February, 1901] 



PSYCHE. 



101 



records three species of Collembola 

 from the mossy nests of sea gulls ; 

 whether birds carry these insects among 

 their feathers, or not, is not known. 

 Mr. .S. R. Williams gave me a specimen 

 of Entcmwhrya griseo-oli-'ata and a Thrips 

 both of which he found deep in the fur 

 of a mole. 



These accidental, or occasional, means 

 of dispersion may be recognized without 

 being assigned too much importance. 

 Certainly, human intervention cannot 

 be held responsible for the distribution of 

 the arctic forms b)' which the European 

 and North American faunae are linked 

 together. 



By far the most important agent of 

 dispersion is running water. During 

 most of the year, some species of Col- 

 lembola are to be found on our streams 

 and ponds and, in some months, as many 

 as a dozen kinds at once. .\ few of these, 

 such as Pfldiira aquatica. Isotoina palus- 

 tris, Smlnt/iunis aquaiiciis, malingrcni 

 and spinatus possess structural adapta- 

 tions for their semiaquatic life, but many 

 other species are met with which, thovigh 

 nonrially terrestrial, are quite at home 

 on the surface of fresh water, which can- 

 not wet them, and in which they can- 

 not sink ; they leap upon the surface 

 film with ease. 



Such species undoubtedly owe their 

 broad distribution mainly to streams of 

 fresh water. 



The snow fleas, which attract interest 

 by their sudden appearance in immense 

 numbers, are distributed by the same 

 means. In the latter part of winter 



they may be found, more or less be- 

 numbed, under the loose bark of trees, 

 especial!)' pine, oak and maple, or else 

 about the roots among dead leaves, in 

 which situations tlieir eggs are laid. 

 Rendered active by the first warmth of 

 spring, the little creatures wander out 

 and sometimes darken the snow by their 

 numbers. Rivulets of melted snow or 

 of rain water carry them to the brooks 

 whence they are borne to the rivers and 

 scattered no one knows how far. 



Many other species are washed from 

 streain to stream in the same way ; 

 floating logs, branches and roots must 

 often transport Collembola, especially of 

 the genus Isotoma, a large proportion 

 of which live under loose bark. In fact, 

 I believe that the most important agents 

 of dispersion for inland Collembola are 

 fresh water streams. 



Marine currents, also, are of vast im- 

 portance in this respect. Wahlgren re- 

 cords nine species of Collembola from 

 barren rocky islands off the Swedish 

 coast and properly maintains that they 

 were distributed solely by sea water. 

 These are Anurida tiiarifimn and /ii/l- 

 I'erg!, Anurophorus /aricis, Xenylla mar- 

 ifinia and Immicola, Achorittes Tiaticiis. 

 Isotoina viridis, Entoinobrya hviiigiiit'sa 

 and Sira biiski. 



It is a significant fact that almost ail 

 those named are Holarctic. 



The species of Anurida, indeed, are 

 restricted to salt water. A. maritiina is 

 common on the coast of Massachusetts, 

 where it occurs in colonies between tide 

 marks. At low tide the insects walk 



