CHAP. XXVII INSECTA : HYMENOPTERA 



441 



within these galls is distinguished as Spathogaster haccarum. 

 From the currant galls, in June, emerge hoth male and 

 female gall insects ; the latter lay fertilised eggs in tender 

 young oak-leaves, causing once more the production of 

 spangle galls, and so the cycle is completed. 



Similar alternation of two generations— one sexual and 

 one parthenogenetic — and of the alternation of two corre- 

 sponding different forms of galls, are known in a great many 

 other cases. Some of these are given in the following Table, 

 and are illustrated in Plates V. and VI. Those mentioned 

 are only a few of the many that are known ; they are chosen 

 for mention here either because they are well-marked, easily 

 recognisable forms, or because they are very frequently found 

 in the south of England. 



OAK GALLS WITH ALTERNATING GENERATIONS ^ 



Galls oooubrikg in Late Sdmmer 

 OR Autumn. 



Neuroterus ( 9 only). 



1. The Common Spangle (W. lentiadaris). 

 Flat hairy discs below the leaves, 

 thickest in the centre, mature in Sep- 

 tember. Plate v., Ai. 



2. The Smooth Spangle (N. laeviuscvlus). 

 Discs pale green, smooth, or with 

 hairs only round the central knob, 

 margin incurved. Plate VI., J**!. 



3. The Silk-hutton Spangle (N. nwmls- 

 tnatis). 



Brown silky button-like discs with 

 a central depression, n^ature in Sep- 

 tember. 



Plate VI., Gj. 



-I- 



Galls occubrino in Spring or 

 Early Summer. 



Spathegaster ( ? and S). 



1. The Currant Gall (S. baemrum). 



On the leaf or on male catkins in 

 early spring. 



Plate v., ^«. 



Schenck's Gall (S. aZbipes). 



A small egg-shaped outgrowth on 

 the contorted leaf margin ; in May. 

 Plate VI., Fa. 



The Blister Gall (S. veslcatrix). 



Inconspicuous green discs, partly 

 embedded in the lower surface of the 

 leaf; with lines radiating from the 

 central knob to the margin. 



Plate VI., Ga- 



The Cupped Spangle (W. fumipennis). 

 Pale or reddish discs with no central 

 knob, but with margin often slightly 

 curved up ; on the surface are delicaiie 

 brown hairs. 



4. The Hairy Pea Gall (S. hmlor). 



Very like the Cnrrant Gall, but 

 lighter coloured and covered at first 

 with short white hairs, found below 

 the leaf in Jtine. 



8. The Oyster Gall * (N. ostrevs). 



Occurs on the mid-rib below a leaf, 

 small and oval, at first enclosed 

 between two brownish scales ; pale 

 yellow, often spotted with red ; small. 



5. April Pea GaU (S. wprilvaus). 



Yellow green swellings within the 

 bud scales, very thin walled, may 

 contain more than one larva. 



* This form should perhaps be removed from the genus Newroterus ; see Adier. 



^ The nomenclature is based on that given in Adler's Alternating Genera- 

 tions, A Study of Oak Galls and Gall Flies, translated Into English and 

 edited by C. R. s'traton (Clarendon Press, 1894). 



