192 SPRING-TAILS chap. 



of restraint by the catch, when the latter is removed the spring 

 extends by reason of its elasticity, and the leap is thus executed. 

 Whether this is really the exact method of leaping is, however, 

 doubtful, for Lubbock says that the catch " only exists in certain 

 genera " ; while in its structure it does not appear to be well 

 calculated to retain in position an organ that by virtue of its 

 elasticity is constantly exerting a considerable force. 



The ventral tube is an anomalous and enigmatic structure. 

 In the lower forms, such as Lipura or Anurida, it consists merely 

 of a papilla (Fig. 100, A, a) more or less divided by fissure into two 

 parts. In the Smynthuridae it is more highly developed, and 

 protects two long delicate tubes that are capable of being 

 protruded, as shown in the outline profile of Smynthurus fuscus 

 (Fig. 99), which is taken from specimens preserved in balsam by 



Mr. J. J. Lister. The nature and use 



of this ventral tube have given rise to 



.-^tH^y'^'^^^<iLf much discussion. Lubbock considered, 



\i-/T^ r^iJi^ ^^^^^ others have agreed with him, that 



M J^^^)i^:~-^^^'^^^'^ ^^ serves to attach the Insect to bodies 



W/f]p 1^ to which it may be desirable the Insect 



/// "^ 4^ should, when in the perpendicular posi- 



% (I tion, adhere. Eeuter ^ assigns a quite 



^ different function to this singular struc- 



FiG. ^^. Smynthurus fuscus, ^^^^ jjg states that the hairs of the 



with exsertile vesicle (a) pro- i i i 



traded from ventral tube; body are hygroscopic, and that the 

 &, the spring extended. peculiar claws of the Insect having 



collected the moisture from the hairs, the ventral tube becomes 

 the means of introducing the liquid into the body. These Insects 

 possess, however, a mouth, and there seems to be no reason why 

 a complex apparatus should be required in addition to it for so 

 simple a purpose as the introduction of moisture to the interior of 

 the body. Haase finds ^ that Collembola can crawl on glass 

 without the aid of the ventral tube ; he considers its function 

 to be physiological, and that it may probably be respiratory as it 

 has been suggested is the case with the vesicles of Thysanura. 

 The function of the ventral tube is certainly not yet satisfac- 

 torily elucidated. The vesicles contained in it are said to be 

 extruded by blood-pressure, and withdrawn by muscular action 

 in a manner similar to that which we have described as occurring 

 1 Ent. Tidshr. i. 1880, p. 159. " Moiyhol. Jahrh. xv. 1888, p. 361. 



