This family includes the familiar silverfish and firebrats, which are common in both natural and domestic habitats. Outdoor species occur in caves, under stones and debris, and some species are associated with the nests of ants. The domestic species occupy a variety of habitats, and feed on starchy substances and plant material.

Acrotelsia collaris Adults are 16-18 mm long, not including the terminal appendages, and about 5 mm wide. The dorsal scales are nearly black, but when lost, the insect is pale gray. The abdominal tergum i0 is triangular, and there is a tuft of bristles on the prosternum. This species occurs in the tropics and in northern Australia.

Common silverfish, Lepisma saccharina Adults are about 12 mm long, not including the long terminal appendages. They are silver-gray, with a metallic sheen. This species is cosmopolitan and a domiciliary pest in most regions of the world. Development and reproduction indoors require high temperatures and humidity. Eggs are laid on successive days or weeks, 2-3 eggs ata time; fecundity is about 100 eggs. Hatching occurs in about 19 days at 32 °C and 43 days at 22 °C; 90% RH is optimal for egg development. Development from nymph to adult takes 90-120 days at 27 °C. Nymphs are pale white for 1-3 months, then scales appear on the body at the third or fourth molt. The external genital appendages appear at the eighth molt. Adults live about3.5 years at27 °C, 2 years at29 °C, and 1.5 years at32 °C. This species may have originated in the tropics, but in Latin America it is only found in the cool highlands of Brazil, Bolivia, and Argentina.

Silverfish consume both carbohydrates and protein material; animal products are eaten readily and are important to the diet. Infood-storage areas they feed on flour, meal, and other similar products. They damage paper products, especially paper that has a glaze or sizing, which consists of various amounts of starch, dextrin, casein, and gum. Paper with high cellulose content is preferred. Cotton and silk fabrics are also attacked.

Four-lined silverfish,Ctenolepismalineata(Fig. 17.1b) Adults are about 15 mm long, not including the terminal segments. They are brownish gray, with four dark lines extending the length of the body. Nymphs are light brown and slightly pink until the fourth molt, which occurs about 4 months after hatching. This species can be found throughout infested structures, including basement, wall voids, and attic, where it often occurs in large numbers. Infestations are usually large in houses with roofs with wooden shingles. In California, it also occurs outdoors under bark. It is distributed in eastern USA, but is also reported from California.

Gray silverfish, Ctenolepisma longicaudata (= Ctenolepisma urbana) Adults attain a length of about 12 mm, not including the terminal appendages. They are uniformly light to dark gray, without a metallic sheen, as in Lepisma saccharina. This species occurs in southern USA, California, and Hawaii; it is a common household pest in Australia, and is the most common household silverfish in Latin America and South Africa. Itis not restricted to humid sites indoors and may occur throughout a building. It is not known to occur outdoors.

Other Ctenolepisma Three other species that occur indoors include C. ciliata, known from California. C. diversisquamis is known from Florida, and C. targionii is known from South Carolina.

Figure 17.1 Thysanura. (a) Thermobia domestica female; (b) Ctenolepisma lineata female.

Firebrat, Thermobia domestica (Fig. 17.1a) Adults are about 14 mm long, not including the terminal appendages. They are silver-gray, with a somewhat mottled gray appearance. Without the gray scales the underbody color is pale yellow. Firebrats prefer indoor locations with temperatures above 32 °C; optimum development occurs between 37 and 39 °C. Firebrats are pests in commercial locations that maintain high temperatures, such as food-processing plants, and equipment rooms. Eggs are elliptical, white, and about 1 mm long; hatching is in 9 days at 44 °C and 77 days at 25 °C. Optimum hatching occurs at 76-85% RH. Females begin ovipositing when 45-135 days old; fecundity is about50 eggs. One clutch of eggs is produced between molts and fertilization is required following each molt. Developmentis abouti day for first-stage young, 4 days for the second, about 6 days for third and fourth stage, and about 8 days for fifth to 10th stage. There are about 13 days between successive molts, and individuals have 45-60 molts in a lifetime. Adults live 2-2.5 years at 32 °C and 1-1.5 years at 37 °C. Molting and growth appear to be continuous during their life, but there is little change in size and shape of adults after the 35th molt. Females begin ovipositing when 45-135 days old, or about the 14th molt. This species is probably cosmopolitan.

Populations persist in hot and dry environments, but fire-brats survive bestwhen there is available water. Withoutaccess to water, development is slower than normal, and oviposition decreases. Atroom temperature, firebrats lose water from their body when the RH is below 45%; above this humidity level they absorb water.

Other Thysanoptera Thermobia campbelli is often associated with stored-food materials, and Nicoletia meinerti occurs in greenhouses.

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