422 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 



to a most intricate lock, the organism is a most intricate key that 

 fits that lock completely. How has this key come to fit this lock? 



First of all, it is to be said that the Collembola in question is the 

 only larger organism that is found in any huge numbers on and in 

 the great portion of the beach. Worms there are that live in the sand 

 of tlie beach ; Crustacea and insects there are that feed on the debris 

 that is thrown up by the sea at the high tide line; but it is only 

 the Collembola that swarm over the beach. (Fig. 2.) 



Why are the Collembola the 

 only organisms that make such 

 use of the beach? The answer 

 seems to be that they are the 

 only group that holds a key 

 approximating the needs of 

 the beach lock. Two other 

 species of Collembola live on 

 the edge of the beach, in rela- 

 tively small numbers. But 

 one, Isotoiaa herseldi^ has the 

 combination of small size, 

 slender form, and greater ca- 

 pacity for jumping that are 

 demanded for successful life 

 on the beach. Another species 

 which has a chunkier form 

 and is less successful as a 

 jumper is found on the beach 

 but is much less numerous than 

 the first. Now the Podurid 

 Collembola have, in general, a 

 structure and reactions that 

 lead them to live in situations 

 not so very different from 

 those occupied by Isotonia. 

 They are found on water or in humid earth or in moist caves, or in 

 crevices of moist walls. If anything is to survive in the sand of the 

 sea beach it must come out of a group wnth instincts and structure 

 that make it possible and preferable to live in such places. However, 

 the interstices of the sand of the beach are especially fine and the 

 period of exposure to the air is so brief that the insect must have 

 movements and responsiveness of such sort as will ensure adequate 

 exercise and oxygenation of the tissues during the brief time that 

 it is exposed to the air. Collembola, in general, have the right form 

 of key ; Isotoma herselsii has precisely the appropriate notches to fit 

 the precise lock of the beach. 



FiGURK 2. — Drawings of three species of 

 Collembola on the beach, (1), (2), and 

 (3), with some details of their structure; 

 (11) shows the relative frequency of 

 Isotoma on the beach ; the abcisaae repre- 

 sent distances from the beach line on top 

 of the beach ; the ordinate's, the pro- 

 portional frequency of occurrence 



