HYMEN OPTERA OAK GALLS. 143 



in man passes its early stage in the pig, and the latter stage in man. 

 Certain plant diseases pass one stage on weeds and the other on trees. 

 These are two typical instances from animal and plant life. 



Alternation of generations is totally different from these phenomena, 

 inasmuch as two distinct generations alternate one with the other. 

 In fact, this may he best understood when we say that the children 

 never resemble their parents, but are always facsimiles of their 

 grandparents. 



Having mentioned two distinct genera resulting from alternat- 

 ing generations, the question arises, Are the two genera widely 

 separated in point of general appearance or anatomical details 1 Adler 

 considers they are, and accordingly adopts Neuroterus and Spathe- 

 gaster as generic terms. But Professor Mayr of Vienna classifies 

 the two genera as Neuroterus, and this classification is adopted by 

 Cameron. 



The main distinguishing features are slight differences in colour and 

 in general stoutness, Neuroterus being stouter than Spathegaster. 

 The abdomen differs in size and appearance, resulting from the size 

 and shape of the ovipositor (fig. 129, e), which must be viewed in the 

 relation of adaptation, a long ovipositor being necessary for penetrating 

 buds, and a short one for leaves. 



la. Spathegaster baccarum (Htg.) 



The galls are found on the flowers and leaves (figs. 132 and 133). 

 They are pea-shaped, smooth, glossy-looking, soft and succulent, of a 

 greenish colour, but have often a few bright-red spots. They are 

 situated on the under side of the 

 leaves, but often penetrate through the 

 upper side and rise slightly above it. 

 The specimens found on the male cat- 

 kins are generally smaller than those on 

 the leaves, and slightly more reddish 

 in colour. / 



The fly (fig. 134) has a black shin- Fig. 134,-Spathegaster baccarum. 



ing body, about 2| mm. in length, 



antennae brownish, 14-jointed in the male. The legs are yelloAv, with 

 black coxae at the base. Tarsi 5-jointed ; wings longish, broad at the 

 apex, and about 8 mm. in expanse. In a comparatively forward 



