706 



Pomona College Journal of Entomology 



often on the undersides of tlie leaves, though not a few are formed from above, and 

 along the margins or midvpay between the margins and the midrib in longitudinal 

 rows. From the photograph (Figure 225) it will be seen that they are arranged 

 indiscriminately also and may be very near the midrib in a single row or the rows 

 may extend transversely to the main axis of the leaf. They are formed by a 

 pushing up of the leaf tissues and the infolding of the tissues around the margins 

 of the galls so as to almost completely close the mouth. The surface of the galls 

 varies from a light green to a dark red in color and is usually rather rough. Each 

 gall is just large enough to conceal a single individual and grows with its occupant. 



Figure 225. Pemphigus populimonilis Riley 



Showing moniliform galls from ventral and dorsal surfaces of leaves. 



Never more than one insect is found in a single gall, but often there are several 

 guests in the form of the larvae of syrphid flies which completely destroy large 

 numbers of the lice before they mature. Figure 226 shows the opened galls and 

 in some are to be seen these maggots. It is difficult to find a single gall that 

 does not contain at least one of them. The adult fly deposits her eggs in the gall 

 while it is being formed and before the mouth closes up and thus defeats the 

 object of protection which the galls should furnish the lice. If anything, the 

 larvse of the syrphid flies are protected from their natural enemies while they are 

 feeding upon them. 



