1907-1908.] Galls J Gall-makers, and Cuckoo Flies, 43 



A group of more importance as gall-makers, however, is the 

 family of mites. Gall-mites are not, properly speaking, insects, 

 since they belong to the Arachnida or spider class ; their galls 

 are exceedingly common, and assume a variety of forms. On 

 the leaves of the lime, alder, beech, plane, bramble, and other 

 plants they occur as little tufts of felted or cottony hairs. 

 Sometimes they project from a leaf in the form of pegs, nails, 

 horns, purses, or warts. The " witches' brooms " of the birch 

 and hazel are due to species of Eriophyes (Phytopus). The 

 disease known as " big-bud," which is rapidly exterminating 

 the black currants of our gardens, is occasioned by another 

 species of mite, E. ribis. Examples may also be found on the 

 hawthorn, rowan, walnut, ash, willow, sloe, aspen, yew, Galium, 

 Poterium, Ajuga, &c. Gall-mites also roll rhododendron leaves, 

 and occasionally make witches' brooms on the privet and lilac. 

 Their little bud-like galls on the wild thyme are familiar to 

 most naturalists. 



These mite-galls, as also those of the Aphides and some 

 even of the dipterous gall-makers, are formed differently from 

 the closed solid galls of the Hymenoptera. In the pocket and 

 embossed forms the leaf becomes gradually bulged so that the 

 grub is enveloped; in other cases an overarching of tissue 

 above the insect or its ^gg occurs, closing it in above. Galls 

 of this description commonly begin to gape at the apex when 

 the time arrives for the occupants to take their departure. 

 The aphis gall on the poplar petiole affords a good illustration 

 of this mode of dehiscence. 



The eel-worms, or Nematodes, so destructive to grass and 

 wheat, occasionally give rise to galls. The leaves of the 

 yarrow, Achillea, are galled by the eel-worm Tylenchus 

 millefolium. 



It only remains to mention the class of fungal galls. One 

 of the most remarkable of these is a cherry-like gall produced 

 on the laurel by a parasitic fungus, Exobasidium Lauri. Fungal 

 galls are also produced by species of Gymnosporangium on the 

 juniper, mountain-ash, and other trees. The fungus Exoascus 

 attacks the plum and cherry, converting the ovary into a kind 

 of gall ; and on the silver fir and other trees species of 

 ^cidium give rise to witches' brooms. The witches' brooms 

 of the birch and hazel, as we have seen, are caused by mites. 



