How and Where Insects Live

a. Ecology and life cycle
Insects are incredibly important due to their large biomass and making up more than half of the world’s discovered species. This means that their ecology is very important with regards to the environment and how they interact with it. Insects are able to inhabit most biomes and can be found on every continent in the world. Insects often act as pollinators; bumblebees can detect the presence of the atmospheric electricity on nearby flowers, from which they can determine if another bee has pollinated that flower already (Clarke et. al, 2013). Many insects consume large amounts of leaf litter, rotting wood, and feces, making them invaluable for waste reduction. Some insects, such as the Dung Beetle, not only eat feces but also lay their eggs in it. The ability to find a home in feces might not sound appealing to us but it is this knack for surviving anywhere and everywhere that has made insects so evolutionarily successful. Insects also have a very important relationship with humans. We compete with insects for food, as insects consume about 10 percent of the food we produce. On average, insects infect one in every six people with a pathogen (Speight et. al, 1999).
Holometabolist insects follow a lifecycle of egg to larvae to pupae to adult, while hemimetabolist insects develop from an egg to a nymph to adult. One example of hemimetabolism is in the Large Milkweed Bugs.

 

Large Milkweed Bugs (Oncopeltus fasciatus) exhibit incomplete metamorphosis. The nymph, or younger bug, resembles the fully grown adult only smaller in size.
Insects may pass through seasons in the egg stage; some grasshoppers have been known to pass through summer droughts in the eggshell, becoming dry and shriveled, and resuming development once moistened (Insects, 2015). Some may pass the winter in a diapause, a specific type of dormancy, in their egg and resume after the winter (Insects, 2015).

A zoomed in view of a cricket egg. Once the egg hatches a nymph will emerge that will grow and look similar to an adult cricket, only miniature. They go through incomplete metamorphosis.
After a holometabolist insect hatches, it enters the larval stage. Throughout this stage a gland in the larvae’s brain secretes neotenin, or juvenile hormone, which delays the metamorphosis from larva to pupa, the pupa developing when the amount of neotenin secreted is at a minimum (Insects, 2015). Similarly, neotenin also acts to prevent the metamorphosis from nymph to adult. As long as neotenin is present in the blood the molting cells lay down a larval cuticle, and in the final larval stage, neotenin is no longer produced, allowing the transformation from nymph into adult to begin (Insects, 2015). This allows for evolutionary success because many insects are able to inhabit more than one niche throughout their lives.

b. Throughout evolutionary time up to extant insects

The oldest known insect fossil, Rhyniognatha hirsti, dates back to the Devonian period. In many insect fossils, evidence exists of their feeding behaviors, such as feeding damage on vegetation and wood, giving clues to their habitats and food sources (Grimaldi and Engel, 2005). Insects have always been able to rapidly adapt to environments and resultantly can fit into many niches. Insect evolution is also closely tied to the evolution of plants, with up to 20% of extant insects relying on flowering plants, pollen, or nectar for food (Insect Evolution, 2007). Insects have not left a particularly strong fossil record, because with the exception of in amber, insects are mainly terrestrial and only preserved
in specific conditions on the edge of lakes (Insect Evolution, 2007)

A 24 cm. Madagascar Hissing Cockroach (Gromphadorhina portentosa). Cockroaches are a great example of an insect that has changed relatively little since they first started appearing.

Remarkably, throughout time, insects have changed relatively little in that an entomologist today could likely identify many species buzzing or flitting around in the Jurassic period. Certainly it is millions of years of success that have prompted many to refer to our age as the age of the insects (Insect Evolution, 2007).
Extant Diversity

a. Abundance and Distribution
Due to the overwhelming success of hexapods in an evolutionary sense, extant or living hexapod abundance is astronomically high. Their high reproductive rates combined with their high variety make them present in extremely high numbers that are almost too large to comprehend. Some studies have shown that in one square yard of soil, up to 2,000 different insects can be found. This means that in just one acre there can be about 4 million different insects. Not only that, but hexapods have adapted to even the most extreme environments that range from deserts to the coldest mountains (Insect, 2015). One member of this extant group is the Australian Stick Insect, a bug that is extremely well camouflaged in its natural environment.

A female Australian Stick Insect (Extatosoma tiaratum) hanging from a small branch. The unique outer layer of the body serves to camouflage the insect.

