Aquatic Crane Fly Larva
Yesterday, K T Cat raised some good questions about the digestive system of insects, gnats in particular, so today I thought it would be good to talk about that. To start with, we need an example, so here is one: a larva that we found in Cole’s Creek in April:
To be honest, when we found this I wasn’t even sure it was an arthropod. On the one hand, it sometimes moved by extending and contracting, kind of like a worm. On the other hand, it sometimes crept along using what appear to be feet, and it wasn’t quite clear whether the two appendages at one end were antennae on a head, or cerci on its tail. I couldn’t tell for sure at first which end was the head, it seemed willing to move either direction, although it seemed to prefer moving as if the end with the tendrils was the tail. Also, when disturbed it tended to lash back and forth vigorously.
When I posted it on BugGuide, the general opinion was that it was probably a larva of one of the Limoniid crane flies, although it is hard to tell with larvae. This was partly based on the fact that it had little legs on part of the body. Unusually, these legs were all prolegs, it didn’t have any visible true legs:
Anyway, it turned out that the head end was the end without tendrils, so at least that was settled:
So, these guys eat debris and fungus that they scrounge up from the bottom of streams. It has practically transparent skin, so we can see some things inside. The most prominent things are the digestive tract, which we can see because it is stuffed with food:
In general, the digestive tract of an insect goes
Buccal cavity (mouth) ==> esophagus ==> crop (rough equivalent of stomach) ==> midgut (where most digestion takes place) ==> hindgut ==> rectum ==> anus.
We can see some of that here, although the stretch from the mandibles to what is presumably the crop is invisible because it is empty. The stored food from the crop then enters the midgut to be enzymatically digested, and nutrients are absorbed. The “Malphigian Tubules” (basically the equivalent of kidneys) then inject various waste products into the hindgut, which then extracts most of the water and any residual nutrients. Finally, the wastes get fully dried to a fecal pellet in the rectum, and out it comes. We can also see some thin lines, that I think are maybe part of the nervous system.
Anyway, KT Cat’s questions were along these lines: 1) Since humans need about 20 feet of intestines to extract nutrients from our food, how do little insects (specifically gnats) manage to do it with maybe a couple of millimeters? 2) What is the benefit for things like fungus gnats to eat fungus, rather than whatever the fungus is growing on?
Taking the first question first, there are two issues here: relative surface-to-volume ratios, and diffusion distances. To start with, consider the digestive tract as a tube. If we take a cross-section of the tube, we get roughly a circle. As we decrease the radius of the circle (going from around and inch for a human, to less than a millimeter for an insect), the circumference of the circle decreases linearly, because it is equal to Pi times twice the radius. However, the area of the circle is Pi times the radius squared, so the area decreases much faster than the circumference does. I would estimate the radius of a human intestine as being about 30 millimeters, and the radius of even a largeish insect’s midgut as being about 0.3 millimeters. That means that the circumference of the insect’s gut is about 100 times less than ours, but the cross-sectional area is 100×100 = 10,000 times less, and so the ratio of gut circumference to gut cross-sectional area is 100 times larger than ours. Right away, this means that they can manage as much nutrient absorption as us with only 1/100th the gut length.
Of course, that would still seem to mean that insects need at least a couple of inches, but this is where diffusion comes into it. Let’s say we have a nutrient molecule in the center of the gut, and it needs to travel all the way to the gut wall in order to be absorbed. This motion basically happens by a combination of thermal vibrations and churning/mixing of the gut contents, and collectively is referred to as diffusion. Mixing is more important while the contents are very wet, but as they dry the mixing drops off and we are restricted mostly to random thermal motions. The point is, that the further sideways the molecule has to travel, the longer it will take, sometimes much longer. So, at the very least, reducing the radius by a factor of 100 also reduces the distance traveled by nutrients at least that much, so we get another factor of 100 improvement in digestive system efficiency. Combining the effects of smaller diffusion distances, and the much better ratio of surface area to volume, and we get insects with digestive tracts that are, conservatively, about 10,000 times more effective than ours for a given length. Which is why they can get away with essentially a straight pipe a few millimeters long, while we have this monstrous serpentine thing that fills most of our abdominal cavity.
Now, for the second question: why eat fungus? Well, mainly because the fungus tend to live on things that are practically indigestible by anything other than fungus. In particular, many fungus decompose cellulose, hemicellulose, and lignin, which make up a large fraction of plant matter and are practically impossible for animals to digest. Once the fungus gets at this stuff, three things happen: First, the miscellaneous proteins, sugars, and other good stuff that animals can digest, but were encapsulated by the indigestible parts, get released. Second, the fungus turns the indigestible things into highly-digestible fungus. And third, the fungus is itself soft and easy to eat, and causes whatever it is growing on to disintegrate into easily-eaten pieces as well. So, the relatively insignificant mouthparts of, say, a gnat larva are now able to actually ingest and eat the fungus, while they would have starved trying to carve indigestible chunks off of the substrate if they were trying to get along without the fungus. Really, what is going on is they are using the fungus as a method for external predigestion of food – they’ve effectively moved part of their digestive tracts completely out of their bodies.
 The original question was about gnats, actually, but crane flies are basically just really big gnats, so that should be OK.
 Of course, since this is an aquatic larva, it doesn’t have to worry much about conserving moisture, so I expect that it just kind of expells its wastes as a more-or-less liquid mass, not a fecal pellet.
 We humans can take advantage of this too, of course. As an extreme example, you can buy mushroom kits to grow mushrooms on water-soaked toilet paper rolls, which essentially allow you to successfully digest paper.