Insect Flight Basics

Q:

What's the difference between direct and indirect flight mechanisms in insects?

A:

Insect wings are driven up and down by flight muscles located in the animal's thorax. There are two types of insect flight mechanisms, based on how the flight muscles transmit force to the wings. They are called direct and indirect flight mechanisms. In the direct flight mechanism, at least one power muscle connects to the wing…DIRECTLY! For example, a wing depressor muscle might attach to a special part of the wing near the wing base, and directly pulls the wing down when the muscle contracts. The direct flight mechanism is more primitive than the indirect mechanism, and so we find it on older insects (evolutionarily speaking), such as dragonflies and mayflies. Make no mistake, however – insects with the direct flight mechanism are not inherently worse flyers than those with the more advanced indirect mechanism. In fact, dragonflies are some of the most impressive fliers out there, due in part because their direct flight mechanism allows them to independently control each of their 4 wings. Check out this high-speed video of a dragonfly cruising around. (Click on the image to open the Quicktime movie.) These babies can turn on a dime!

In the indirect flight mechanism, on the other hand, the flight muscles invest their energy into deforming the insect’s thorax, which in turn causes the wings to move up and down. This mechanism in particular involves a pretty complicated hinge system at the wing base (see movie below), which scientists are still trying to understand. The majority of flying insects have indirect flight muscles, including butterflies and moths, beetles, grasshoppers and crickets, flies and bees. The big benefit of the indirect flight mechanism is that the thorax is able to store substantial elastic energy from each half-stroke (like a spring), and return it to help propel the subsequent half-stroke, in a process that involves mechanical resonance. Such a system makes flapping flight more energy-efficient.

Since I couldn't find any physically accurate representations of an indirect flight mechanism out there on the web, I went ahead and designed one using Google’s Sketchup application. Below is a (somewhat simplified) version of the indirect flight muscles at work in the thorax of a hawkmoth! (Click on the image to open the Quicktime movie.) Note that there are two opposing sets of muscles, the dorsolongitudinal muscles and the dorsoventral muscles, which contract alternately to deform the thorax, which drives the wings up and down.

Q:

But how can some insects, like bees and flies, beat their wings so quickly?

The current world record holder for wingbeat frequency, weighing in at a ridiculous 1,046 beats/second, is the mighty midge: Forcipomyia!
A:

The evolution of the indirect flight mechanism, with its mechanical resonance properties, set the stage for a further adaptation in the flight muscles of some insects: asynchronous flight muscles. In asynchronous muscles, the muscle fibers no longer require a nerve impulse for each and every contraction (as they do in typical synchronous muscles). Instead, a single nerve impulse elicits multiple consecutive muscle contractions. Insects with asynchronous flight muscles, such as beetles, bees and flies, have wingbeat frequencies that are no longer limited by the firing rate of nerve cells, and many of these representatives have incredibly high flapping frequencies as a result.

Q:

Ummmmm, what?

A:

Direct, indirect, synchronous, asynchronous, resonance, nerve impulse... Confused? Okay, here's an analogy that might help clarify things a bit. In this case, let's focus on the indirect flight mechanism, which can be either asynchronous or synchronous (the direct flight mechanism is always synchronous). The insect thorax can be thought of as something like a boxer's punching bag - one of those speed bags, for example. Just as the insect must alternately contract it's flight muscles to drive the thorax (and thus the wings) up and down, the boxer must punch the bag to launch it in motion. Both the muscles' contractions and the boxer's punches are considered the driving force of their respective system. However, the driving force is not the whole story in either case; both systems also exhibit mechanical vibration, with energy being stored and released during each stroke reversal to set up a natural oscillation. If the boxer repeatedly punches the bag at the right moment during it's back and forth oscillation, he not only maintains it's natural vibration, but actually amplifies it. Similarly, the insect's flight muscles contract at the right time in order to enhance the natural oscillatory motion of the thorax. This, in a nutshell, is what is meant by mechanical resonance in the indirect flight mechanism.

Taking the boxer analogy one step further, here's a stab at incorporating the synchronous/asynchronous issue...Imagine that the boxer must take slurps from his water bottle in order to gain enough, I don't know, hydration, in order to punch the bag. This would be analagous to the flight muscle receiving a nerve impulse (or action potential) in order to contract. In synchronous muscle, a nerve impulse is required for each and every contraction. This limits the rate at which the muscles are able to contract, because it takes time for the physiological processes associated with a nervous stimulus to run their course, and refresh for the next contraction...just as it takes the boxer time to re-slurp from his water bottle. Here's an animation of this (increasingly bizarre) analogy. Pretend the boxer is analagous to the dorsolongitudinal muscles, which are driving the downstroke in the insect. (For the analogy to be more precise, we'd need another boxer on the opposite side of the punching bag, modeling the dorsoventral muscles.) Click the image to view the movie:

Now, for the case of asynchronous muscles. Asynchronous muscles do not require a nerve impulse for every contraction. Instead, a nerve impulse now and then is all that is needed to keep these muscles contracting with exquisit regularity. How do they do it? Well, it turns out these special muscles are triggered to turn on by the very action of being stretched out. That is, give them a stretch (like the kind caused by the contraction of the opposing muscle set in the thorax), and they are somehow triggered to subsequently contract themselves, after a short delay, thus continuing the wingbeat cycle. You can see how mechanical resonance of the thorax plays a critical role in ensuring that this stretch/activation conversation between opposing muscle sets continues through multiple wingbeats. Scientists are still trying to understand how asynchronous flight muscles work, but suffice it to say, they are a pretty impressive product of insect evolution! Free from the physiological contraints on activation rate of synchronous muscles, asynchronous muscles can, and do, achieve ridiculously high wing beat frequencies. Here's the boxer animation again, this time modeling asynchronous flight muscles: