Galls caused by the larvae of Gecidomyia persicariae 

 Linn. 



on 



the leaves of Polygonum amphibium Linn. 



PLATE 8-2. 



Synonymy of Insect. 



Cecidomyia persicarm Theobald. 



Habitat of the Amphibious Polygonum.— In ponds and 

 ditches, around the edges of lakes fringed with reeds and 

 rushes, and other similar spots throughout Britain. 



The Gall is formed by the thickening of the leaf, the margins 

 of which are caused by the larvae to roll under and inwards 

 until they meet under the mid-rib. 



Manner of Growth.— Glabrous ; glossy. 



Colour. — Pale green suffused with pink and purple. 



Average Dimensions of a Mature Specimen. 



Length, 45 mm.; breadth, 15 mm.; girth, 35 mm. 



May be sought during the months of July, August, and 

 September. 



The Growth is complete by the end of August. 



The Typical Condition of the gall is multilarval. 



The Larv^ pupate in the gall. The imagines emerge 

 during the spring. 



This is a very interesting gall, and one which cannot be 

 mistaken. The rolled and distorted leaves are usually those 

 near the top of the stem ; and although this plant grows in dense 

 patches, the galled leaves are not difficult to discover, especially 

 after they have assumed the beautiful rose-pink or purple patches 

 of colour with which most specimens are decorated. Attention 

 is directed to the spiral conformation of the galled leaves. 



The illustration is of specimens gathered at Guestling by the 

 Rev. E. N. Bloomfield, M.A., F.E.S. 

 196 



