THE MEALY PLUM APHIS. 9 



Antennae four-fifths as long as body, on frontal tubercles. Comparative 

 measurements as follows : I, 0.08 mm. ; II, 0.0G ; III, 0.405 ; IV, 0.26 ; V, 0.21 ; 

 VI, 0.48 (0.10 plus 0.38). Beak reaching a little beyond anterior border of 

 mesosternum, 0.26 mm. long. Wings 2.5 mm. long. Cornicles faintly imbri- 

 cated, shorter than in wingless female, barely twice as long as broad at base, 

 slightly constricted near base, in length 0.08 mm. Style ensiform, 0.15 mm. 

 long. Length of body, 1.61 to 2 mm. ; width, 0.66 mm. 



Lateral tubercles as in wingless form ; first antennal joint somewhat gibbous. 



Sensoria. — On III, 23 to 30 ; on IV, 4 to 10 ; on V, 1 to 2 ; on VI, usual apical 

 group. Sensoria of unequal size, not at all arranged in longitudinal rows, but 

 rather in spirals. 



Duration of Nymphal Stages. 



Eight winged spring migrants developed in an average of 14J days, 

 the developmental period ranging from 13 to 18 days. The winged 

 form therefore develops more slowly than the wingless, due to the 

 increased duration of the fourth instar. 



Repbodtjction. 



The migrants commenced to reproduce on the cat-tail rush (Typha) 

 a few days after they settled. In many cases migrants were found to 

 settle, remain for several days, and finally die without reproducing 

 and only a small percentage of those settling reproduced. Most of 

 them remained a few days and then departed. Spring migrants in 

 only one instance out of 76 deposited young on caged plums. In this 

 instance the three young born refused to feed on the plum (Myroba- 

 lan). In this connection it might be said that all attempts to induce 

 wingless forms of earlier generations to settle on Typha failed. Mi- 

 grants placed in small dishes and provided with plum and cat-tail 

 foliage in no instance deposited young and in extremely few instances 

 did migrants placed on cat-tail deposit young. Unfortunately it was 

 not possible to obtain Phragmites for similar tests. 



The migrants may remain on the winter host foliage for several 

 days before taking flight, especially if the weather be cool and cloudy. 



Field observations indicated that the migrants produced young 

 at the rate of about five every two days at first, and later at a slower 

 rate. On Typha it appeared that the maximum number of young 

 per migrant rarely exceeded 20 and, discounting all migrants which 

 failed to deposit, averaged not much over five. This must have been 

 abnormal, as examination of individuals disclosed the presence of 

 many more embryos than were extruded. It is possible that Typha 

 does not prove an invigorating food for the migrant and this point 

 is perhaps elucidated below in the paragraphs on migrations. 



MlGEATIONS. 



Phragmites and Arundo have long been known as alternate hosts 

 of the mealy plum aphis; in fact the species has been described as 

 new from both of these hosts. In California enormous infestations 



