1912] Morphology and Biology of Insect "Galls 305 



Dimensions: — Average length including stalk 21 mm.; average 

 width 10 mm. 



This gall is covered by an unusually small-celled epidermis. The 

 spines that are so noticeable a feature are found to consist of projections 

 of the epidermis, filled with cells in continuity with the mesophyll. The 

 cells of the gall are almost perfectly circular in outline and packed to- 

 gether very closely. This tissue is very uniform except in the four or 

 five layers adjoining the gall cavity. In that zone the cells are smaller 

 and richer in protoplasmic contents, constituting a fairly well marked 

 nutritive layer. 



In cross section of the gall about thirty main fibro-vascular bundles 

 are cut; these are comparatively large and situated near the larval cavity. 

 Two of these have been cut in the section shown in Fig. 10. Other 

 smaller strands are cut further out. The gall receives all the fibro- 

 vascular strands that, under normal conditions, would have passed up 

 into the flower. The interior of both this and the preceding species is 

 almost perfectly glabrous. 



Aphid Corrugations on Birch. 



Tj JBetula lenta L. 



[Betula alba var. papyrifera (Marsh) Spach. 



The primary folds in the leaf that form this gall run parallel to the 

 main veins, with the latter as boundaries between them. Their crests 

 are on the upper side of the leaf, while the hollows which form the larval 

 chambers are on the under side. The primary folds are divided into 

 secondary folds, and these again into depressions resembling minute 

 Acarina dimple-galls. This complex arrangement is conditioned entirely 

 by the veining of the leaf, since each fold, primary or secondary, is sup- 

 ported along its edges by veins. The folding can be seen in Fig. 9. 



The anatomical characteristics of these galls show that the folding 

 of the leaf has not entirely changed the structure of its normal mesophyll. 

 Around the gall cavities the spongy parenchyma is nearly normal through- 

 out and the palisade layer is recognizable in different places. The cells, 

 however, are considerably larger than the cells of the normal mesophyll. 

 The cells of the lower epidermis, that form the lining of the gall cavities, 

 are well filled with food materials for the larvae. The supporting veins 

 on each side of the fold send out branches that supply the gall with an 

 adequate vascular system. 



Hormaphis hamamelidis Fitch and Hamamelistes spinosus Shimer, 

 as worked out by Pergande,"" show that they inhabit alternately Betula 

 nigra L. and Hamamelis virginiana L. 



