656 CLASS IV. INSECTA. 



life they can be bent forward under the abdomen and are then 

 held in position by a " catch " which is carried on the third 

 abdominal segment. If the catch be released the downward 



blow of the processes projects 

 the insects suddenly into the 

 air. The catch only exists 

 in certain species, and this 

 explanation of the action of 

 the springing apparatus is not 

 universally accepted. The 

 second structure peculiar to the 

 Collembola is the ventral pro- 



Fio. 410. Corynothri.t borealis. a ventral tmsible Organ of the first ab- 



tube ; b the spring (after Tullberg). 



dominal segment (-big. 410, a). 



This in the Lipuridae and Anuridae is little more than a protru- 

 sible papilla, in the Smynthuridae, however, it is a long, caecal 

 tube which splits into two limbs near its base. Its function is a 

 matter of dispute, but the usual view is that it is an organ for 

 attaching the small insect to a desirable spot. Development 

 shows that the ventral tube, the catch and the spring are formed 

 from the embryonic appendages borne on their respective seg- 

 ments. The remaining three abdominal segments lose their 

 appendages. 



The tracheal system is often reported as absent, but Smyn- 

 ttyurus is described as having a pair of stigmata situated between 

 the head and the pro thorax ; a very unusual position.* The 

 mouth parts are withdrawn into the head. Some authorities 

 consider that there is an extra or fourth pair of " jaws," the 

 maxillulae, t and although this view is not universally accepted 

 the mandibles are regarded as very archaic in structure. The 

 antennae have but four to six segments and at their base is a 

 post-antennal sensory organ. The legs have no tarsus, the 

 tibia simply ending in one claw or in two unequal claws. 



The spring-tails, as the Collembola are often called, when 

 they do occur, usually do so in great numbers. They mostly 

 haunt concealed retreats, hiding under bark, stones, etc. Some 

 are aquatic e.g. Podura aquatica, which sometimes occurs in 



* Willem, Recherches sur les Collemboles et les Thysanoures ; Mem. 

 Cour. VAcad. Sc. Belg., T. Iviii, 1900. 



f H. J. Hansen, Zool. Anz., xvi, 1893; op, cit. on p. 604; see also 

 p. 605. 



