Galls caused by the larvae of Hormomyia millefolii Lw. 



on 



the leaves of Achillea millefolium Linn. 



PLATE 97. 



Synonymy of Insect. 



Cecidomyia achillea; Inchbald. 



Hormomyia millefolii Theobald. 

 Habitat of the Common Yarrow. — One of the most 



plentiful of British wild plants, varying considerably in 



size according to situation. It grows on waste ground, 



meadows, hedge-banks, and roadsides. 

 The Gall is formed by the larva causing swellings on the 



pinnatifid segments of the leaf, and also on the mid-rib 



of the leaf. 

 Manner of Growth. — Gregarious ; pedunculated ; irregular 



in shape. 

 Colour. — Pale green, unrelieved by any colours. 

 Average Dimensions of a Mature Specimen (globular 

 form). 



Girth, 9 mm. 

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



August. 

 The Growth is complete by the end of July. 

 The Typical Condition of the gall is unilocular. 

 The Larva pupates in the ground. The imago emerges 



during spring. 



In order more easily to discover these galls, the fingers 

 should be drawn along the leaf while it is attached to the plant ; 

 the little swellings will then be more easily detected than if 

 simply looked for, as their similarity of colour to that of the 

 leaf makes them difficult to be seen quickly. They are very 

 local, but usually very abundant where they occur. The natural 

 size of the leaf in the centre of the illustration is ■j\ in. long, 

 and it bears ninety galls. 



The illustration is of specimens gathered at Hastings by 

 the author. 



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