Nematus gallicola. 



The species of saw-fly which causes these galls on the leaves 

 of various willows is double-brooded. The willows it principally 

 attacks are S. alba, S. caprea, S. cinerea, and 5. fragilis ; 

 more frequently on the last named than on any other. It 

 is the most plentiful of all the willow galls. During April 

 or May the parent insect oviposits in the leaf-buds, and as 

 the leaves unfold the gall-structure develops. For several 

 weeks it is solid, containing the ovum in a small cavity in or 

 near the centre ; ultimately the larva hatches and feeds upon 

 the store of material, thus producing and maintaining an ample 

 amount of space around itself, until the walls of the cell are 

 reduced to a minimum of thickness. In the meantime a small 

 hole has been eaten at one end of the gall, with the opening 

 on the underside of the leaf, through which the fras is ejected. 

 The swelling is equal in proportion on both surfaces of the leaf, 

 and is situated indiscriminately along the margins and either 

 side of the mid-rib, but never upon it At first the colour is pale 

 green, afterwards becoming suffused with red or purple. The 

 surface of the upper portion is glabrous, irregularly wrinkled ; 

 the other portion may be glabrous or pilose, pale or dark 

 green, coloured or not, all varying much according to the 

 species of Salix. In shape they resemble a bean, and hence 

 their popular name of "bean gall." They vary in number, as 

 many as fifteen being found on a single leaf. The imagines of 

 those which have passed the larval stage during the summer 

 lay eggs on leaves in the early autumn, thus giving rise to the 

 second brood, which pass the winter within the gall. 



The illustration is of specimens gathered at Hastings by 

 the author. 



