Cheshire

Google books is a wonderful thing. You can find pretty much anything on there, at least in part. Quite a bit of the stuff you’ll find is missing some pages, but it’s definitely enough to learn from…or at least find a title of something you’re looking for or interested in. Today’s example: Insect Hearing
. Seriously…give it a whirl.

A lot of insects use auditory cues to look for mates. Today, we’ll be taking a look at another function of those same ears…we’ll be looking at their use as host-location devices in parasitic flies. We’ll also be explaining what appears to be happening in this video:

I had originally intended this as an evolution post, but I’m not against turning it into an anti-creationism post and moving it up by a week or so if the opportunity presents itself. Strangely enough, today it has in the form of the perpetually vapid Ray Comfort. He has a post up at his blog which is genuinely interesting…it involves a military project looking at the structures in flies and trying to adapt them for military use. He concludes:

There is an alternative to the Air Force actually remaking the Fly Ear. Let’s play pretend for a moment. Imagine if they had the time to wait for evolution make one for them. I know that we are speaking of a huge amount of time, but let’s pretend anyway. After all, this is how evolution did it in the beginning.

Evolution made everything over millions or even billions of years, for no reason. But the Air Force Fly Ear has something more going for it than the process of evolution had in the beginning. The Air Force want it to happen, so that may help.

Imagine being a fly on the wall and watching when the process starts. I wonder how long it will take for something to begin, from nothing.

Here’s where evolution comes in. The ears of flies aren’t exactly a case of something from nothing. Nor did they happen without a reason. There’s a perfectly good evolutionary reason for them in parasitic Diptera.

Parasitoids detect their hosts in a variety of ways, and there are advantages to each way as well as disadvantages. Some look for host waste products and chemicals given off by plants being fed on by their hosts. Host derived chemical cues may be very reliable, but they’re not that easy to detect. Plant derived cues may be easy to detect, but they’re not very reliable. There are a variety of ways insects get around this, and one is by looking for mating cues.

Mating cues are both reliable and detectable for many species. The whole point of sending them out is so another one of your species can find you reliably. Sometimes, this invites some unwanted company in the form of predators or parasitoids. One very cool example of this is the evolutionary loss of stridulating organs in crickets in response to parasitoid flies.

Even though that’s not exactly what we’re talking about, it still serves as a good introduction. Crickets chirp. Go outside on a warm summer evening and this will suddenly become apparent to you. They’re doing this to attract mates. Different species of cricket chirp in a slightly different way and this attracts mates. The flies who parasitize them can use those chirps to find their hosts and can also differentiate between them using those chirps. In fact, scientists collect these flies by playing the mating songs of their hosts over a tape recorder. That’s what’s most likely happening in the video above…those flies are gravid females looking for a host to parasitize.

Here’s how they do this:

Lateral view of the head and the thorax of a female E. auditrix. The arrowhead points towards the ear, located directly behind the head. Note: I've slightly modified the picture from the original article. The black arrow was difficult to see in the black and white image, so I colored it red using MS paint.

It turns out that ears aren’t particularly difficult to evolve for insects. Many different groups have evolved ears many different ways. The mechanism I’m discussing here is probably one of the more common ways to do this. Most can actually pick up vibrations from the environment without any sort of specialized structure. There are pre-adaptational things which can pick up vibrations like the air sacs insects use to supplement airflow in their breathing apparatuses. Many orders of insects have their own ear designs which evolved independently from one another, this includes different groups of parasitoid flies.

You don’t actually need ears to feel vibrations. Sure, it helps…but many species of animals that actually have no sense of hearing and use other means to pick up vibrations from their environment. Scorpions, for example pick up vibrations from their substrate. In Drosophila some organs pick up wind vibrations which help them stabilize while flying.

Now…in response to Comfort’s post, there’s something vital he’s missing…go figure. Parasitoid flies didn’t evolve tympanic hearing membranes for no particular reason, they did it to move into uncharted territory. If you can get yourself into a nice ecological niche that doesn’t involve any competition, your reproductive capacity is gonna increase greatly. That’s why this system originally evolved…it’s there and it’s an untapped food source. Untapped resource means less competition. Less competition means your babies have an easier time making it to adulthood. That’s a damn good reason. It’s kind of like getting a new job that pays a lot without a whole lot of work being involved.

Let’s look at how really basic insect ears are set up for a minute. Again, MS paint is gonna come in handy because I don’t like the illustrations in the paper I’m discussing for this purpose. This is a very, very simplified version.

Teaching diagram designed rather intelligently by yours truly

A tympanic membrane captures vibrations from the air. These vibrations are transferred to an airsac, not unlike mechanical transmission of sound by our stapes to the vestibular window. From this airsac, these vibrations are transferred to a set of nerves where they transmit the information to parts of the nervous system where it’s integrated. The path of sound is signified by the bold black arrows.

Upon hearing the host’s attempts to get laid (some of these guys also parasitize cicadas…so it’s not just Orthoptera who are the victims here), the female orients her body towards the sound and starts to fly. Locating crickets or cicadas isn’t that difficult. If you give it a whirl on a summer day, you can usually find the males with only a slight bit of difficulty…only since we’re big they tend to shut up when we get close.

Anyways…the anatomy of a non-hearing fly surprisingly isn’t that different. They have organs called chordotonal sense organs which are kind of set up like ears but serve a different purpose. They detect vibrations (but not sounds) and also function like stretch receptors, telling the fly how far a certian part of the body has moved, what position it’s in…that kind of thing. Here’s a 20+ page paper which describes the organ in detail in locusts who have also evolved ears from this organ.

Now…right behind this organ just so happens to be an air tube in many species of parasitoid fly. Now, before the creationists start shouting ‘teh preadaptatunshunz meenz teh frunt loadinz’…you need to learn something basic about insect physiology. This is how they breathe. They breathe through air tubes which run throughout their bodies to various tissues which directly exchange gasses with the environment.

The ears in parasitoid flies, as most things are by evolution, are fashioned from bits and peices of parts already present. Just moved around a bit. A vibration receptor here, a stray air sac there…a few nerves to detect the sound…and you’ve got yourself a working ear.

I should note that the ear needn’t come up first in order for the parasitoids to parasitize singing insects. These guys also leave behind plenty of chemical cues for the flies to pick up on. Many parasitoids detect host vibrations made through normal activities (eating, moving, etc) as a final-approach stimulus but in many groups these cues aren’t very detectable over longer distances and thus chemical-based sensory is a much better way to detect their hosts.

Since the vibrations made by singing insects are quite loud, it’s very possible that they evolved to sense sounds by becoming more and more sensitive to vibrations and being able to sense these vibrations over a longer distance through co-evolution. Organs grew, shrank and gradually re-arranged themselves because those who were better equipped to sense vibration left more offspring and passed on the winning combinations for the most sensitive organs to their offspring so they could experiment through their own variations. Your standard evolutionary trial-and-error when it comes to trying out new features.

After any vibrational organ becomes sensitive enough, it will be able to pick up sound from the environment because that’s all sound is…it’s a wave transmitted through the air instead of a solid medium. The same basic equipment used to pick up vibrations in the environment can be modified to pick up sound.

R. Lakes-Harlan, H. St lting, A. Stump (1999). Convergent evolution of insect hearing organs from a preadaptive structure Proceedings: Biological Sciences, 266 (1424), 1161-1161 DOI: 10.1098/rspb.1999.0758