United States or Anguilla ? Vote for the TOP Country of the Week !
In Hymenoptera and in Daphnia, etc., the whole reduction takes place in the parthenogenetic ovum, and in the mature male, though reduction divisions occur, no separation of chromosomes takes place: at the first division one cell is formed with N chromosomes and one with none, and the latter perishes.
In Corixæ the posterior wings, which lie under the hard and horny anterior wings, are a little shorter than the anterior wings; they are not folded up on themselves and are not so delicate and transparent as the wings of the beetle. Such small creatures as Daphnia pulex, Cyclops quadricornis and Rotifera should be introduced into ponds.
Both Daphnia and Cyclops are bred in stagnant water in which there should be a good stock of weeds. Corixæ and other small insects should also be given as often as possible. The fresh-water shrimp is bred in running water, Corixæ in still or slow running water. Weeds are necessary to the well-being of both. The boxes must be kept carefully covered, as I have already pointed out.
Fraser, in 'Proc. Zoolog. Soc. 1869, p. 3. I am indebted to Mr. Bate for Dr. From M. Bert's observations on Daphnia, when placed in a vessel illuminated by a prism, we have reason to believe that even the lowest crustaceans can distinguish colours. I am informed by Fritz Muller, that in the female of a Brazilian species of Gelasimus, the whole body is of a nearly uniform greyish-brown.
The difference of sex merely corresponds to duplex and simplex conditions of nucleus, but it is curious that the simplex condition in the gametes occurs in both ova and sperms. In Daphnia and Rotifers the facts are different. Parthenogenesis occurs when food supply is plentiful and temperature high.
Daphnia breeds at the rate which is almost inconceivable. The female produces her first brood of young when she is ten days old, and goes on breeding at an average of three or four times a month. The female and her progeny are rendered fertile by one act of coition, probably for fifteen generations at least, without any further intervention of the male.
Here again parthenogenesis continues for generation after generation so long as conditions are favourable, i.e. in summer, and the eggs are in the same condition as in Daphnia, etc., that is to say, reduction does not occur, and the number of chromosomes is 2N. Under unfavourable conditions males are developed as well as females by parthenogenesis, but the males arise from eggs which undergo partial reduction of chromosomes, only one or two being separated instead of half the whole number.
"The pond should be six feet deep in the center and two feet around the edge; the bottom should be of natural sand; water plants should be growing in profusion, particularly such aquatic plants as the Daphnia, Bosmina, and the Corix, to furnish food for the young bass. A good size for a breeding pond is 100 X 100 feet." For spawning, artificial nest frames are built in rectangular form.
Another member of this group is Daphnia, although the appropriateness of this name yet remains to be discovered; Daphnia being a chunky-bodied little being, with a double-branched pair of oar-like appendages, with which he darts swiftly through the water. Although covered with a hard crust like a crab, this is so transparent that we can see right through his body.
A little water-crustacean like Daphnia may swim against the tentacles of Hydra; it is stung to death by the minute cell-batteries which the animal possesses, and then in a mechanical way the tentacles transport the food to the mouth, through which it is passed inward to the digestive cavity.
Word Of The Day