A zoomed out view of the Australian Stick Insects hanging from small tree branches.

Another extant member is the Praying Mantis, an insect that exhibits many of the defining traits of the subphylum hexapoda.

http://youtu.be/MC3inhf3CL4

A praying mantis (Mantodea burmeister) shows off his mouth parts and movement of his compound eyes.
The subphylum hexapoda can be broken down into various clades. First, there are the basic insects known as Class Insecta. This group makes up most of the winged insects that have been studied. According to Dr. Terry Erwin, an entomologist at the Smithsonian museum, this class may make up nearly 90% of all documented species on the planet (Erwin, 1982). Global populations of extant insects are estimated to range from 4 to 6 million different species (Novotney et al., 2002). Members of Class Insecta have also been found to be very adaptable, as shown by the fact that they are found in nearly every freshwater or terrestrial habitat. Some members have even found to be living in marine environments, though this habitat is primarily found to be home to crustaceans (Myers, 2001). There are also 3 groups of wingless insects or Class Entognatha: Collembola, Diplura, and Protura. Collembola, or springtails commonly, are the largest group and can be found essentially anywhere where soil or soil related habitats are found (Ponge, 1993). Populations of over 750 million individuals have even been found in certain grassland regions (Collembola, 2009). The groups Diplura and Protura are still abundant, but not nearly at the scale of Class Insecta. According to Dr. John Meyer, Diplura have around 800 species recognized and Protura have around 500 species recognized. Diplura are also common in grassy areas, but due to their small size they can sometimes be overlooked, meaning species estimates might be lower than reality (Diplura, 2009). Protura are less commonly found and can usually be associated with soil habitats that are damp or moldy (Protura, 2009).
b. Economic Importance
Hexapod insects can have a profound impact when it comes to the human economy. One of their biggest impacts come from natural resources. Insects can be used as a food source or can help produce other products. These products include can includes dyes and waxes, but the two primary products used by humans are honey from honey bees and silk from domesticated silkworms. Honey bees are also important due to their previously mentioned ability to pollinate plants, including human crops. Certain hexapods also have the ability to damage human products, such as wood homes that can be destroyed by insects like termites or furs and skins that can be broken down by insects like tineid moths. Hexapods also have the ability to help crops by assisting in pollination, or competing with humans by devouring said crops due to the herbivorous nature of most insects. Hexapods have a very important role in the medical community, due to the fact that many insects can be vectors, or carriers, for harmful microorganisms that causes diseases. Some of these diseases include malaria, yellow fever, sleeping sickness, and typhoid (Insect, 2015). This makes hexapods and other insects a large focus in biological medicine in order to help prevent such tragic illnesses. On that note, certain insects may also be used to assist in genetics research as well. The common fruit fly (Drosophila melanogaster) is one of these organisms. Due to its short generation time and high rate or reproduction, the fruit fly is considered to be a model organism that is used to study links between genes, evolution of behavior, and development in eukaryotic organisms (Pierce, 2006). Finally, insects can be a food source for human populations. Due to their high protein amounts, some believe that insects may have a high potential to be used in human diets, although diets that include insects are seen as abnormal in some higher developed countries (Gullan, 2005).

Conclusion

As our blog shows, hexapods are more than just the creepy crawly critters you squish under your feet. They have a rich evolutionary history dating back as far as 412 million years ago. Being one of the first insect groups to inhabit terrestrial habitats, hexapods served as a very important role in selecting land based plant life. Hexapods have many interesting characteristics including, the hexapod bauplan, wings, metamorphosis, and highly developed sensory systems.
If you still are not convinced that hexapods are a fascinating taxa of animals, don’t forget they are one of the largest and most diverse groups of animals on the planet. There are over 750,000 species of these guys! Hexapods ensure the success of many ecosystems, and we benefit from a lot of the ecological work they do. If you have ever stopped to admire a beautiful flower, chances are you have a hexapod to thank. Don’t worry if it is still hard for you to see insects as friendly little helpers, but we hope this blog gives you the insight to appreciate how amazing and important they are for our planet.

Works Cited
Clarke, D.; Whitney, H.; Sutton, G.; Robert, D. (2013). “Detection and Learning of Floral Electric Fields by Bumblebees”. Science 340 (6128): 66–9.

