GALLS CAUSED BY FLIES 63 



Mel. T. Cooke, who makes a special study of the galls, 

 and he advises me as follows : ' There is but one species of 

 gall reported on Taxodium distichum, and I have specimens 

 of that species. It is entirely different from the one you 

 send me. The gall which you send is of insect origin 

 without doubt, and apparently belongs to the genus 

 Cecidomyia.' " 



One of the commonest of British galls is caused by the 

 presence of the larvae of Pernsia ulmariae on the leaves of 

 the Meadow Sweet. Wherever this plant occurs — it ranges 

 throughout the British Isles and Europe — the observer will 

 find the galls in abundance during the summer. The fly 

 attacks the leaf in late spring, and the galls appear soon 

 after as small, glabrous, light green, umbonate pustules on 

 the upper surface, with whitish projections on the lower 

 one. They are usually situated on the ■ midrib or the larger 

 lateral veins, and are often densely gregarious, causing the 

 leaf to pucker, but not otherwise producing very marked 

 distortion. Over 200 galls have been counted on one leaf. 

 Later, the gall assumes a reddish-brown or pinkish tint 

 on the upper surface, the lower remaining greenish-white. 

 The umbo vanishes from the upper part, and the lower 

 assumes the form of an elongated cone covered with a felt 

 of whitish hairs. The cavity is somewhat triangular. The 

 larva is yellowish-orange. It pupates in the point of the 

 cone. . A circular separating line is formed in the tissue, to 

 enable the cap to come away easily when the fly emerges. 

 This is easily demonstrated ; by seizing the point of the cone 

 with forceps it comes away quickly, and always in exactly 

 the same way. The larva frequently bears an external 

 parasite, a minute, active, hyaUne, worm-like creature. 



Pernsia ulmariae also attacks Spiraea filipendnla. It is 

 noteworthy that the galls on this plant differ very markedly 

 from those on S, ulmariae ; the truncated cone opens on to 

 the upper surface of the leaf, the pustule being on the 

 inferior one. As a rule the galls caused by any species of 

 insect on closely allied plants present very slight differences 



