78 WILLIAM H. COLE 



DISCUSSION 



The responses, other than mechanical ones, of animals to 

 centrifugal force and air currents have not been thoroughly 

 investigated. Only a few references to centrifuging experiments 

 are found in the literature. Loeb ('91) stated that Cucumaria 

 cucumis responds to centrifugation by contracting its body and 

 remaining motionless. This condition persists for from one- 

 quarter to one-half an hour afterward, when crawling is begun 

 again. Although he studied the geotropic reactions of certain 

 caterpillars, ephemerid larvae, coccinellids and blattids, no refer- 

 ence is made to testing the effect of centrifugal force on them. 



Jensen ('93), having found that Paramoecium was negatively 

 geotropic, discovered that it moved centripetally with weak cen- 

 trifugation. Davenport and Perkins ('97), after concluding that 

 ' gravity acts as an irritant to which the organism makes a 

 response, belonging to the category of adaptive responses," say 

 that this irritating pressure " may be replaced by a centrifugal 

 pressure, when the same geotactic orientation will occur." Har- 

 per ('11) also reported that Paramoecium reacted negatively 

 with weak centrifugation. He believes, however, that " the 

 response of Paramoecium to gravity is a purely mechanical 

 tropism." 



On the other hand, geotropism of animals has been exten- 

 sively studied, and many theories put forth for its explanation. 

 It is generally accepted that the ear or some ' static " organ 

 controls this tropism in certain forms. But for insects there is 

 much doubt as to how the stimulus is received. Kafka ('14) 

 reviews this question and summarizes the different theories, as 

 follows: Loeb believes that the chordotonal organs at the base 

 of the halteres of some Diptera are the organs of reception. 

 Pfliigstaedt and Weinland describe other structures which might 

 serve as sense organs. Similar organs have been described by 

 Hochreuther for Dytiscus, by Janet for Hymenoptera, and by 

 Baunacke for nepid larvae. But conclusive proof that any of 

 these organs, the functions of which are little understood, control 

 the response to gravity is entirely lacking. 



The reactions to the three kinds of forces described above 

 suggest an explanation as to how the stimuli are received. When 

 the fly is creeping upward against gravity the weight of the 

 body is on the legs. There is, therefore, a tension on the leg 



