Origin of the insects

Crock · Newbie Joined: Apr 04, 2005 Posts: 1

In an interesting article about the discovery of Rhyniognatha, pushing back the origin of winged insects many million years.
This means the wings must have evolved very shortly after the ancestors of the insects adapted to land.
But isn't that just logical?
Crustacean epipodites are as we know dorsally located limb branches with respiratory and osmoregulatory functions, precisely the type of structures that would have given rise to insect wings. And the insect wings and legs have the same embryonic origin.

If wings evolved by modification of limb branches already present in multibranched ancestral appendages where some of them first functioned as gills, the epipodites and endites would end up as pretty useless rudiments once on the animals adapted to a terrestrial life style. Rudiments disappears, or at least are very reduced, if there are no use for them or they are in the way. The bristletails don't have them any more, except from the reduced limbs on the abdomen that are still used in locomotion.
So the early insects had to evolve the epipodites into wings before they became too reduced to be modified to something more than just rudiments or a remote memory.

Just a thought about the origin of the closing apparatus in the insect trachea. The main rule about primary aquatic animals with gills is the fact that they are still breathing with gills when they are adapting to breath in air instead of water. We see this in horseshoe crabs and crayfish, crabs and so on. To protect the gills they are covering them inside a chamber, where the evaporation also is reduced as much as possible. In scorpions and spiders the book gills have been modified into book lungs and are inside a chamber, and in some spiders and most mites the book lungs are lost and replaced by trachea.

When the ancestors of the insects started to live on land, they still had branched appendages with gills. As time passed by, the gills adapted to take oxygen from the air by withdrawing into a depression in the body wall by the basis of the appendages (where the gills already were). This depression evolved into a chamber with just a small opening, a gill chamber.
The gills inside the chamber were reduced because the thin walls surrounding them were able to take care of the gas exchange themselves. The chamber was elongated and branched into a lot of tubes stretching inside the body, reaching every part of the body tissues. The heart was no longer needed for oxygen transport. With an oxygen supply directly to the cells, a small body and an atmosphere much more rich in oxygen than today, the animals could become much more active, improving their biochemistry, senses and anatomy.
Another thing which must have happened when the basis of the limbs were withdrawn a bit into the body, the branched appendages became separate units, free to migrate away from one another. Just like when you pull your hand up your sleeve till just the fingers are sticking out. But because the insects don't have an internal skeleton, there is no bones conncting them, making the appendages near the basis free when they "met the wall".

As mentioned there were still some epipodites and endites at the basis of the appendages. Two pairs of them (or maybe even three pairs in the beginning, one for each segment) on the thorax evolved into wings. The others disappeared except an already reduced pair laying close and opposite to each other near the opening of the trachea. These were modified to a closing apparatus, making the insects able to close their spraicles in contrast to arthropods like millipedes and centipedes.

(Or maybe they evolved in another way .The spiracles were surrounded by muscle tissue from the multibranched appendages. By tighten some of this tissue, the spiracles could be closed, and then prevent the body to lose too much water or drown if they was surrounded by water. Some if this muscle tissue specialized to close the spiracles and tighten the trachea to make them pump air in and out. The soft external part of the spiracles were sclerotized to make them even more effective.)

Later the spiracles migrated some, so they were less in the way for the legs and the wings.

I guess the only way to find out is to study the embryonic origin of the closing apparatus of the spiracles. If they evolve from the same cell clusters as the legs end wings, they are probably modified epipodites.

When insects could close their spiracles, some of them could return to the water and evolve secondary gills by modifying the trachea system.

The special form of paedomorphosis where some parts of the body are growing faster than the rest and some part slower than the rest (post-displacement), may have been the key to the separation of the insect body into a thorax and an abdomen. This is most probably why the Tyrannosaurs had such small arms and big heads. If this occurred in the early insects, it would explain why the segments and appendages (including the epipodites) on the thorax was so well developed and the abdomen less pronounced.

Some thoughts about how the insects evolved when the trachea and spiracles had come into being.

The first insects did not copulate, the males just produced spermatophores which they then left behind on random, hoping the females would pick them up. And when the female had laid her eggs (on land), she would not die, but continue to live on to have more offspring later in life.
Archaeognatha was the first to branch off, and then the Thysanura. They are both still reproducing by using spermatophores, so this is most likely how the ancestors of the winged insects were reproducing too.
But even if Archaeognatha and Thysanura are the most primitive, their evolution have obvious not stood still. They have lost the branched appendages, maybe even before the winged insects.

