14 



BULLETIN 100, U. S. DEPARTMENT OP AGRICULTURE. 



The maximum number of aphides found on a single leaflet through- 

 out the counts was 90, of which 64 were sexual females. This oc- 

 curred on the first date of collection. 



It will be noticed from Table V that on the first two dates the ovi- 

 parous forms were predominant but that on all later dates these were 

 outnumbered by the viviparous individuals. On the date of the 

 fourth collection (October 7) numerous sexual females were found on 

 the lirnbs of the tree, and their number was more and more augmented 

 each succeeding week. About October 1 the males appeared in num- 

 bers, very few of them having been in evidence previous to this time, 

 although the first male of the season was noticed July 10. Table VI 

 indicates the preponderance in numbers of the sexual female over 

 the male. 



Table VI. — Preponderance of the sexual female of Chromaphisjuglandicolaover themale. 



Date of collection or count. 



Number of 



sexual ' 

 females to 

 each male. 



Date of collection or count. 



Number of 



sexual 

 females to 

 each male. 





62 



71 



24 

 7.3 

 5.3 



October 24 



13 



23 



November 2 



28 



30 



9 



7 



October 7. . . 



16 



5 



14 



23 



0) 







1 None of either sex seen on leaves. 



Table VI was compiled from the same material as that used for 

 Table V. On November 9 nearly all the sexual females were clus- 

 tered on the limbs, and two weeks later they and all other plant lice 

 at the experimental trees succumbed to a severe frost, which had 

 at the same time withered all the leaves. This clustering of the 

 sexual females or sexuparse about the limbs explains the small per- 

 centage of this form as compared with the males on November 9 

 and 16. 



Copulation seems to occur only on the leaf, and the females are not 

 fertilized until they have passed through the last molt. A single 

 male may fertilize several females — probably quite a large number 

 when it is considered that the latter sex so greatly outnumbers the 

 former and that very few eggs prove infertile. Copulation in all 

 instances observed by the writer occupied some 30 seconds of time — 

 a very short period for an aphis. If the male be disturbed, he will 

 immediately retract his genital organ and move off. In 1912 the 

 males appeared in comparative abundance in the vicinity of San 

 Jose as early as August 26. 



In general appearance the adult oviparous female differs from the 

 viviparous form in that it is wingless, has a wider body, and bears 

 three conspicuous transverse brown or black bands on the dorsum 

 of the abdomen. The male is greenish-yellow, winged, with black 



