CURRENTS OF AIR AND OF WATER 



157 



to keep the model in position in a steady 

 current. Care was not taken to maintain a 

 constant cross section in the different mod- 

 els, and hence the data obtained are only 

 approximate. It is worth noting that sharp 

 edges increased the pull. 



Bottom-dwelling animals in rapid cur- 

 rents or in similarly disturbed coastal waters 

 of lakes or oceans often have flat perhaps 

 sucker-Uke ventral sides and streamlined 

 dorsal halves. An interesting example of 

 streamlining and a test of the whole set of 

 relations we are discussing is furnished by 

 black fly larvae of the genus Simulium. 

 These larvae are found only in running 

 water, where each is attached by a self- 

 spun thread in such a manner that the 

 posterior end is upstream while the head 

 end dangles with the current. Despite the 

 reversed orientation, here, as usual in 

 streamlined forms, the widest bulge is near 

 the rounded upstream portion of the body, 

 and the head lies at the end of the tapering 

 "after-body." StreamHning remains, but the 

 usual morphological relations are reversed. 

 D'Arcy Thompson (1942) gives a more 



head and long, gently sloping posterior 

 body of the heterogeneous lot of bottom- 

 dwelling fishes, known collectively as the 

 darters (Boleosoma nigrum is an example), 

 emphasizes the multiple adaptation of these 

 animals to the turbulent waters that are 

 their characteristic habitat. Rather than be- 

 ing strictly bottom-dwellers, as are flounders 

 in the sea, the darters five in the current 

 close to the bottom and between stones, 

 where the velocity of the current is reduced. 

 Bottom-dwelhng animals in rapidly flow- 

 ing streams, whether brooks or rivers, par- 

 ticularly forms such as the darting fishes, 

 may fly nymphs, and damsel fly nymphs, 

 tend to be more or less automatically ori- 

 ented to face upstream. Their organs of 

 attachment, tlie large pectoral fins for the 

 fishes and the thoracic legs for the nymphs, 

 are placed well forward. The current acts 

 on the long posterior body, as wind does on 

 the after-body of a weather vane, to turn 

 the animal with its head upstream. Similar 

 morphological relations are also found in 

 related nymphs that five in ponds, hence, 

 as with many other adaptations, the auto- 



Tahle 10. The Effect of Streamlining on Resistance to Flowing Water 

 {Data from Clemens. 1917) 



Form Orientation Pull in Grams 



Cone, rounded edges Base upstream 12 



Cone, rounded edges Base downstream 50 



Sunfish Head upstream 15 



Trout Head upstream 6 



Trout Head downstream 10 



Chirotonetes (May fly Head upstream 9 



Chirotonetes (nymph Head downstream 16 



complete, interesting discussion of these 

 problems. 



In addition to being streamlined, fishes of 

 rapidly flowing streams have rounded, mus- 

 cular bodies that contrast strikingly with 

 the flattened and often shorter body form 

 of those that live in less pronounced cur- 

 rents. The contrast between brook trout and 

 the common sunfish (Lepomis) makes the 

 point. The darting habit is also character- 

 istic of many fishes that live in rapidly flow- 

 ing waters, and this is normally correlated 

 with the possession of enlarged pectoral fins 

 that serve to help anchor the fish to the bot- 

 tom. The streamlining shown by the blunt 



matic orientation in a rapid current is prob- 

 ably secondary rather than primary.** 



** A short chapter on electricity as an en- 

 vironmental factor has been eliminated to save 

 space. The interested reader is referred to the 

 series of essays edited by Fleming (1939). The 

 chapter included a brief summary concerning 

 atmospheric electricity (Gish, 1939; Schonland, 

 1939; Wenstrom, 1942); the electrical fixation 

 of nitrogen (Clarke, 1924; Ernst, 1928); fires 

 started by hghtning (Heyward, 1939); ter- 

 restrial magnetism (Fleming, 1939); earth 

 currents (Rooney, 1939); and the possible 

 effect on homing and migration in birds 

 (Warden, Jenkins, and Warner, 1936, 1940) 