First of the two main theories about how the wings evolved is that it happened on the water surface, the other that the wings evolved by gliding among early trees.
Why would early insects adapt to a life on water? Maybe because of mats of algae, spores from the primitive plants that covered the surface, small animals like collembola and to live among and on the water plants. In surroundings like the ones the raft spider is living in. This would explain why the limbs on the thorax would have to be long to lift the abdomen up in the air. A silverfish would not be able to run on water. The epipodites on the appendages, with their own set of muscles, could be used for water skimming. The problem is why they would start to fly. Real flight demands much more energy than just sailing on the water. The first primitive flight could not have been much faster than water skimming either. It does not make sense to start flying when it was possible to do the same task with much less energy. Maybe there was a carnivorous relative chasing them into the air, and then followed them into the air themselves? Or maybe the Neoptera evolved on water and the Palaeoptera among trees?
The water skimming we see in stone flies seems to be more like a secondary feature than a primary one. To live on water you have to be very small, and the first insects were pretty big (of course the first winged insects could have started out very small and then grow larger later).

But even more probable is it that the wings evolved by gliding among the vegetation on land or in swamps. If there is few arthropods to adapt to a life on the water surface, there is plenty of species to evolve to a life in high vegetation. There they are safer from the larger predators, they can eat from the plants, smaller animals, the cover of small micro-algae on the tree trunks, the sporangium and so on. The first insects were opportunistic, as we see among all the most primitive insects living today.

It is evidently that it would be much more effective to glide among the vegetation than to waste energy by walking up and down. Some rodents and marsupials have figured it out too, and they will for certain not be the last. And when they already had the potential with the plate-like appendages above their legs, the protowings, is was only a matter of time. First functioning as parachutes to damp down the fall when they jumped, and later adapted to gliding.

If it was two or three primitive wings to start with I'm not sure about, but only two pairs are much more easy to control and use than three, so it is no wonder one of the pairs disappeared if they really had an "extra" pair.

When they were gliding through the dense vegetation it was important to navigate between the plants, to suddenly interrupt the flight for some reason (they spotted an enemy or they decided to land on a tree some meters closer) and to land. They improved these abilities to the point when they one day "discovered" if one pair of the wings was beating downwards when they were in a certain position, and the other pair were on their way up when twisted in a certain position, real flight would be possible.
And from there on it continued to evolve and improve over the generations. In a world with no other flying animals, there is no doubt true flight would be of big importance for those who made it to the air, even if the first wing flight was primitive and weak compared with many of the insects living today.

All this happened on land, but later many would return to water. The first and most primitive among these (that are still alive today), are the Palaeoptera and the stone flies.

As you know, Palaeoptera are special in many ways. Mayflies are most primitive. The nymphs looks like aquatic Thysanura. And of course we have the sub-imago.
Why they returned to water is not known, but suggestions about competition in terrestrial environments seems likely.
Where did the gills in mayflies come from? Their gills are unique among aquatic insects.
All the members in Neoptera have tracheal gills that looks like threads and tufts, outgrowths from the body, growing from the thorax, near the basis of the appendages, on the head and jaws in stoneflies, directly from the abdomen or the insects are simply breathing through their skin. This is without a doubt secondary evolved gills. And very different from the gill plates we see in Palaeoptera.

The gills in mayflies have perhaps evolved from the abdominal appendages (styli), appendages that are lost in all others insects with wings. When their primitive ancestors returned to the water, and still had styli on the abdomen, they would have appendages that were free to be flattened and used as gills since they had no other important function. And these gills would be free to migrate away from away from the belly to a more dorsal position.
But early mayflies had gills even on the abdominal segments where the genitals are found, which are modified styli. So the gill plates must have evolved from some other kind of appendages.
As mentioned earlier the epipodites and endites on the branched legs in crustaceans where the gills were attached, became separated units when the adapted to land and evolved trachea. On the thorax the lower appendages became the legs, and the upper ones became wings.
On the abdomen the appendages became reduced, but they did not disappear, and as you know we can still find styli among the bristletails and silverfish. The other branched parts of the appendages are gone however. This may not have been the case with the early ancestors of mayflies. And if these appendages already had a flat and wide shape, it would be hard to avoid them from absorbing some oxygen from the water even if they tried. The next step was to make these plates bigger and more efficient water breathers. If this is the origin of the mayfly gills, then they are truly primitive.

