230 R. W. PENNAK 



one "rich" lake to another "poor" lake. Over a period of time these 

 activities will undoubtedly assume considerable importance in 

 spreading various invertebrates over wider areas of the West. 



A further and unintentional means by which aquatic inverte- 

 brates are greatly increasing their ranges and abundance is through 

 the extensive system of thousands of farm ponds and stock tanks 

 which have been constructed in the West, especially during the 

 past twenty-five years. In areas formerly characterized by great 

 stretches of unbroken arid or semi-arid land, we now have an 

 extremely effective system of "stepping stones" by which inverte- 

 brates may disperse much more effectively than was the case many 

 years ago. This is no exaggeration. Anyone who will take the trouble 

 to examine the invertebrate population of a stock tank or ranch 

 pond in the middle of a large semi-arid tract is bound to be impressed 

 with the abundance of species occurring there. (Incidentally, this 

 facet of aquatic biology seems to have been completely neglected.) 



During the past seventy years the West has become densely 

 criss-crossed with an extensive system of irrigation ditches, many of 

 them carrying water at all times of the year. Such ditches connect 

 with a host of artificial irrigation reservoirs. In our estimation, 

 these waterways also provide an enormous transportation and 

 dispersal system by which our aquatic invertebrates are becoming 

 much more generally distributed. 



