Ctenolepisma longicaudatum
Ctenolepisma longicaudatum, generally known as the gray silverfish, long-tailed silverfish or paper silverfish, is a species of Zygentoma in the family Lepismatidae. It was described by the German entomologist Karl Leopold Escherich in 1905 based on specimens collected in South Africa, but is found worldwide as synanthrope in human housings.
In recent years, gray silverfish have increasingly become an issue in indoor environments in Europe, especially in newly built houses with a stable climate beneficial for the growth and reproduction of this species. As a food generalist with the ability to digest the cellulose contained in paper and cellulose-based textiles like rayon, Ctenolepisma longicaudatum is considered a pest species in cultural heritage institutions like libraries and archives.
Nomenclature
Most authors have historically treated the nomenclatural gender of Ctenolepisma as feminine, but in 2018 the International Commission on Zoological Nomenclature issued a formal ruling stating the gender of Lepisma is neuter, following ICZN Article 30, which resulted in changes to the spelling of several well-known species, including Ctenolepisma longicaudatum.Description
The nymphs and adults are slender, agile, fast-moving insects with long, paired antennae and three long appendages at the posterior abdomen: a pair of cerci and the single central epiproct. Apart from the earliest instars, the body is covered with scales, giving the light- to dark greyish animals a glimmering appearance. With their brushes and bristles along the body sides, gray silverfish somewhat resemble the related firebrats.The eyes consist of twelve stemmata, which are rather more rounded in early instars.
Developmental stages
The eggs have an oval shape with dimensions of about 1.15 x 0.83 mm. Two to twenty eggs are laid per lot, and they are normally deposited about 2 mm into crevices or cracks, or under the edge of paper. Freshly laid eggs are smooth and cream-coloured; after three days the chorion turns yellow and exhibits shallow reticulate markings.First instar nymphs have a special hatching organ on the frons of the head that helps them break free from the egg shell; this organ is shed with the first moulting. The air-filled crop pulsates vigorously in the hatching process, which takes about five minutes. The hatched first instar nymphs have a pale cream body colour and lack hairs and scales, the appendages are short and soft, and the anus seems to be closed.
The 2nd instar nymphs exhibit a firmer, darker cream sclerotisation, and the longer appendages can be freely vibrated. A few bristles mark the position where the "brushes" of the mature stages will be. This number of bristles increases with subsequent moultings, and the bristle pattern might be indicative for each instar.
The 3rd, very active instar shows the body colour pattern of the succeeding instars: a dark cream colour with the edges of the thoracic terga and the anal lobes tinted purple.
The first three instars also have an increasing number of tarsal segments, by which they can be distinguished: the 1st instar has legs with two tarsal segments, whereas the 2nd instar exhibits three-segmented tarsi on the metathoracic pair of legs. The 3rd instar exhibits the three-segmented tarsi of all following stages.
In the 4th instar, the first pair of styli appears on the ninth abdominal sternum, as well as the scales covering the body.
Instars five to seven exhibit no particular distinguishing characters.
In the 9th instar, the second pair of styli appears on the eighth abdominal segment in males; in female, these appear in the 11th instar.
The genitalia first appear in the 8th nymphal instar, developing from two small lobes on the intersegmental membrane at the base of the cleft in the ninth sternum. The shape of this cleft, which first appears in the second instar and becomes more pronounced until the eighth instar, allows the distinction of the sex. A small cleft in the female's eighth sternum, which develops in early instars and completely divides this sternum in later instars, further facilitates sex determination.
The genitalia lobes remain short in males until the shape of the penis can be distinguished in the 11th nymphal instar, when the internal reproductive organs will also have developed, including seven large testicles. The two short vasa deferentia, which fuse immediately anterior to the penis, are thin-walled and slightly dilated at their distal ends; they lengthen in the next instar and form two loops between the two cercal nerves. The seminal vesicles form in the 13th instar, when also the penis reaches is final adult form by the ventral fusing of its rolled edges.
