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. "Ctenolepisma longicaudata" 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 (bleached or unbleached) 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 (1940) states that gray silverfish do not actively take up (i.e., drink) water, but obtain it from the ingested food and from oxidation of food, Heep (1967) shows that free, dyed water is taken up into the gut by dehydrated gray silverfish. Dehydrated "C. longicaudata" are able to replenish their body water content from the water vapour in air of 60 to 100% relative humidity. 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.