[Occasional Invaders] A New Look at an Old Group

When is a silverfish not a silverfish? Modern silverfish resemble some of the first insects that lived 390 million years ago. Many consider silverfish to be the most primitive group from which the winged insects evolved. Fifty years ago taxonomists included all silverfish within the insect order Thysanura. Today, advanced methods for studying insect relationships support the existence of two orders.

One is the Archaeognatha (Microcoryphia) which includes the non-pest, outdoor-living "jumping bristletails" (see photo above). Archaeognaths have small elongated bodies bent into an arch shape and three long tail-like structures. They have flexible antennae, large compound eyes that meet at the top of the head, one mandibular articulation, and three ocelli. Their mouthparts are partially retractable, with simple chewing mandibles and long maxillary palps. Archaeognatha are unique in being able to use their tail to spring up to 30 centimeters (12 inches) into the air. There are about 450 named species in the world.

The second order is the Zygentoma, which includes the family Lepismatidae containing the firebrat (Thermobia domestica), the common silverfish (Lepisma saccharina) and other pest silverfish species (see photo on page 96). Zygentoma have two mandibular articulations, small or absent eyes, and from one to three ocelli. They often hold their two lateral cerci out at right angles to the body. They cannot jump. About 380 species occur worldwide.

PEST STATUS. Silverfish consume many things such as starches and dextrin in adhesives, book bindings, paper, photos, carpet and textiles such as cotton, linen, silk and synthetic fibers. They also eat dead insects, including other silverfish, silverfish eggs and their own exuvia (moulted exoskeleton). The feeding habits of silverfish have been observed and recorded as far back as 1665 (Robert Hooke’s Micrographia).

Ebeling (Urban Entomology, 1978) speculated that silverfish harbor organisms that aid in the digestion of cellulose materials. However, Lasker and Giese, (1956, Journal of Experimental Biology 33, pp. 542-553) demonstrated that the silverfish Ctenolepisma lineata can digest and grow on pure cellulose without requiring gut microorganisms as do termites. And Zinkler and Götze (Comparative Biochemistry and Physiology Part B: Biochemistry and Molecular Biology, Volume 88, Issue 2, 1987, pp. 661-666) found that the fire-brat gut contains cellulases and other enzymes that make cellulose digestion possible. These and other studies suggest that bait control approaches aimed at starving silverfish by elimination of cellulose-digesting symbiotic microorganisms would be doomed to failure.

ALLERGEN POTENTIAL. In addition to the contamination and damage caused by silverfish, new information indicates they are also a threat to human health. Silverfish can produce human allergens. An allergen from extracts of the common silverfish (Lepisma saccharina) has been identified as an allergenic tropomyosin protein and named Lep s 1. The silverfish tropomyosin is cross-reactive to the invertebrate-sensitive IgE antibodies in patients with allergic rhinitis and asthma.

GROWTH. Silverfish growth is relatively slow and life spans are long — individuals of some species have been known to live for more than six years. They continue to molt throughout life, even after reaching adulthood, and may complete 50 (or even more) molts. It has been suggested that frequent molting is an adaptation that reduced the risk of infection by parasitic fungi.

The first two stages of pest silverfish species lack scales. After that, silverfish bodies are covered with flexible and slippery scales that may help protect the insect from capture by predators such as spiders and house centipedes.

REPRODUCTION. Silverfish have an elaborate courtship ritual to ensure exchange of sperm. The male spins a silken thread between the substrate and a vertical object. He deposits a sperm packet (spermatophore) beneath this thread and then coaxes a female to walk under the thread. When her cerci contact the silk thread, she picks up the spermatophore with her genital opening. Sperm are released into her reproductive system, and then she ejects the empty spermatophore and eats it.

ECOLOGY AND PHYSIOLOGY. Firebrats and silverfish are found in different locations within a home. Firebrats tend to congregate near warm and dry areas like water heaters and furnaces. Silverfish require cooler and more humid locations and are commonly found near sinks, tubs and showers. Where you find one silverfish, you’ll often find many more. These aggregations of silverfish result, in part, from species-specific, non-volatile, contact pheromones that are found in their frass. Since silverfish cannot climb smooth surfaces, the first silverfish a homeowner might see is often in a bathtub.

Firebrats have an interesting adaptation that allows them to survive in dry environments: they can absorb water directly from the air. They are adapted to suck air into their rectal system, compress the air and absorb water into their blood.

SILVERFISH CONTROL. It helps if your inspection shows you the extent of the silverfish infestation. Control the silverfish by using a combination of habitat modification efforts and chemicals. Always try to reduce or eliminate areas that are providing harborage (especially clutter) where silverfish live. Because silverfish are often found in areas with high humidity, dehumidifiers can also be used to create a less-than-optimum environment for silverfish development. Insecticide use should include targeted crack and crevice applications of residual sprays or dusts to areas where the silverfish were found during inspection.

Photos in this article are by Lisa Ames of the University of Georgia Department of Entomology, Griffin. Steven R. Sims, Ph.D., is a senior research entomologist with BASF Pest Control Solutions. Daniel Suiter, Ph.D., is with the Department of Entomology, University of Georgia, Griffin. Arthur G. Appel is professor and chair of the Department of Entomology and Plant Pathology at Auburn University, Auburn, Ala.
 

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Why Are Silverfish Silver?

This was a question that Hooke (1665) attempted to answer. He noted the presence of "several shells or shields that cover the whole body, every one of these shells are covered or tiled over with a multitude of transparent scales, which, from the multiplicity of their reflecting surfaces, make the whole animal a perfect pearl color." It is reasonable to assume that the "silver" reflection that we see is produced by the ribbed structure of the scales that cover the body of the insect. However, this is not the case. Below the scales there is a complicated multilayer structure composed of high index layers of chitin interspersed with low index layers. Most of the silverfish reflectance is due to the multilayer stack that Hooke didn’t see, rather than the scales that he thought accounted for it.