Erwin, Terry L. (1982). “Tropical forests: their richness in Coleoptera and other arthropod species”. Coleopt. Bull. 36: 74–75

Grimaldi, David; Engel, Michael S. (2005). Evolution of the Insects. Cambridge University Press. ISBN 0-521-82149-5

Gullan, P.J.; Cranston, P.S. (2005). The Insects: An Outline of Entomology (3 ed.). Oxford: Blackwell Publishing.

Insect. (2015). In Encyclopædia Britannica.

“Insect Evolution”. Virtual Fossil Museum. 2007.

Meyer, John R. (2009). “Collembola”. North Carolina State University.

Meyer, John R. (2009). “Diplura”. North Carolina State University.

Meyer, John R. (2009). “Protura”. North Carolina State University.

Myers, P. 2001. “Insecta” (On-line), Animal Diversity Web. Accessed April 20, 2015 at http://animaldiversity.org/accounts/Insecta/

Novotny, V., Basset, Y., Miller, S. E., Weiblen, G. D., Bremer, B., Cizek, L., & Drozd, P. (January 01, 2002). Low host specificity of herbivorous insects in a tropical forest. Nature, 416, 6883, 841-4.

Pierce, B.A. (2006). Genetics: A Conceptual Approach (2nd ed.). New York: W.H. Freeman and Company. p. 87.

Ponge, Jean-François (1993). “Biocenoses of Collembola in atlantic temperate grass-woodland ecosystems” (PDF). Pedobiologia 37 (4): 223–244.

Speight, Martin R.; Hunter, Mark D.; Watt, Allan D. (1999). Ecology of insects: concepts and applications (4(Illustrated) ed.). Wiley-Blackwell. p. 350.

Introduction to Hexapods

 

Welcome to the Hexapod Blog! What really is the story behind those creepy crawly critters? They may not be as disgusting and scary as you think! Hexapods have a rich evolutionary history and are one of the most diverse groups of animals on earth. Follow us as we dive into hexapods, and learn all about their biology. Maybe before you squish one under your shoe, you will remember how awesome and beneficial they can be!

 

We have everything you need to know about hexapods right here! First, we provide some history. How long ago did they first appear? What does their family tree look like? We have all heard of dinosaur fossils, but what about hexapod fossils? In fact, hexapods do have their own fossil record which we also discuss. Second, we dive into what makes a hexapod a hexapod. The insect body plan, wings, wing folding, and complete metamorphosis are the main key evolutionary innovations we focus on. For instance, the mealworm beetle, Tenebrio molitor, undergoes complete metamorphosis changing from a mealworm larvae, pupa, and finally an adult beetle. The field cricket, Gryllus pennsylvanicus, is a good example of the insect body plan. They have a clear head, thorax, and abdomen. Of course it would not be a hexapod if it did not have six legs! The Madagascar hissing cockroach, Gromphadorhina portentosa, is one such critter having three sets of legs. Find all this and more in the Hexapod Blog!

Phylogeny

 

There are four different supergroups that help to classify Eukaryotes and each of their lineages. These groups are known as the Chromalveolates, the Excavates, the Archaeplastida, and the Unikonts. Hexapods can be found later down the Unikont lineage (Tree of Life Web Project, 2002).

 

The Unikonts split into various lineages, including Fungi, Amoebozoa, and others. However, Hexapoda is found in the Animal lineage. The lineage of Hexapoda continues to another group deemed Bilateria that includes most groups that are considered to be animals (Tree of Life Web Project, 2002). Scientists have determined that there are two separate lineages that make up Bilateria, which are Deuterostomia and Protostomia. Deuterostomes and Protostomes are defined by their embryonic development. Deuterostomes develop their anus before their mouths, and Protostomes develop their anus after their mouths. Scientists now recognize two lineages within Protostomia that are Lophotrochozoa and Ecdysozoa (Halanych et. al, 1995). Hexapods can be found in the Ecdysozoan or molting insect lineage under the phylum Arthropoda (Tree of Life Web Project, 2002).

