Flying made insects more diverse

The appearance of insect wings represented an adaptive improvement in the evolutionary history of these organisms, since they allowed them to spread and diversify across all kind of habitats. It is precisely for these events that wings are very diverse organs which have undergone a lot of changes.

In the following article, I will talk about the appearance of wings as elements that have ensured the diversification of insects, and also about the evolution of these organs and about their subsequent changes.

Introduction

Insects form the most diverse and successful group among the current fauna, and they’re also the unique invertebrates capable to fly. Even though they almost haven’t change since their appearance during the Devonian era (395-345Ma), the appearance of wings and of the ability to fly (alongside with other events that took place at the same time) allowed them to diversify rapidly.

Timeline of geological eras. Hexapoda and also insects appeared during the Devonian era (Picture from buglady.org).

Nowadays, there are almost 1 million of species of insects identified, and it’s known that there are lots of them waiting to be identified.

When winged insects appeared?

As you probably know, not all insects worldwide have wings: there are apterous insects (that is, insects without wings), which form the Apterygota group, and winged insects or Pterygota (is interesting to say that some organisms of this group have lost their wings later).

The most ancient winged insect is probably Delitzchala bitterfeldensis, an organism from the Palaeodictyoptera group dated from early Carboniferous in Germany (50Ma after the appearance of insects during the Devonian era, more or less).

Approximated representation of a Palaeodictyoptera. In contrast with current insects, these ones had three pair of wings instead of only one or two (the first one was probably a couple of little lobes located near the head) (Picture from Zoological excursions on Lake Baikal).

However, the fossil remains of the most ancient insect known nowadays, Rhyniognatha hirsti (dated from the early Devonian in Scotland, which was found in the “Rhynie Chert” sedimentary deposit), which has no wings, reveal that this insect shares some traits with winged insects (Pterygota). According to this, the origin of insect wings could be more ancient (probably from the Devonian or even more ancient).

We are still far from knowing the exact moment when the appearance of winged insects took place. But, despite of this, we can affirm that the ability to fly allowed them to reach new habitats, looking for more and better food and also run away from predators more easily. These events have provided a huge evolutionary advantage to insects and allowed them to diversify.

How did wings appeared?

Discrepancies toward the origin and evolution of insect wings is not limited only to “when ” , but also “how”: How did they appeared? Which structures from ancient insects have been modified to become wings?

There exist 4 hypothesis that try to explain the way wings were formed from different ancient organs: branchial hypothesis, stigmatic hypothesis, parapodial hypothesis and paranotal hypothesis.

First of all, and in order to understand all these hypothesis way better, we need to know the basis of corporal structure of insects. Let’s see the body scheme of a cricket (Orhoptera order):

Body scheme of a generic insect. There are 3 principal segments: 1) Head, where central nervous system and feeding functions are located, 2) Thorax, which has a locomotor function (here we can find all the appendices, including wings in winged insects); it’s divided in three parts: prothorax, mesothorax and metathorax; 3) Abdomen, in this segment we can usually find all the visceral organs. Moreover, we can also find the spiraculi located at both soft sides of its body, that is, holes that connect with the tracheal system and through where the exchange of gases takes place (Picture from Asturnatura).

 

Representation of the tracheal or respiratory system of an insect. This system is branched into the organism (Picture by M. Readey, Creative Commons).

 

So now, which are these hypothesis?

1) Branchial hypothesis 

According to this hypothesis, ancient Pterygota insects were aquatic organisms that were derived from terrestrial insects that got adapted to live underwater. Those ancestors breathed, as current insects, through spiracles connected to a net of internal pipes or tracheas. During the adaptation process to aquatic environment, these insects developed branchial or gill sheets on those spiracles in order to breathe underwater. Then, when they migrated back from aquatic to terrestrial environment, these sheets lost their ancient function and became a kind of wings.

According to recent data, it’s considered one of the most plausible hypothesis.

2) Stigmatic hypothesis

In the thoracic region, that is, where legs and wings born, the respiratory spiracles tend to be closed. According to this hypothesis, wings could be tracheal pipes expeled to the outside of the body in the thoracic region.

3) Parapodial hypothesis

This is a very simple hypothesis: it tells us that wings were formed by modified legs.

4) Paranotal hypothesis

A few years ago it was considered the most  plausible hypothesis, but now it competes with the brancial hypothesis. This is the most accepted hypothesis about the origin of insect’s wings. According to this hypothesis, wings were formed by the expansions of the tegumentary membrane located at both sides of the body, that is, the space located between the dorsal and the ventral surface of the body.