When the carnivorous ancestors of the Dragonflies and Damselflies returned to water, they had already lost these appendages, so if they would evolve some gills they had to come up with something else. They could have done the same as the stoneflies and other aquatic insects; grow some threads or tufts from the body wall, but instead they modified some other parts of the abdomen that were no use for the nymphs; the cerci and an unsegmented tail filament in the middle. They became flat and bigger, somehow repeating what happened in mayflies. These two cases in the Odonata and Ephemeroptera are the only examples I am aware of that have evolved plate like gills from abdominal appendages. Making them even more special.

Another trait among insects where the nymphs have adapted to a very different environment than the imago, is the habit of first reproducing and then dying shortly after. A secondary specialization.
Cockroaches and other primitive insects that lives their whole lifes in the same surroundings as both nymph and imago do not usually die after mating. This is yet an indicator that the terrestrial environment is the primary one among insects.

Also the stoneflies and other aquatic insects can move their gills back and forth, but they are using other muscles than the insects with plate like gills. And the abdominal legs in caterpillars are also evolved after the styli disappeared, so the ability to move the gills back and forth is no confirmation that the insects evolved directly from aquatic animals.

Some more comments about the Odonata.
When the dragonfly nymphs started to pump water in and out from their hind gut to move quickly by jet propulsion, they could also take up oxygen from the water in the gut, and the gills on the abdomen became redundant and vanished.
Their unique labial mask evolved very early, but most interesting is the way they copulate.
All the other winged insects have a direct sperm transfer. Their ancestors on the other hand, reproduced by males placing the spermatophores on the ground. And they were moest likely still doing this even after the wings had evolved.
Among the early Odonata the male placed a spermatophore on the ground and then grabbed a female's head and dragging her over the sperm packet. Later they started to attach the spermatophore on their own abdomen, a much safer place for the sperm, yet they kept the habit of grasping the female's head when mating. This penis on the anterior part of the abdomen are as far as I know not found in other insects, probably making them the only pterygote insects with primary appendages on the abdomen that are not (the usual) genitals or cerci.
If the Odonata males copulated the same way the other pterygote insects does, why would they start placing a spermatophore on their abdomen? The most credible explanation would be they never copulated the same way the other insects does. There would be no reason for them to start mating in a different way if the first one worked fine. Just like there is no spiders that have evolved a penis instead of using their palps. Once a penis has evolved and are used for mating, animals rarely or never evolve a substitute for this.

The other insects probably did much the same thing as the Odonata, but instead of grabbing the head of the female, they grabbed her the other way round, from behind. From there on it would be a short way to evolve a direct sperm transfer. The genitals used for this are much the same we see in the Odonata when they are placing the sperm packet under their abdomen. Why? Maybe because the genital opening in males was surrounded by the same styli and had the same potential to evolve into a penis. In Odonata the styli evolved to a tool used for placing a spermatophore under their abdomen, while it in all the other winged insects evolved to fertilize the females directly.

This can mean one of two things.
If direct sperm transfer (directly from the male to the female without being attached to the male's abdomen first) evolved only once, Odonata are the oldest flying insects, they just lost the subimago and most of the other abdominal appendages earlier than the rest of the pterygote insects. Just like Chondrichthyes evolved advanced characteristics like inner fertilization long before the Actinopterygii, even if they are considered more primitive and ancient as a whole.
Or it means that direct fertilization evolved two times independent of one another among pterygote insects. Once in the Ephemeroptera and once in the Neoptera.

Either way, Ephemeroptera and Odonata are very old.

Who are the oldest ones among the Neoptera? Maybe stoneflies, but they have none of the ancient signs we see in Ephemeroptera and Odonata. Then why are they being studied to find out how the wings evolved if the ancestors of Orthopteroidea were terrestrial? Maybe because their wing muscles are very primitive and so are the rest of the animal's body plan. But it does not mean the stoneflies are primary aquatic animals and the wings evolved from gills, particular not from the gills we see on stoneflies. It is more likely the insect wings, legs and the gills (only in mayflies) evolved from the same structures, and the original gills gave rise to the tracheal system we see in most insects today (except from some insects like a few species of silverfish that have lost their trachea and are breathing through their skin), just like the book gills gave rise to the book lungs and trachea we see in spiders today.

But as the new fossils are telling, all this must have happened very early in insect evolution, shortly after they adapted to a life on land.