of oostegites of the female's brood pouch; the poste- 

 rior part included the fifth and last pair. There was 

 an interval between the shedding of the two parts 

 (females: i = 6.5 ± 6.1 SD hours, n = 35, range 

 = 0-12 hours; males: x = 8.3 ± 6.9 SD hours, n = 

 36, range = 0-24 hours). There was no significant 

 difference between the intervals of the two sexes 

 (Student's f-test: t = 1.41, df = 69, P < 0.05). Final- 

 ly, the observations suggest that there is no terminal 

 molt in either sex, because all individuals molted 

 until they died. 



Amplexus 



Males picked up and held onto females until the 

 females molted. During amplexus, females were 

 held ventral to the male, and only the males' move- 

 ments resulted in locomotion. There was no spe- 

 cialized point of attachment on the female. The male 

 inserted the angle formed by the dactyl held slight- 

 ly extended from the palm of its second gnathopod 

 into the space between the curved, lateral edges of 

 the female's first and second pereonal segments. 

 Sometimes males held onto females with posterior 

 pereiopods as well, but this was only for brief periods 

 of time. 



The occurrence of amplexus was correlated prin- 

 cipally with female intermolt stage. Amplexus began 

 about ^ = 1.9 ± 1.4 SD days before the female 

 molt(n = 15, range = 0.0-4.5 days) and all couples 

 separated within 12 hours after the female discarded 

 its anterior cast. Thus, females were in amplexus 

 for approximately the latter 14.2% of their inter- 

 molt periods. 



Most females were in amplexus 24 hours before 

 their posterior molts and separated from the males 

 24 hours after their anterior molts (Table 1; Fisher's 

 exact probability test, P < 0.001). In contrast, most 

 males were not in amplexus either before or after 

 their molts (Table 1; 83.3% and 64.3% not in am- 

 plexus before and after the molt, respectively). 

 However, there was a significantly greater propor- 

 tion of males in amplexus after than before their 

 molts (xi = 3.94, P < 0.05). Thus, amplexus was 



Table 1. — Frequencies of occurrences of amplexus 12 hours 

 before the posterior molt and 12 hours after the anterior molt in 

 Idotea baltica maintained in heterosexual pairs. 



principally correlated with female intermolt stage, 

 but male intermolt stage had an affect as well. 



Finally, amplexus was never observed in either 

 male or female isosexual pairs. 



Copulations 



During copulation the male held the female's body 

 in a perpendicular, ventral position, and inserted its 

 pleopods into the posterior part of the female's 

 brood pouch about five times in rapid succession. 

 This was followed by a rest period of about 5 sec- 

 onds during which the male retained its hold on the 

 female. Then the sequence was repeated two or 

 three more times. 



Copulations occurred within minutes after males 

 were introduced to females that had shed both parts 

 of their exoskeleta, regardless of how many days 

 ago the molt had occurred. In contrast, no copula- 

 tions occurred in the five females that had just 

 molted the posterior portions. Amplexus was ini- 

 tiated with these females instead, and copulation 

 occurred only after the anterior portion of the exo- 

 skeleton was shed. 



Ovulations 



Isolated females ovulated about 2.9 + 0.5 SD days 

 after their molts {n = 14, range = 2-3.5 days). In 

 contrast, females maintained with males in hetero- 

 sexual pairs ovulated x = 0.12 ± 0.2 SD days after 

 their anterior molts (n = 26, range = 0-0.5 days). 

 The difference was significant (Student's ^-test: t 

 = 25.92, df = 38, P < 0.001). Another difference 

 between isolated females and females maintained 

 in heterosexual pairs was that none of the broods 

 of the former group developed, while most of the 

 latter broods did (Table 2: of 14 vs. 24 of 26 broods, 

 respectively; Fisher's exact probability test: P < 

 0.001). Eggs ovulated by isolated females were no 

 longer observed in the brood pouches about 5.0 + 

 1.8 SD days after the females' molt (n = 12, range 

 = 3-6 days) or about 2.0 ± 1.5 SD days after ovula- 



Table 2.— Frequencies of ovulations and viable broods of isolated 

 females and females paired with males. 



Number of 



Number of 



ovulated 



broods that 



378 



