38 Galls y Gall-makers, and Cuckoo Flies. [Sess. 



solid, but as the larva gradually devours the internal substance 

 it becomes hollowed out into one or more chambers. The 

 walls of the marble gall and of the commercial gall-nut remain 

 very thick, so that the mature insect has to tunnel its way 

 out. Some of the saw-flies, on the other hand, so completely 

 eat away the interior that only a thin external shell remains, 

 which is ultimately perforated to admit of the escape of the 

 grub. In rare cases the gall opens by means of a circular lid 

 or plug. A monothalamous gall has but one chamber, which 

 may be tenanted either by a single larva or by several living 

 together. In a polythalamous gall, on the other hand, there 

 are a number of separate chambers, each inhabited by one or 

 more grubs. Many of the gall-flies undergo their entire meta- 

 morphosis within the gall : the egg is hatched ; the larva feeds, 

 moults, pupates, and only emerges after the winged condition 

 has been attained. It is otherwise with some of the saw-flies ; 

 the larvae, when full fed, escape from the gall and descend to 

 the earth, where they pass into the pupa stage. 



The oak is a favourite tree with gall-making Hymenoptera ; 

 on the oak alone they make in this country more than fifty 

 kinds of gall. A saw-fly, ]N'ematus gallicoli, causes the well- 

 known egg-shaped swellings on the willow leaf ; the mossy 

 " bedeguar " of the wild rose is the gall of a cynipid, Khodites 

 roseae ; the red berry-like gall on the leaves of the same plant 

 is the work of another species, R eglanteriae ; and the pretty 

 pear-shaped gall on the stem of the spiny rose is also due to 

 a member of the same genus, E. spinosissimse. The mossy 

 rose - gall was formerly used in medicine, and its name 

 " bedeguar " is of Arabic origin. A pillow of these galls is 

 supposed to be good for inducing sleep. The elegant tuft of 

 pink- coloured fibres conceals a woody core, in which are several 

 larval cells — sometimes as many as thirty — each occupied by 

 a grub of the gall-fly Ehodites. On the sweet-brier a similar 

 but smaller gall is produced by E. rubiginosas. 



Each gall-maker not only confines itself as a rule to one 

 particular species of plant, but selects by preference certain 

 parts of the plant, such as the leaf-blade, petiole, flower, buds, 

 twigs, or roots. In locating its punctures the rose gall-fly in 

 particular displays extraordinary sagacity. The bedeguar is 

 a transformed leaf-bud, the moss-like fibres representing the 



