To the majority of species, however, delicate odours seem to make little or no appeal, since excessive development of the olfactory apparatus, such as is seen in the aberrant Water-flea (_Leptodora_), is rare. This is perhaps explained by the fact that Leptodora is a species which does not herd together in vast numbers, hence, probably, the need of some exceptional means whereby the males may discover the whereabouts of the females, while in the case of the swarming hosts formed by Water-fleas and Brine-shrimps, for example, no such highly specialized aid is necessary. Instead, the males have developed powerful arms for the capture and retention of the females. In the case of the Brine-shrimp these arms are of quite formidable proportions. The males of the Copepoda, remarks Weismann, “possess on their anterior antennæ an arrangement which enables them to throw a long, whip-like structure like a lasso round the head of the female as she rapidly swims away. The antennæ of the male Daphnids, too, are in one genus (_Moina_) developed into a grasping apparatus; ... the first antennæ ... are not only much longer and stronger than those of the female, but they are also armed with claws at the end, so that the males can catch their mates as with a fork, and hold them fast. And even that was not enough, for, in addition, the males of most Daphnids possess a sickle-shaped but blunt claw on the first pair of legs, which enables them to cling to the smooth shell of the female, and to clamber up on to it to get into the proper position for copulation. [Illustration: Plate 40. SOME REMARKABLE DEVICES. 1. A Water-flea (_Moina rectirostris_): male showing the claspers-the front pair of “legs,” for grasping the female. 2. The female of the same, in which the “claspers” appear as mere stumps. 3. The aberrant Water-flea (_Leptodora kindlii_): the male showing the long comb-like antenna for the discovery of the female (the left only is drawn), and the female, just beneath, lacking this olfactory organ. 4. An extraordinary species of Bug in which the upper surface of the thorax has been produced backwards to form an overhanging pent-house, of unknown function, and illustrating the theory of “Hypertely.” Face page 262.]