant insects of horror films are a fantasy.

So, how does the predaceous diving beetle solve the problem, you ask.

A common way insects and some spiders store oxygen to dive below the surface is to carry a bubble of air. Many aquatic beetles have a hydrofuge pubescence, clusters of hairs on the ventral (underside) that repel water while allowing a bubble of air to build and exist. Because these beetles are glabrous, (almost hairless) as seen in the above photograph they use a different method.

The elytra is indicated by the letter "A"
and below it is the wing "B".

On the interior of the body just below the wing covers  (elytra) and wings is a tracheal system for storing air that connects to abdominal spiracles and the two most important are the largest ones at the posterior end. 

The red arrows indicate the spiracle locations. There are a total of twelve, ten smaller ones and the two important abdominal ones that take in air. When the beetle is underwater, carbon dioxide comes out the ten smaller spiracles and collects under the wing (B) and the elytra just above it. When the beetle comes to the surface, it shifts the elytra and carbon dioxide flow out. Because it move up the water column with the large anterior abdominal spiracles breaking the surface first oxygen diffuses in through them quickly. Note the hair like structures near these two large spiracles. They allow a bubble of air to form quickly and make taking it in easier.

When the beetle is under water, carbon dioxide migrates out of the spiracles into a space below the wing cover (elytra). This spiracle has an oil gland associated with it so water is repelled and cannot enter the respiratory system when it is submerged. To rid itself of the carbon dioxide and get oxygen the beetle surfaces with its posterior (rear end) facing upwards. As it breaks the surface of the water, oxygen floods in through the abdominal spiracles and a slight shift in the wing cover (elytra) releases stored carbon dioxide.

The red arrows indicate the spiracle locations. This close up depicts the fact the spiracles are more than just simple pipes that open to the air. In fact, beneath them is a complex network of tracheoles interwoven with organs and muscles. Because of this, it is likely that muscular movement during swimming and flight facilitates the movement of air in. Specifically, the flexing muscles, in turn flex these tubes. Therefore, they are likely to function in some ways like lungs.