546 ALTERATION OF FORM BY GALL-PRODUCING INSECTS. 



Clustered galls may be divided into two classes, those which develop in the 

 region of the flowers and those in the foliage region respectively. The most 

 noticeable and best known forms of the galls occurring in the foliage region en 

 rudimentary leafy shoots are the following: First, those peculiar structures on 

 the tops of Willow twigs (Salix aurita, Caprea, grandifolia, &c.) which are popu- 

 larly termed "Rose Willows". They are caused by the gall-gnat Cecidomyia 

 rosaria. The leaf-bud from which they arise keeps its axis quite short and 

 develops on it numerous green leaves arranged like the petals of a double rose. 

 The lowest leaves of the "rose" differ but slightly from the normal foliage of 

 the particular species of Willow. Usually there seems to be only a shortening 

 and broadening of the petiole and leaf -sheath, the green blade being almost un- 

 altered. In the upper inner leaves, however, the sheath-like part of the leaf is 

 much increased in size, and nearer the centre of the "rose" the leaves become 

 scale-like. The leaf-blade has entirely disappeared, and the end of the contracted 

 axis possesses only the remains of leaf-sheaths. It is worth noting that the 

 number of leaves in a Rose Willow is always greater than would be found on 

 an unaltered shoot of the same species. For example, if the number of leaves 

 on the one-year-old shoot of the Sallow (Salix Caprea) is 25, the number in 

 a "rose" on the same species would be at least twice as large. This can only 

 be explained by supposing that a "prolepsis" has occurred, i.e. that not only the 

 shoot laid down for the current year has developed, but also one originating from 

 a bud of this shoot, which, under normal conditions, would not have developed 

 until the following year. When autumn comes the rosette-shaped galls on the 

 Willow bushes show up conspicuously at a distance because the leaves forming 

 them do not fall off like the rest, but remain behind as brown dried structures 

 at the ends of the branches. They are also found associated with the catkins. 

 The rosette-shaped galls produced by the gall-gnat Cecidomyia cratcegi at the 

 tips of Hawthorn twigs (Cratcegus Oxyacantha and monogyna) also claim atten- 

 tion. They are full of bristles and resemble tiny birds' nests. The stimulus of 

 the gnat larvae excites a deeper and more frequent segmentation in the leaves 

 and stipules. Narrower points and fringes which are much bent and which 

 resemble the antlers of reindeers replace the broad lobes. Also soft spines with 

 capitate ends rise up from the green cortex of the twigs and from the tissue of 

 the leaf- blade, especially above the vascular bundles, and 3-5 of them often fuse 

 together into cock's-comb-like structures. These bristling rosettes on Hawthorn 

 branches also remain long after the time the ordinary foliage falls off. 



In marked contrast to the rosette-like cluster-galls are others whose leaves 

 all fold together in a ball something like the leaves of a cabbage, the whole 

 gall having a button-like appearance. The outer leaves are round and hollowed 

 on their upper side, and they usually fold together like mussel-shells. The inner 

 leaves have a similar form, but they are much smaller and more concave, and 

 they have become succulent and paler in colour. The galls produced by Ceci- 

 domyia genisticola on Genista tinctoria and those which Cecidomyia Veronicce 



