APTERA 



189 



concealed by 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 there are six' 

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

 Lepisma, 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 

 be respiratory, as suggested by Oudemaiis. 



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 2 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 Collembola agree 

 with the former group. The German 

 author therefore proposes to divide our 

 Aptera, not into Thysanura and Collembola, 

 but into Ectognathi and Entognathi, the 

 former group consisting of Machilidae and 

 Lepismidae, the latter of Campodeidae, 

 Japygidae and the various families of 



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

 the older division into Thysanura and Collembola. 



cles of Machilis. A, 

 appendage ; I', vesicles 

 protruded ; P, basal por- 

 tion ; R, muscles, x 70. 



Collembola. 



The sub-order Collembola, 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 

 1 MorpJi. Jahrb. xv. 1889, p. 363. 2 SE. Ak. Wien, c. 1891, Abtb. I. p. 216. 



