Diversity and the reason of their success.

Insects are an incredibly successful group, about 90% of all species are insects. About 1 million species of insects are described, but the total number of species is estimated to be between 2.5 and 10 million. To place this in perspective, the number of described chordates (vertebrates, of which we are 1 species) is about 43000. So there are about 20 times as many species of insects as there are chordates, and we are only talking about the number of different species! If we also look at the number of individuals of each species the total number of insects that walk the earth at any time is estimated at 10 quintillion (10,000,000,000,000,000,000). So that are a whole lot of insects!

But why are insects so incredibly successful? There is not one specific reason for their success; multiple factors are involved. One of the proposed reasons is the fact that insects are able to fly. Because they can fly they are very mobile and can easily reach different habitats. Once they reached a different habitat they can adapt to the local circumstances.
A different reason is their co-evolution with plants. Co-evolution means that when one species adapts, a different species (dependent on the adapting species) also adapts. As it is always the case with evolution, we are talking about variation between individuals. For example, if there is a large population of white flowers that are being pollinated by 1 type of butterfly, it is possible that by chance one flower has a mutation by which it start producing red flowers instead of white flowers. If there is also a butterfly, in a population of butterflies that prefers white flowers, which deviates from the rest of the population and prefers red flowers, this red flower and butterfly that prefers red flowers can co-evolve. After multiple generations this could lead to different species, 2 species of plants (with white flowers and with red flowers) and 2 species of butterflies (preference for white flowers and preference for red flowers). This example is illustrated in the figure below; of course the butterfly doesn’t have to be red in order for it to prefer red flowers but it I made it red for the illustration. Through this mechanism of co-evolution, the speciation events in plants also led to speciation in insects, so more species of plants means more species of insects.

There are more factors that can partly explain insect success. But one aspect that is different in insects when we compare them to their sister group the crustacea (lobsters etc.), is that in general insects live on land and with few exceptions crustacean live in water. The number of crustacean species is about 40.000. Still a lot of species, but compared to the insects there are very few species of crustacea. But how are insects able to live on land but crustaceans not? Very important in the transition from water to land was the development of a waterproof cuticle (insect skin). Changes in the composition of the cuticle provided insects with a waterproof cuticle that allowed them to explore terrestrial habitats without dehydrating. Because of the high mobility of insects they were able to move towards water to replenish the water they still lost. But the ability of adult insects to live on land is not enough. This is because when their eggs are not able to survive the terrestrial habitat, the adult insect has to move back to the water to reproduce. This would prevent insects from wandering very far into terrestrial habitats. This involvement of the insect egg in their success is exactly what I am researching. (also take a look at this article about the beauty of insect eggs and for some awesome pictures)

Insect eggs, the road to success.

Most insects are bad parents. Very different from us people, insects in general don’t show parental care. Eggs are deposited and the egg has to cope for itself from that moment on. Due to this lack of parental care insects are able to reproduce quickly, as soon as an egg is laid they can go on to lay the next one or produce more eggs. But of course the eggs have to be able to survive. As soon as the larva hatches from the egg it is able to move and so it can move towards water, flee from predators or hide, seek shadow and so on. But as long as it remains inside the egg it is vulnerable. To survive the terrestrial habitat the insect egg has several adaptations. One of these adaptations, the most beautiful one to me of course, is the formation of an extra-embryonic epithelium, named the serosa (extra-embryonic means that although it is developed by cells in the egg, these cells will form no part of the embryo itself). This epithelium is unique to insect eggs in that it completely enfolds the embryo. So it could potentially provide protection to the developing embryo inside. So this serosa is an epithelium (red in the figure below), it consist of a single layer of cells. These cells could theoretically produce compounds that could protect the egg. The goal of my research is to find out why insect eggs develop this layer and in what way this layer protects the embryo. Below you can see a picture of an embryo of a flour beetle (Tribolium castaneum) inside the egg. This picture is magnified, the real egg only measures about 0.5 mm in length. Next to this picture I drew a schematic representation of the same egg, in which you can see where the extra-embryonic serosa is located, and below that there is a picture of an embryo near the end of development.  This embryo was almost ready to hatch; you can still see the eggshell on the left.

Egg Embryo

The extra-embryonic serosa as protective barrier.

The question is, does this layer actually protect against anything? In short the answer is, yes. These serosal cells secrete a cuticle (insect skin) which resembles the adult insect cuticle. As described earlier, the cuticle protects against dehydration. I found that also the cuticle secreted by the serosa protects the embryo against dehydration. As soon as my research is published I will write more about this topic. This protection against dehydration ensures survival of insect eggs in locations where they would otherwise not be able to survive. Butterflies can lay their eggs on the underside of leaves without those eggs succumbing to dehydration and flour beetles can lay their eggs in dry flour without large numbers dying before they hatch. This resistance against dehydration of the insect egg opened up many new habitats for colonization, and this could partly explain the great success of the insect lineage. I also look at other protective aspects of the serosa but I will write about this in the future.

Why is this research so interesting?

A question that I get a lot when talking to people about my research is: “What is the use of your research?”. I could discuss the benefits of fundamental science vs applied science but I will not do that. Besides that it is very interesting to understand how a group as the insects could become as successful as they are and what role their eggs play in this story, there are other more practical matters at hand. The first one is this; the flour beetle that I’m researching is a pest species. As the name implies, the flour beetle eats flour. So do its larva. Flour infested with beetles and larva is not something you can sell and so it loses its value. So they, and other insects, cause for billions of dollars of damage every year on our food supplies. And as they say: “Know your enemy”. The more we know about why insects are so successful, the better we understand how we can fight them.
There is another aspect in which insects influence our lives. They (especially mosquitos) carry diseases like Malaria, Dengue, Yellow fever, and many more. The control of mosquito population size with pesticides has proven to be an effective method for reducing the number of people that fall ill with these diseases. But mosquitos gain resistance to our pesticides and we constantly have to find new ways to fight them. Also here, knowledge is power. The more we know about why they are successful the better we can fight them. (mosquito eggs are well known for their ability to withstand long periods of drought)

So, in a nutshell that is the type of research that I do. Do you want to know more about a specific subject or are certain things unclear? Don’t hesitate to send me a message at [email protected]