 

Hexapoda are traditionally shown to be monophyletic or descended from one evolutionary group. They are one of the most diverse groups, with over 750,000 species described so far (Tree of Life Web Project, 2002). Recent evidence suggests that the closest relatives to Hexapoda are Crustacea (Giribet et. al, 2001). While Crustacea are dominant in aquatic environments, Hexapoda are dominant on the land. The Hexapods split into lineages that include Diplura, Insecta or the true insects, Protura, and Collembola or the springtails (Tree of Life Web Project, 2002). Of each of these groups, Diplura is seen as the most unstable and it’s place in the phylogeny of Hexapoda changes often (Tree of Life Web Project, 2002). A final taxonomy of insects would be ordered as Eukaryotes, Unikonts, Animals, Bilateria, Ecdysozoa, Arthropoda, Hexapoda.

Phylogenetic tree of Hexapoda. Created using Class Notes and J.E. Blair, 2009. Created by Daniel Pinto

 

Fossil Evidence

 

Hexapods are one of the earliest known lineages of terrestrial based insects, dating all the way back to the Early Devonian period about 412 million years ago or possibly even to the Late Silurian period (Misof et. al, 2014). These early fossils are that of Rhynies (Rhyniognatha hirsti), or springtails (Collembola). Some basal insect fossils could date to 379 million years ago, however the fossil evidence is sparse and therefore scientists are unable to say definitively what lineage these fossils are from. However, fossils of other early insects include those of bristletails (Archaeognatha) that date to about 390 million years ago (Engel et. al, 2004). Most of this evidence suggests that insects were one of the first terrestrial organisms and were major selectors on land based plant life (Engel et. al, 2004). Fossil evidence of key structures have helped scientists to determine if such structures allowed for the massive speciation of the Hexapod lineage (Nicholson et. al, 2014). The fossil record can be a good way to show early existence of particular groups and can help to infer earlier origins if some groups are more diverse or abundant than others (Thomas & Ware, 2011). However, the fossil record is not as complete for certain groups of Hexapods, thus making it not as reliable as other dating methods (Thomas & Ware, 2011).

Molecular Clock

 

Unlike the fossil record, molecular clock evidence can be seen as more reliable in some cases. Using genomes of living Hexapods, estimates for divergence dates of certain groups can be found even in the midst of a sparse fossil record (Thomas & Ware, 2011). In a study on the evolutionary history of insects, molecular clock data led scientists to conclude that Hexapods may have originated earlier than the Silurian period in the Cambrian or Early Ordovician periods (Misof et. al, 2014). These results have been controversial, as there are little to no actual fossils of Hexapods from the Cambrian to the Silurian periods (Misof et. al, 2014). Indeed, some scientists have hypothesized that Hexapod evolution happened drastically earlier than paleontological evidence based on molecular evidence (Thomas & Ware, 2011).

 

Though there are benefits to using molecular clock data, there are also drawbacks as well. Using molecular clock data can help to fill gaps in the fossil record, but data can only be obtained from living or recently extinct species. This is due to the fact that molecular clock data relies on using DNA sequencing to infer genetic changes over time, but genetic material cannot be obtained from ancient species (Thomas & Ware, 2011). Despite these limitations, molecular clock data has supported the idea of Hexapods being a monophyletic group, and even helped to confirm previous hypotheses about the close relation between Hexapoda and Crustacea (Misof et. al, 2014).

Key Evolutionary Innovations of Hexapods

 

Hexapods are one of the most diverse classes in the animal kingdom. In fact, Hexapods alone make up over half of all recorded species. With such diversity and success, you can bet there a few innovations that have warranted their success.

 

1. Hexapod bauplan: The importance of the bauplan, or body plan, of Hexapods cannot be overlooked when it comes to contributing to their success. Hexapods exhibit metameric, or repeated, segmentation and have one pair of appendages per segment. These segments are organized into three tagmata, or specialized segments: the head, thorax, and abdomen. Their appendages are jointed, which aids in walking, swimming, and feeding, similar to the joints in our bodies. Hexapods also have exoskeletons, meaning their skeletons are on the outside of their bodies, providing them with protection (Nicholson et. al, 2014).

