Bristletail

What is bristletail?

A bristletail is any of approximately 350 species of primitive, wingless insects belonging to the order Archaeognatha. These insects typically measure between 5 and 20 mm in length and are characterized by three slender, bristlelike appendages at the tip of the abdomen.¹

Key features of bristletails include:

• Physical traits: They possess small compound eyes, external mouthparts, and some species are covered in scales.¹
• Life cycle: They are long-lived, with some species surviving up to seven years. They continue to molt throughout their lives, sometimes as many as 35 times.¹
• Diet: They consume starchy materials, which can lead to damage in books and papers.¹

Historically, the term was associated with the order Thysanura, which has since been split into Archaeognatha and the order Zygentoma, the latter of which includes silverfish and firebrats. Notably, bristletails are among the oldest known insects, with fossils dating back approximately 390 to 392 million years to the Devonian period.¹²

How do silverfish differ from bristletails?

While both bristletails and silverfish are primitive, wingless insects historically grouped together in the order Thysanura, they are now classified into distinct orders: Archaeognatha (bristletails) and Zygentoma (silverfish and firebrats).¹

The primary differences between them include:

• Body Shape: Bristletails typically have a more cylindrical body, whereas silverfish are characterized by a flattened, slender body shape.¹³
• Classification and Anatomy: Bristletails (order Archaeognatha) possess large compound eyes and three tail filaments, with the middle filament being the longest. Silverfish (order Zygentoma) may have small compound eyes or be entirely eyeless, and their abdominal styli are unjointed.¹²
• Habitat: Bristletails are generally found in outdoor environments. In contrast, silverfish are commonly found indoors, where they are considered pests due to their tendency to consume starchy materials like bookbindings, wallpaper, and fabrics.¹³
• Life Span and Molting: Bristletails are notably long-lived, with some species surviving up to seven years and molting as many as 35 times. Silverfish typically have a life span of two or more years.¹³

Why do bristletails molt so many times throughout their lives?

Bristletails, like other arthropods, must molt because they are encased in a rigid, nonliving exoskeleton that cannot grow along with their internal tissues. To accommodate growth, they undergo ecdysis—the periodic shedding of the old skeleton—which allows them to expand and develop a new, larger, and initially flexible covering.¹

While many insects stop molting once they reach sexual maturity, bristletails and other primitive, wingless insects continue to molt throughout their adult lives. This ongoing process is essential for their reproduction; because females store sperm in a receptacle that is lost with each shed of the exoskeleton, they must engage in the deposition and pickup of sperm packets during every adult stage to remain fertile.¹⁴

Molting is a complex process initiated by the steroid hormone ecdysone, which triggers the underlying cells to digest the base of the old exoskeleton and secrete a new one beneath it.⁵¹

What makes bristletails one of the oldest known insect groups?

Bristletails (order Archaeognatha) are considered one of the oldest known insect groups because of their significant presence in the fossil record, which dates back to the Devonian Period.¹

Specifically, researchers have discovered a bristletail fossil estimated to be 390 to 392 million years old on the north shore of Gaspé Bay in Quebec, Canada. This specimen is notable for retaining significant structural details, including fragments of the head and thorax, providing clear evidence of their ancient lineage.¹

As members of the subclass Apterygota, bristletails are regarded as survivors of primitive insect stock that have persisted with relatively little change over hundreds of millions of years. Their evolutionary history highlights them as one of the earliest examples of the terrestrial branch of the phylum Arthropoda to successfully colonize land.¹⁹¹⁰