SCHIZONEURA ULMI. 101 



between tlie exclusion from the egg and the appearance 

 of the perfect sexes. 



With slight variations these stages of development 

 may be regarded typical of the genus. 



Numerous workers have, however, been on the same 

 track, and have greatly added to our knowledge ; 

 amongst whom may be mentioned Messrs. Lichtenstein, 

 Courchet, Kessler, and Cyrus Thomas, of Illinois. 



Briefly the life-history is comprised in the following 

 remarks : 



A careful examination towards winter of the crevices 

 of the bark of such elm trees as have been infested 

 during the past summer will sometimes lead to the 

 discovery of small, dull yellow or brownish ova about 

 0*05 millimetre in length. Some of these eggs will be 

 found free of all downy matter, others are wholly con- 

 cealed by such, whilst here and there will be seen some 

 partially enclosed, and protected by the exuvijB of 

 their dead mothers, who have died before accomplish- 

 ing their task of oviposition. 



These impregnated single eggs — for only one is laid 

 by each individual — hatch and produce the queen Aphis, 

 stock-mother, or fundatrix of Lichtenstein. These 

 commence the distortion of the young leaves in spring. 

 They moult their skins about three times, and after 

 the expiration of a few days, dependent on temperature 

 and moisture, they begin to people the cavities of the 

 curled leaves with young of the second series. 



Some of these migrate to other leaves, and eventually 

 the third generation arrives, which goes through the 

 pupal stage, acquires wings, and then spreads over 

 other trees. 



This last is the " emigrante " stage of Lichten- 

 stein. The life of these winged females appears to be 

 but short. They produce about twelve young each at 

 intervals perhaps of half an hour on the average, and 

 then shortly after they die. 



According to Prof. Riley, the American ScMzoneura 

 ulmi is not so prolific as the European. 