Field Cricket, example of insect bauplan

 

1. Wings: Hexapods are thought to have begun flying sometime in the Carboniferous Period, about 350 million years ago. While it is not fully understood why wings developed, the hexapod wing and the flight capabilities it provides are extremely advantageous. Hexapod wings are paired, and the pairs are generally referred to as the fore- and the hind-wings. They are heavily veined, and are surrounded by a cuticle for rigidity and protection. Most Hexapods have their fore- and hind-wings coupled, but dragonflies do not; they can move each of their wings completely independently of the others, allowing them to perform amazing flight maneuvers (Kesel, 2000). The benefits of flight are many, and wings have no doubt lent to the overwhelming success of Hexapods and other insects (Yanoviak et. al, 2009).

Cicada exhibiting folded-over, heavily veined wings.

1. Metamorphosis: There are two types of metamorphosis, hemimetabolism and holometabolism. Hemimetabolism, known as incomplete metamorphosis, characterizes Hexapods that go through a series of molts, shedding the exoskeleton that has become too small for them. Holometabolism, or complete metamorphosis, is a four stage cycle in which the organism goes from egg to larva to pupa to adult. Hexapods that undergo hemimetabolism typically resemble the adult stage as nymphs, but holometabolous Hexapods do not. A unique advantage to metamorphosis is that it allows them to inhabit different niches throughout the course of their life (Class Notes). For example, a grounded caterpillar will not occupy the same niche when it becomes a winged adult butterfly.  In addition, it has been suggested that holometabolous Hexapods enjoy lower extinction rates than other groups (Nicholson et. al, 2014).

 

4.  Sensory systems: Hexapods have acute, highly developed senses. They have compound eyes, and each eyeball is comprised of tiny units called ommatidia. Each ommatidium contains its own lens. In addition, most Hexapods also have simple eyes, or ocelli, to further their sense of sight (Mayer, 2006). Many Hexapods, such as wasps and mantids, have two large compound eyes and three smaller ocelli on top of their heads. In addition to their developed eyesight, Hexapods were among the first creatures to sense sounds. Most sound is produced through repeated stimulation of appendages. Some species of moths can even hear ultrasound, helping them avoid predation by bats (Kay, 1969).

A review of key evolutionary innovations of Hexapods, including metamorphosis, wing folding, and body segmentation.

References

Engel, M. S., & Grimaldi, D. A. (January 01, 2004). New light shed on the oldest insect. Nature, 427, 6975, 627-30.

 

Giribet, G., Edgecombe, G. D., & Wheeler, W. C. (January 01, 2001). Arthropod phylogeny based on eight molecular loci and morphology. Nature, 413, 6852, 157-61.

 

Halanych, K. M., Bacheller, J. D., Aguinaldo, A. M., Liva, S. M., Hillis, D. M., & Lake, J. A. (January 01, 1995). Evidence from 18S ribosomal DNA that the lophophorates are protostome animals. Science (New York, N.Y.), 267, 5204, 1641-3.

 

J.E. Blair. Animals (Metazoa). Pg. 223-230 in The Timetree of Life, S.B. Hedges and S. Kumar, Eds. (Oxford University Press, 2009).

 

Kay, Robert E. (1969). “Acoustic signalling and its possible relationship to assembling and navigation in the moth, Heliothis zea”. Journal of Insect Physiology 15 (6): 989–1001.

 

Kesel, A.B. (2000) Aerodynamic characteristics of dragonfly wing sections compared with technical aerofoils. J Exp Biol 203: 3125–3135.

 

Mayer, G. (2006), “Structure and development of onychophoran eyes: What is the ancestral visual organ in arthropods?”, Arthropod Structure and Development 35 (4): 231–245.

 

Misof, B., Liu, S., Meusemann, K., Peters, R. S., Donath, A., Mayer, C., Frandsen, P. B., … Zhou, X. (November 06, 2014). Phylogenomics resolves the timing and pattern of insect evolution. Science, 346, 6210, 763-767.

 

Nicholson, D.B., Ross, A.J., Mayhew, P.J. Fossil evidence for key innovations in the evolution of insect diversity. Proc. R. Soc. B: 2014; 282(1803).

 

Thomas, J. A., & Ware, J. L. (January 01, 2011). Molecular and Fossil Dating: A Compatible Match?. Entomologica Americana, 117, 1, 1-8.

 

Tree of Life Web Project. 2002. Hexapoda. Insects, springtails, diplurans, and proturans. Version 01 January 2002 (under construction). http://tolweb.org/Hexapoda/2528/2002.01.01 in The Tree of Life Web Project, http://tolweb.org/