APTERA 189 



concealed liy the ventral plate. The abdominal appendage is not 

 retractile. In Machilis there are twenty-two of these vesicles, 

 arranged either two or four on one ventral plate of the hind 

 body. They are also present in Campodea, where tliere are six 

 pairs. They are usually said to be absent in Jajiyx and in 

 Zepisma, but Haase shows ^ that Japyx possess a pair placed 

 behind the second ventral plate of the abdomen. The vesicles 

 appear to be exserted by the entrance of blood into them, and to 

 be retracted by muscular agency. Much difference of opinion 

 prevails as to their function ; it appears probable that they may 

 he respiratory, as suggested by Oudemans. 



The scales found on the bodies of the Ectotrophous Thysanura 

 may be looked on as modified hairs, and are essentially similar to 

 those of the Lepidoptera, and they drop off 

 as readily as do those of the Lepidoptera. 



Stummer-Traunfels, who has recently 

 published^ the results of his researches on 

 the mouth-organs of Thysanura and Col- 

 lembola, confirms the division of the 

 Thysanura into Entotrophi and Ectotrophi, 

 and considers that the CoUembola agree 

 with the former group. The German 

 author therefore proposes to divide our 



.. , ■ , rtM ^ r\ It ^ 1 FiG. 95. — Abdominal appen- 



Aptera, not into ihysanura and Lollembola, ^^„^ ^^^ exsertiie vesi- 



but into Ectognathi and Entogiiathi, the cies of MacHUs. A, 

 former group consisting of Machilidae and protruded ;'p, basal por- 

 Lepismidae, the latter of Campodeidae, tion ; ij, muscles. x70. 

 Japygidae and the various families of 



CoUembola. We think it far more natural, however, to retain 

 the older division into Thysanura and CoUembola. 



CoUembola. 



The sub-order CoUembola, which we have defined on p. 182, 

 consists of small Insects, many of which possess the capacity of 

 leaping, or springing suddenly, and when disturbed or alarmed 

 naturally make use of this means of escaping. Their leaps, how- 

 ever, appear to be made quite at random, and very frequently do 

 I Morph. Jahrh. xv. 1889, p. 363. ^ SB..AI: JVien, c. 1891, Abth. I. p- 216. 