In comparison to males, the female's genitalia lobes elongate in succeeding moultings. In the 10th nymphal instar, a second, anterior pair of lobes develops from the intersegmental membrane between abdominal segments 8 and 9 and extends to the ninth sternum's cleft in the 11th instar. In the following 12th instar, both pairs of genitalia lobes are almost of equal length. In the 13th instar, the ovipositor of the adults is formed by fusion and interlocking of the posterior lobes with the anterior ones. The complete ovipositor extends circa 1.2 mm beyond the sternum. The spermatheca first appears in the 10th instar as a short lobe directed anteriorly from the gonophyses. In the 12th instar the two side sacs and the central neck are still thin-walled and rather undifferentiated, whereas in the following 13th instar, well-marked, soft walls have formed. The internal reproductive organs are developed until the 13th instar, although the accessory glands and the "yellow" glands still lack pigmentation and the ovarioles contain not yet differentiated ova.
From the 14th instar on, no further development apart from a gradual increase in size takes place.
At 24 °C, eggs hatch after 34 days, and nymphs develop to the 13th instar within 11 months, with sexual maturity probably reached at 18 months of age. Gray silverfish can reach ages of about eight years, and unlike the hemi- and holometabolous insects, the ametabolous silverfish undergo further moultings even as imagines, with three to five moults per year.
Digestive tract
Gray silverfish have a simple digestive tract, consisting of a hypopharynx, a large, thin-walled crop that occupies more than half of the body length, followed by the toothed gizzard, the mid-intestine, anteriorly with sacculi and further posteriad with a peritrophic membrane enclosing the ingested food mass, the hind intestine with an anterior dorsal loop and ending in the rectum, with the anus being surrounded by two rows of papillae. The anterior region of the midgut exhibits gastric caeca, bladder-like extensions that absorb the nutrients of the digested food. Histologically, the midgut epithelium consists of a single layer of columnar cells that border the midgut lumen with a brush border membrane. Interspersed in the midgut epithelium are nests of stem cells. The surface of hind intestine and rectum is greatly increased by deep longitudinal folds, presumably extracting water from the faeces. The hypopharynx is flanked by a pair of large salivary glands which open into its lumen.The mid-intestinal cells of early instars are already as differentiated as in mature stages, and the gizzard is of the same form, although with fewer serrations and hairs on the teeth. The malpighian tubules are relatively large until approximately the twelfth larval instar.
Biology
Diet
Ctenolepisma longicaudatum is a synanthrope in human housings, and its natural food sources are unknown so that information on the biology of this species comes from indoor observations and rearings. Gray silverfish feed on a wide spectrum of substrates, ranging from plant remains like dried grass over insect remains to bread crumbs, paper, and artificial silk and cotton fabrics. They also eat cast skins from previous moults, as they prove rich in nutrients, containing 1% of the fat and 6% of the nitrogen stored in the body. Gray silverfish will not feed on wool felt, flannel, carpet, fur felt and natural silk. Paper made from mechanical pulp is not attacked, and that from Kraft and Esparto pulps is slightly attacked; only paper produced from sulphite pulps is readily eaten. Paper consisting of 80% sulphite pulp and 20% mechanical pulp greatly reduces the attack by gray silverfish as compared to 100% sulphite pulp paper. Papers with 45% or more of mechanical pulp content are not attacked. Starvation experiments showed that gray silverfish can survive without food for up to around 250 to 300 days.While Lindsay states that gray silverfish do not actively take up water, but obtain it from the ingested food and from oxidation of food, Heep shows that free, dyed water is taken up into the gut by dehydrated gray silverfish. Dehydrated C. longicaudatum are able to replenish their body water content from the water vapour in air of 60 to 100% relative humidity.
Rearing
For rearing purposes, gray silverfish nymphs and adults can be fed on tissue paper, gummed paper, artificial silk, ground whole wheat and yeast, or oat flakes. The related firebrat, Thermobia domestica, can be fed whole wheat or plain flour; thoroughly dried and pulverised meat can be used as a strong attractant.In a dry environment, gray silverfish will die within one month, therefore a high humidity of 70–85% must be maintained under rearing conditions, e.g. through open containers of water next to the rearing containers. In addition, a moist cotton wick or a shallow tray of sand that has to be kept always damp might be provided in the rearing containers. The cultivation temperature should be around 24 °C.
For egg deposition under rearing conditions, cotton wool can be provided. Since silverfish are nocturnal and photonegative, the light regime for a successful culture should be kept in favour of darkness, e.g. eight hours of light and 16 hours of darkness.