DEHISCENCE OF CERTAIN GALLS. 



539 



SO embedded in the leaf that it projects on the upper side as a small pointed 

 cone, and on the lower side as a disc covered with a thick coating of hairs. In 

 the autumn a circular piece like a lid becomes detached from the lower side of 

 the chamber. It corresponds exactly with the extent of the hairy disc, and is 

 so sharply defined that it looks as if it had been cut out with a knife (see figs. 

 362 ^ and 362 ^). The operculum falls off, and the larva which had emerged from 

 the egg and which has lived all the summer in the gall-chamber tumbles out 

 and makes its way into the ground, where it begins to spin. By the next spring 

 it has entered the pupal stage, and the gall-gnat creeps out of the chrysalis 

 about May. 



Still more peculiar are the galls produced by Oecidoses Eremita on the green 



;.— Solid Galla. 



;ipsule-like galls on a leaf of the Broad-leaved Lime (Tilia grandifolia) produced by Hormomyia Eiaumuriana. 2 Longi- 

 tudinal section through one of the galls, showing the maggot in the interior; x 2. » Longitudinal section through a 

 capsule gall from which the inner gall is just being extruded ; x 2. * Outer gall after the extrusion of the inner gall; x 2. 

 5 Inner gall at the moment when the operculum is thrown off ; x 2. ^ Capsule-galls on the leaf of a Brazilian species of 

 Cetastrus. 7 Longitudinal section through one of these galls; x 2. 8 xhe same after the inner gall has fallen out; X 2. 

 1 and ^ natural size. 



cortical tissue of young twigs of Buvaua longifolia, a South American represen- 

 tative of the Anacardiacese (see figs. 362^ and 362^). The gall is quite spherical 

 and very hard, and its large cavity conceals the caterpillar which has been 

 hatched from the egg. When the time draws near for the formation of the pupa, 

 a plug with a projecting rim is developed on the side of the gall furthest from 

 its point of attachment. When the plug is pushed out a circular hole is left 

 which leads into the gall-chamber through which the caterpillar escapes from its 

 dwelling. People who have not seen these galls with their own eyes might almost 

 think this description was the work of imagination. And yet there are still more 

 wonderful forms in this class of gall-structures. On the foliage of the Lime (Tilia 

 grandifolia) a growth arises round the eggs of the gall-gnat HorTnomyia Be'au- 

 muriana which at first has the form of a flat lens inserted in the green tissue of 

 the blade, but which gradually enlarges until it projects from the upper side like a 



