. 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. 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==