PRESIDENTS ADDRESS. 3 



partly responsible for the great increase in the average 

 length of life, amounting to 25 per cent, in the last 50 years. 

 Observations of the plankton, or the mass of low and small 

 animal life found in the sea, off the Isle of Man, shew that the 

 bulk of it is composed of comparatively few species, though 

 the number of individuals must be enormous. A new species 

 of the crawling medusae, in which group the modified 

 tentacles are used as legs, is worth recording on account of 

 the rarity of these small jellyfish. It was found in an aquarium 

 at the Cape, and is only about J^in. in diameter. A large 

 collection of Crustacea of the family Sergestidas was made by 

 the Danish Siboga Expedition, and has greatly increased our 

 knowledge of them. As is often the case, the number of species, 

 founded on fewer specimens, has had to be reduced, in one 

 genus from 12 to 3, as intermediate forms have been found. 

 Observations of the common limpet have shewn that the small 

 ones are nearly all males, and there seems reason to believe 

 that a large proportion of these change their sex later on. A 

 similar fact has been noted in the slipper limpet (Crepidula 

 fornicata), which has invaded so many of our coasts. It has 

 been arranged in Staffordshire that, at the various flower shows, 

 an exhibit should be made of the life histories of some of the 

 most troublesome insect pests as well as other things relating 

 to plant diseases, which will help in spreading the knowledge 

 of them through the county. This might well be imitated 

 elsewhere. It would appear that the habits of the Aphis 

 known as American blight are different in America from this 

 country. Here there is no sexual generation, and the species 

 lives all the year round on the apple tree, whereas there, the 

 sexual generation is sustained on the American Elm, while for 

 the rest of the year the insects are found on the apple. 

 Plant lice are said to be comparatively scarce in the tropics, 

 owing perhaps to the violent rainstorms and the greater number 

 of insect enemies. The mystery of how the common housefly 

 passes the winter does not seem yet to be solved. None have 

 yet been seen hibernating, though many flies, some very like it 

 in appearance, are common enough in this condition. But 



