882 



SCIENCE 



[N. S. Vol. XXXVII. No. 962 



THE EFFECT OF MOLTING ON" RHEOTAXIS IN 

 ISOPODS 



In an earlier pape/ I showed that the 

 strength of a current in which stream isopods 

 can maintain themselves is determined by the 

 period of weakened responses during the 

 breeding season. More recent studies have 

 shown that the shorter but more frequently 

 recurring molting period is also of impor- 

 tance in this respect. 



The molt in the isopod, AselJus communis 

 Say, usually occurs at intervals of from 13 to 

 25 days, although over 70 days may elapse be- 

 tween molts. Normally the covering breaks 

 between the fourth and fifth thoracic seg- 

 ments. Either the anterior or posterior part 

 may be shed first. Both may be molted the 

 same night or as long as four days may inter- 

 vene between the molting of the two halves. 

 At times parts or all of the covering may be 

 molted, segment by segment, or even parts of 

 a segment may break off; as long as nine 

 days has been observed to be spent in molting 

 the abdomen alone. 



The influence of this period on the rheotactic 

 reaction is typically shown by the following 

 account of the responses of an isopod during 

 one molting period. The isopod under obser- 

 vation was a male, 12 mm. long, which averaged 

 normally 86 per cent, positive rheotactic reac- 

 tions. The molt began just after a test of 11 

 trials which averaged 35 per cent, positive, 35 

 per cent, negative and 30 per cent, indefinite. 

 During each minute's reaction the isopod 

 moved an average distance of 40 centimeters. 



The covering broke between the fourth and 

 fifth thoracic segments and the forward part 

 was worked off over the head by a series of 

 undulating motions of the body and by move- 

 ments of all the legs. It took 70 seconds to 

 complete the process. One of the antennas 

 that had been dragging before the molt oc- 

 curred was dropped off with the exuvia. Im- 



^ Allee, W. C, 1912, ' ' An Experimental Analysis 

 of the Relation between Physiologieal States and 

 Eheotasis in Isopoda, " Jour. Exp. Zool., Vol. 13, 

 pp. 269-344. 



mediately after the molt the forelegs were 

 smaller than usual. 



During the process the isopod stayed in one 

 place and disregarded all currents. A current 

 set up immediately after the molt was com- 

 pleted was also disregarded. After about two 

 minutes in the same place the isopod moved 

 across the pan and stopped in an angle 17 cm. 

 away. It rested there quietly for seventeen 

 minutes, when it was again tested for rheo- 

 tactic reaction. No movement occurred. At 

 thirty minutes after molting 10 trials showed 

 30 per cent, positive, 60 per cent, indefinite 

 and 10 per cent, no reaction at all. The aver- 

 age reaction distance per minute was 38 cm. 



At this time the isopod was more sensitive 

 to touch stimuli than usual. In the response 

 just given it twice ran into another isopod 

 with its antennas and jumped back over a 

 centimeter each time, although normally there 

 would have been almost no negative reaction. 



Three hours after molting, 70 per cent, of 

 the reactions were positive, and two hours 

 later 90 per cent, were positive and the reac- 

 tion distance was 47 cm. This last trial was 

 characterized by the steady movements and 

 rapid, definite orienting that mark the nor- 

 mal response of stream isopods. 



In this case the molt of the posterior part 

 occurred two days later, after nine p.m. At 

 nine the response was: 40 per cent, positive, 

 40 per cent, negative and 20 per cent, indefi- 

 nite. The next morning 80 per cent, of the 

 responses were positive and the other 20 per 

 cent, were negative. The reaction distance 

 was only slightly greater in the morning read- 

 ings. On the morning after the next molt 31 

 days later, this isopod would start positive 

 and turn negative as though the current pres- 

 sure against the more sensitive covering was 

 painful. 



An isopod stands higher from the bottom 

 when nearing molting time, which is probably 

 due to the increasing stiffness of its legs. At 

 this time the posterior legs appear harder to 

 move and may become tangled, thus throwing 

 the isopod as it tries to crawl. Impiediately 

 after the molt it is more easily swept off its 



