62 The Substratum 



with which these animals can navigate in darkness was clearly demon- 

 strated on one occasion when a small group undertook the exploration 

 of a cave in Cuba under the guidance of a local plantation supervisor. 

 No sooner had the party passed through the entrance into the inky 

 blackness of the cave than hundreds of bats sprang into the air and 

 flew madly about. The air was thick with bats. At this point the 

 wife of one member of the group demurred. Not wishing to have the 

 lady of the party miss the trip, our guide offered her a dollar for each 

 bat that struck her while passing through the cave. Not a single 

 dollar had to be paid. Bats have been shown to emit supersonic 

 vibrations which are reflected from obstacles and are detectible by the 

 bats' ears. In this unique manner bats can avoid obstructions in their 

 paths during flight through dark forests and tortuous caves. 



The Variety of Substrata 



Many different substrata exist in the natural community. Almost 

 every object is a potential substratum. One often hears the state- 

 ment, "Nature is relentless." This saying springs from the fact that, as 

 soon as any manmade object is abandoned, animals or plants attach 

 to it, bore into it, or grow over it. Thus nature is rentless in re- 

 ducing man's artifacts to natural conditions. 



The most common substrata are the many derivatives of rock; these 

 will be considered shortly, but first let us take note of the great variety 

 of substances which can serve as substrata. It is not even necessary 

 for the substratum to be a hard surface. The surface film of water 

 serves as the substratum for a category of organisms called the neuston. 

 Many algae, certain higher plants such as the duck weed (Leinna) 

 water striders, and "whirligig beetles" which inhabit ponds are sup- 

 ported by the tension of surface film and use it as their regular sub- 

 stratum. Flatworms and pulmonate snails are able to employ the 

 underside of the water surface as a substratum. Under quiet condi- 

 tions the observer can see these forms progressing steadily along the 

 interface between the air and water. Mosquito larvae similarly 

 can attach to the underside of the surface film (Fig. 3.1). The par- 

 ticular ecological needs of the mosquito in this stage make it possible 

 for man to exterminate the pest, locally at least, by applications of oil 

 or poison to the surface of the water. When undisturbed by public 

 health agents, however, the mosquito larva grows rapidly on food 

 which has accumulated at the water surface. Particles sinking 

 through the air accumulate on the water and other materials floating 

 up from deeper layers come to rest under the surface. Hence the 



