IXSECTS AFFECTING PARK AND WOODLAND TREES 247 



s p i n o s u s Shimer, occurriiiijf on birch, the forinatioii of what art- known 

 as pseudo-galls, which are nothing more than deprc;ssions in the underside 

 of the leaf and corresponding elevations on its upper surface. These 

 gradually become more and more marked till the lateral veins nearly 

 coalesce and form admirable shelters. 



This may be continued further as in the case of the cockscomb elm 

 gall, Colopha ulmicola Mtcli. Here we have eventually a beau- 

 tiful cockscomblike structure on the upj)er surface of the leaf, while beneath 

 there is only a longitudinal slit marking the eilges of the orifice. These 

 galls are sometimes so abundant as to deform the leaves very seriously and 

 they mark a more advanced stage of leaf curling. There are a number of 

 plant lice which make more or less defined galls, many of them being simply 

 abnormal growths from the leaf stalk or developing bud. The species, 

 Pemphigus p o p u 1 i - 1 r a n s v e r s u s Riley, making a gall on the leaf 

 stem of our common poplars is rather abundant in the vicinity of Albany. 

 The abnormal growth is very characteristic, being about the size of a 

 walnut, and on examination there will be found a conspicuous transverse 

 scar on one surface. The growth of this gall is in all probability substan- 

 tially as follows. The parent insect establishes herself about the middle of 

 the developing leaf stalk, begins to draw sustenance therefrom and her 

 operations excite an enormous development of tissue which gradually rises 

 over and finally incloses the insect and her young. A still more advanced 

 stage is exhibited in the gall of the hickory gall aphid, P h y 1 1 o .x e r a 

 caryaecaulis Fitch, the orifice of the gall being closed. 



Another very interesting gall is that produced by Pemphigus 

 r h o i s Fitch, on sumach, another. Pemphigus u 1 m i f u s u s Walsh, 

 equally unique, occurs on slippery elm. The galls of plant lice may be 

 distinguished from those of hymenopterous gall flies by the external orifice, 

 though it may be much obscured by the abnormal growth. This opening 

 is a necessity because aphids never lay eggs within plant tissues. 



Prolificacy. The protective devices of one kind or another enumerated 

 above would never be sufficient for the maintenance of so many of these 



