On the abortion of the hairs on the legs of certain coddis-flies. 



775 



considered, as in many other cases, as a direct consequence of disuse; for at the 

 time when the pupae leave their cases and when the fringes of their feet are 

 proving either useful or useless, these fringes as well as 

 the whole skin of the pupa, ready to be shed, have no con- 

 nection whatever with the body of the insect; it is there- 

 fore impossible that the circumstance of the fringes being 

 used or not for swimming, should have any influence on 

 their being developed or not developed in the descendants 

 of these insects. As far as I can see, the fringes, though 

 useless, would do no harm to the species, in which they 

 have disappeared, and the material saved by their not 

 being developed appears to be quite insignificant, so that 

 natural selection can hardly have come into play in this 

 case. The fringes might disappear casually in some indi- 

 viduals; but, without selection, this casual variation would 

 have no chance to prevail. There must be some constant 

 cause leading to this rapid abortion of the fringes on the 

 feet of the pupae in all those species in which they have 

 become useless, and I think this may be atavism. For 

 caddis-flies, no doubt, are descended from ancestors which 

 did not live in the water, and the pupae of which had no 

 fringes on their feet. Thus there may even now exist in 

 all caddis-flies an ancestral tendency to the production 

 of hairless feet in the pupae, which tendency in the com- 

 mon species is victoriously counteracted by natural selec- 

 tion, for any pupa, unable to swim, would be mercilessly 

 drowned. But as soon as swimming is not required and 

 the fringes consequently become useless, this ancestral ten- 

 dency, not counterbalanced by natural selection, will pre- 

 vail, and lead to the abortion of the fringes. 



I do not remember having seen, in any list of clei- 

 stogamic plants, the Podostemaceae. These curious little 

 aquatic plants, which Lindley placed near the Piperaceae, 

 Kunth between the Juncaginese and Alismaceae, and which Sachs considers 

 as being of quite dubious affinity, cover densely the stones in the rapids of our 

 rivers ; on the branches which come above the surface of the water, there are 

 pedunculated, open, fertile flowers; but there are numerous sessile flower-buds 

 also on the branches, which probably remain submerged for ever; I have not 

 yet ascertained whether these submerged flowers are fertile; if they are so, they 

 can hardly fail to be cleistogamic. 



Blumenau, St. Catharina, Brazil, January 21, 1879. 



Fritz Miiller. 



Fig. 2. 



Fig- 3- 



Fig. 2. Tibia and tarsus 

 of the two pairs of legs of 

 the pupa of a species of 

 Leptoceridse, inhabiting Bro- 

 melise. 



Fig 3. The same of a 

 nearly allied species inhabi- 

 ting rivulets. 



