The remaining family of Thysanura, the Lepismidae, is in certain respects the most highly developed of the Order. The covering of scales found on the body is very remarkable in some of the species, especially in the genus Lepisma (Fig. 93, L. cincta); the thoracic segments are different from one another and from those of the abdomen, and the tracheal system is more highly developed than it is in the Machilidae. Several genera are known, but only two members of the family have yet been detected in Britain. One of them (Lepisma saccharina), occurs only in houses, and is sometimes called the silver fish; it is, when full grown, less than half an inch long, and is covered with scales that give it a feebly metallic lustre. Like the other Thysanura, its movements are very perfect. It is said that it is occasionally injurious by nibbling paper, but the writer's observations lead him to doubt this; its usual food is doubtless farinaceous or saccharine matter. Thermobia furnorum, our other British Lepismid, has only recently been discovered; it is found in bakehouses at Cambridge and elsewhere. The bakers call these Insects fire-brats, apparently considering them to be fond of heat.
Much valuable information as to the anatomy of Thysanura has been obtained by Grassi and Oudemans, and is of great interest. Taking four genera, viz. Campodea, Japyx, Machilis, and Lepisma, to represent the four families constituting the sub-order, we will briefly enumerate some of the more remarkable of the characters of their internal anatomy. Campodea has a very inferior development of the tracheal system; there are three pairs of spiracles, which are situate on the thoracic region; the tracheae connected with each spiracle remain distinct, not uniting with those coming from another spiracle; there are thus six separate small tracheal systems, three on each side of the body. Japyx solifugus has eleven pairs of spiracles, of which four are thoracic; the tracheae are united into one system on each side by means of lateral tubes; thus there are two extensive tracheal systems situate one on each side of the body, there being a single transverse tube, placed near the posterior extremity, uniting the two lateral systems. In Machilis there are nine pairs of stigmata, two of them thoracic, seven abdominal; the tracheae from each spiracle remain unconnected, so that there are eighteen separate tracheal systems, some of which are considerably more developed than others. The Lepismidae have ten pairs of stigmata, and the tracheae connected with them are completely united into one system by longitudinal and transverse tubes. Besides these differences there are others, of considerable importance, in the position of the stigmata.
All the Thysanura possess salivary glands. In Campodea there are about sixteen extremely short Malpighian tubules, or perhaps glands representing these organs; Japyx is destitute of these structures; Machilis maritima has twenty elongate tubules; in Lepisma also they are long, and apparently vary in number from four to eight in different species. The proportions of the three divisions of the alimentary canal differ extremely; there is a very large proventriculus in Lepisma, but not in the other families; coecal diverticula are present on the anterior part of the true stomach in Machilis and in Lepisma, but are wanting in Campodea and in Japyx.
The dorsal vessel seems not to present any great differences in the sub-order. Grassi says there are no alary muscles present, but Oudemans describes them as existing in Machilis, but as being excessively delicate.
The ventral chain of nerve-ganglia consists in Campodea of one cephalic ganglion, one sub-oesophageal (which clearly belongs to the ventral series of ganglia), three thoracic, and seven abdominal. In the other families there are eight instead of seven abdominal ganglia.
The structure of the internal sexual organs is very remarkable in the Thysanura. In Campodea there is one extremely large, simple tube on each side of the body. In Japyx there are seven small tubes on each side, placed one in each of the successive abdominal segments, and opening into a common duct. In Machilis there are also seven tubes opening into a common duct, but the arrangement is no longer a distinctly segmental one. In Lepisma there are five egg-tubes on each side, the arrangement being segmental in the young state but not in the adult condition. In Campodea nutrient cells alternate with the eggs in the tubes, but this is not the case in the other families. Fig. 94 shows the ovaries in three of the Thysanura; in the drawing representing this part in Machilis (C), one of the two ovaries is cut away for the sake of clearness.
The male organs in Campodea are very similar in size and arrangement to the ovaries, there being a single large tube or sac and a short vas deferens on each side of the body. In Japyx there is a sac on each side, but it is rendered double by a coecum at its base, and there are long and tortuous vasa deferentia. In Lepisma there are three pairs of coeca on each side, segmentally placed and opening into a common duct. In Machilis there are three retort-shaped sacs on each side opening near one another into a common duct, the vasa deferentia are elongate, and are very curiously formed, being each double for a considerable length, and the separated portions connected at intervals by five transverse commissural ducts.
Fig. 94.—Ovaries of Thysanura: A, of Campodea; B, of Japyx; C, of Machilis. (After Grassi and Oudemans.)
One of the characteristic features of Insect structure is the restriction of articulated legs to the thoracic region. In the Thysanura there exist appendages occupying a position on the hind body somewhat similar to that of the legs on the thorax. These appendages are quite small bodies, and are placed at the hind margins of the ventral plates of the abdomen, one near each side; they are connected by a simple joint to the sternite and are provided with muscles. They are found in Campodea on segments 2 to 7; in Lepisma on 8 and 9, in the allied Nicoletia on 2 to 9; in Japyx on 1 to 7, being, however, more rudimentary than they are in Campodea. In Machilis they attain perhaps their greatest development and exist on segments 2 to 9; moreover, in this genus such appendages occur also on the coxae of the second and third pairs of thoracic legs. Oudemans thinks they help to support the abdomen, and that they also assist in leaping; Grassi considers that they are supporting agents to some extent, but that they are essentially tactile organs. He calls them false legs "Pseudozampe."