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Zygentoma

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Zygentoma is an order of insects that includes silverfish, fishmoths, and firebrats. There are approximately 550 known species within this group. These insects are very ancient. They belong to one of the most primitive living lineages of insects. They diverged from other insects before the evolution of winged insects, known as Pterygota. This makes them part of the Apterygota, which are insects that do not have wings.

Silverfish 2007-2
Silverfish 2007-2

Most Zygentoma have bodies that are flattened from top to bottom. They are often shaped like an oval or an elongated shape. A very common feature is the presence of three long filaments at the end of the body. Two of these are lateral filaments called cerci. The middle filament is called an epiproct, or an appendix dorsalis. In most species, these three filaments are nearly equal in length. However, in the family Nicoletiidae, the cerci are much shorter and hard to see.

Silverfish 2007-2
Silverfish 2007-2

Many people call them silverfish because of their shiny appearance. The family Lepismatidae has scales that cover the body with a silvery glitter. When they move, they look as if they are swimming like fish. Most living species are quite small, with a body length of less than one centimeter. However, fossils from the Carboniferous period show that some were much larger, reaching about 3 centimeters. Most species have slender, mobile antennae. Their eyes are often small, and some species in the families Nicoletiidae and Protrinemuridae lack eyes entirely.

Silverfish 2007-2
Silverfish 2007-2

The history of how we name these insects has changed over time. The name Zygentoma was given by Börner in 1904. The name comes from Greek words meaning "yoke" and "insects." Scientists once thought Zygentoma was a suborder of Thysanura. Later, they realized Thysanura was paraphyletic, meaning it did not include all its descendants. Because of this, Zygentoma was raised to its own independent order. This helped scientists better understand the phylogeny, or the evolutionary history, of all insects.

Silverfish 2007-2
Silverfish 2007-2

Zygentoma live in many different environments. Some species like moist, humid places. Others can live in very dry conditions. In homes, they might eat paper, starch in clothes, or dried meats. In nature, they eat organic detritus, which is decaying organic matter. Some species are commensals, meaning they live near other animals. For example, the species Allotrichotriura saevissima lives inside fire ant nests in Brazil. While most species are safe, some troglobites are limited to only a few caves. These cave-dwelling species face a very high risk of extinction.

Silverfish 2007-2
Silverfish 2007-2

Scientists have discovered interesting ways these insects gather together. It was once thought they used contact pheromones to group up. Instead, research shows they respond to an endosymbiotic fungus called Mycotypha microspora. This fungus and a bacterium called Enterobacter cloacae live in the feces of the firebrat, Thermobia domestica. The firebrats detect the presence of the bacterium by its glycocalyx, which is an outer layer of polysaccharides. The fungus is only detected when cellulose is present. This suggests that the insects respond to the chemical signals created when the fungus digests cellulose.

Silverfish 2007-2
Silverfish 2007-2

Zygentoma also have a unique way of reproducing. The male spins a silk thread between a surface and a vertical object. He then places a sperm packet, called a spermatophore, under the thread. He coaxes a female to walk under the silk. When her cerci touch the thread, she picks up the spermatophore with her genital opening. After the sperm is released, she eats the empty spermatophore. These insects are ametabolous, which means they continue to molt throughout their entire lives. They are slow growers and can live for four to eight years.

Silverfish 2007-2
Silverfish 2007-2

Finally, these insects may help us create new energy. Zygentoma eat lignocellulose, which is the material found in wood. Their guts act like natural bioreactors. This means they use chemical processes to break down cellulose. Scientists are studying them to see if we can use these processes to produce biofuel. Biofuel is a type of energy made from organic matter. Understanding how these insects digest wood could make producing biofuel more cost-effective for humans.

Silverfish 2007-2
Silverfish 2007-2
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