1 86 AFTER A CHAP. 



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 observa- 

 tions lead him to doubt this ; its usual food is doubtless farin- 

 aceous 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 con- 

 siderably 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. 



