Distribution, Food and reproductive Capacity of some fresh-water Amphipods. 25 

 Table XII. Youngest Breeding females. 



E. gracilis, to be further increased by the appearance of a second, 

 a third, or even later generations during the yearly reproductive period. 



X. The Possible Progeny from a Single Pair in one Year. 



With the data now at hand it is possible to calculate approxi- 

 mately the total number of young resulting directly or indirectly during 

 the period of one year from a pair of each species. In order to facili- 

 tate these calculations and to make them more easily comprehended, 

 the essential data have been condensed in the following table. 



Table XIII. 



G. limnaeus 



G. fasciatus 



E. gracilis 



Hyalella 



Total reprod. pd.*) . . 



Ave. temp.^) 



Recurrence of moult.^) 

 „ of oviposit* 



Pd. of incubation^) . . 

 Age of sex. maturity®) 

 Ave. no. of eggs^) . . 



266 



90 C 



38 dys. 

 38 „ 

 35 „ 

 93 „ 

 25 



199 

 17,9 C 

 12—13 

 12,5 

 11 

 39 

 22 



299 dys. 



17,16" C 



15—16 



15,5 



12 

 497 



45 



152 

 20,140 c 

 10 

 10 

 8 

 35 

 18 



By the use of these data together with tlu)se in Table IV, p. 12, 

 and by reference to the four curves of growth (p. 24), the following 



^) This value was obtained by continuing the curve for E. gracilis until it 

 reached a point indicated by the length 5,6. The corresponding abscissa was 497. 

 This assumes that the rate of the increase in length remains constant as the young 

 reaches maturity. This rate probably decreases which would make the age of the 

 smallest sexually mature female greater than 497 days. 



*) Although some fifty small females were measured and this one was the 

 smallest by only a small margin, younger ones may yet be found breeding in other 

 localities. 



») Table II, p. 6. 



*) Table III, p. 7. 



^) Table VIII, p, 19. 



*) Equal to periods between moults. 



') Equal to periods between moults less brooding period (Table VI, p. 16). 



8) Table XU, p. 25. 



») Table IV, p. 12. 