The expansions are known as “paranotes” (these structures gave the name to the paranotal hypothesis).

Ancient vs modern: Paleoptera and Neoptera

Nowadays, mostly of insects presents only one or two pairs of wings located, respectively, in the mesothorax and in the metathorax (middle and posterior segments), and not three pairs, as ancient insects usually had.

The way the two pairs of wings are articulated with the thorax, together with their position, allow us to differentiate two main groups of winged insects or Pterygota: Paleoptera and Neoptera.

Paleoptera

Generally, the Paleoptera insects can’t fold up the wings over the abdomen (this is an ancient condition). Moreover, the two pairs of wings are similar both in size and function, and also in the disposition of the veins that travel under their surface. Inside this group we find organisms from the Ephemeroptera order (for more information, take a look to my article about bioindicators), from Odonata order and also from the Palaeodictyoptera group, now extinguished.

An specimen of Odonata with its four wings unfolded because it has no way to fold up them over the abdomen (Picture by Ana_Cotta on Flickr, Creative Commons).

Neoptera

This group contain the rest of winged insects. Contrary to the ones explained above, Neoptera insects possess articulations that allow them to fold up the wings over the abdomen. Moreover, their wings are not always equal , and they can develop another functions (and new ones as well).

The wings of many groups of Neoptera insects have undergone a lot of secondary modifications, which allowed flying insects to diversify even more. Next, I will talk you about these secondary modifications.

An specimen of Diptera with its wings folded over its abdomen thanks to their articulations (Picture by Sander van der Wel on Flickr, Creative Commons).

Secondary modification of Neoptera’s wings

Generally, one of the two pairs of wings assumes the flying function (the ‘main wings’) while the other pair subordinates to the main one. This subordination can be expressed in two ways: 1) without external modifications (the subordinated pair of wings is limited to assist the main pair during the flight), 2) with secondary modifications, so the modified wings assume a new function.

Some Neoptera insects have undergone drastic modifications in one of the two pairs of wings. Let’s see some examples:

COLEOPTERA (beetles): the forewings, known as elytra, are a very hard structures that protect the rest of the body when they’re folded up. In this case, the hind wings are the main ones, so they assume the function of flying.

An specimen of a longhorn coleopter taking off. In this picture we can appreciate the forewings transformed into elytrum and the hind ones assuming the flying function (Picture by Matthew Fang on Flickr, Creative Commons).

HETEROPTERA (greenflies, cicadas, bedbugs): the forewings, known as hemelytra, aren’t completely hardened as in the case of beetles: only de proximal part is hardened, while the distal part has a membrane texture.

An specimen of Kleidocerys reseda (Picture by Mick Talbot on Flickr, Creative Commons).

POLINEOPTERA: in both cases that I’ve explained above, the hardening process of the forewings entails the loss of their veins; in Polineoptera insects (for example, cockroaches), the forewings are harder than the hind ones, but they retain their veins.

An specimen of Periplaneta americana (american cockroach). Its wings are plenty of veins (Picture by Gary Alpert, Creative Commons).

DIPTERA and HIMENOPTERA (flies and mosquitoes; wasps, bees and ants): in this case, the forewings assume the flying function; on the other hand, the hind wings get reduced or modified, and sometimes they don’t appear. The hind wings of flies became equilibrium organs, the halteres.

An specimen of crane fly (Tipulidae). The halteres (red circle) are located behind the forewings (Public domain picture).

ALTRES MODIFICACIONS: we can also talk about the changes in the shape, color, presence of filaments or scales, or even about the variations according to sex, hierarchy or geography location (for example, thats the case of ants or termites).

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The origin and evolution of insect wings is still a fact waiting to be solved. Even so, independently of the moment and the way this event took place, is undeniable that wings have become key organs for the evolution and diversification of insects.

REFERENCES

• Notes taken from the subject “Biology and Diversity of Arthropodes”. Environmental Biology Degree (Universitat Autònoma de Barcelona).
• Rivas P., Martínez M.L. Paleontología de Invertebrados. Sociedad Española de Paleontología, Instituto Geológico y Minero de España, Universidad de Oviedo, Universidad de Granada. IGME, 2009.
• Information about fossils: palaeos.com.
• Vargas P., Zardoya R. El árbol de la vida: sistemática y evolución de los seres vivos. P. Vargas Gómez, 2012.

Top picture by USGS Bee Inventory and Monitoring Lab (Creative Commons).