1 907-1908.] Galls, Gall-makers, and Cuckoo Flies. yj 



these, all theories of evolution must of necessity be more or 

 less tentative. 



Foremost among gall-makers are the Cynipidse, a hymenop- 

 terous family. These insects belong to the same order as bees 

 and wasps, and in common with them have four clear wings ; 

 they dijBfer, however, in their smaller size, and in the sting 

 being replaced by a slender ovipositor for drilling holes and 

 conveying eggs into the tissues of plants. The cynipids are 

 the gall- wasps or true gall-flies ; they produce the most perfect 

 galls, and there are in this country some forty or fifty different 

 kinds. The galls of the Hymenoptera are completely closed, 

 and consist in some cases of as many as seven distinct layers. 

 The central portion constitutes the food of the larva ; as the 

 cells are devoured they are replaced by a fresh crop from time 

 to time, the insects' larder being replenished by this renewal 

 of the gall-pith. Where the outer layers of the gall are hard 

 they may afford protection against birds and other enemies, 

 although it is stated that the unpalatable taste of galls repels 

 birds, and that they only have recourse to them when other 

 food is scarce. Both in outward appearance and in internal 

 structure galls present not a little resemblance to fruits. The 

 galls produced by Neuroterus baccarum on the male catkins of 

 the oak are red and succulent, and might easily be mistaken 

 for the berries of the red currant. One of the beech galls 

 resembles a small plum, the outer layer being soft and the 

 inner hard. On several labiate plants species of Aulax make 

 galls which resemble cherries, having an external succulent 

 layer surrounding a stone in which the larva of the gall- 

 producer takes the place of the kernel. Parasitic insects may 

 find difficulty in penetrating these layers, and a similar purpose 

 may be served by the moss-like covering of the rose-gall. 

 Probably, however, the chief use of the coverings is to prevent 

 the dessication of the galls. An obscurely coloured gall has 

 the advantage of escaping notice, but brilliant tints may also 

 be protective if they serve to warn away the depredators. 



The saw-flies or Tenthredinidse, a group very closely allied 

 to Cynips, also produce numerous galls of like character. 



At different stages of development galls contain eggs, cater- 

 pillars, pupae or winged insects, according to the degree of 

 advancement. The gall of a hymenopterous insect is at first 



