ANT'RIDA. 



419 



The abdomen never consists of more than six seg- 

 ments ; the first segment always bears the ventral tube, 

 which represents a pair of fused appendages. Appendages 

 may also be present on the third (the " catch ") and fourth 

 (the springing organ) segments respectively (text-fig. 1). 

 The ventral tube is the most constant of all Collembolan 

 structures. It attains its highest degree of development in 

 the Sminthurida?, where it can emit a pair of long, tubular 

 vesicles (text-fig. 3). These may exceed in length the 



Text-Fig .3. — Smintlmrus viridis. I., Springing organ: m, manu- 

 brium ; d, dens ; t, mucro. II., Extremity of fore foot. 

 III., Animal viewed from the side: vt, ventral tube; 

 k, ' catch ' ; m, manubrium ; d, dens ; /, mucro. 



whole animal, and they are frequently studded with small 

 papillae. Almost every transition can be found between 

 this greatly developed condition to where the ventral tube 

 exists only in the form of a bilobed tubercle and capable 

 of only a very limited amount of protrusion. 



By far the majority of Collembola possess a pair of 

 partially fused .--ppendages in relation with the fourth 



