HYMENOPTERA OAK GALLS. 



l r, r, 



very often be found by workmen when trenching or uprooting trees in 

 winter. They are of a coffee-brown colour, and vary in size, but the 

 average is about the bulk of a common pea 

 (fig. 153). They are generally procured in 

 clusters, which very much resemble the honey- 

 combs of the moss-bee, Bombus nmscorum. 

 Adler says that, after many observations, he 

 found that the flies issue in December and 

 January, but adds that other observers found 

 the time of their appearance rather variable. 

 This last remark I can quite endorse, inas- 

 much as galls found by workmen in February 

 did not hatch until well on in spring. In 

 this case there is no mistaking the species or 

 confounding with parasites, inasmuch as the 

 specimens are all apterous. 



The fly varies in size from 4 to 7 

 mm. long, wingless, with a rather slender 

 thorax. The form is somewhat spider-like, 

 and the whole body of a uniform brownish 

 colour. 



With regard to the alternate form, Adler 

 made many observations, and found that the 

 eggs deposited by Biorhiza aptera result in the formation of the 

 well-known oak-apple. 



Adler watched the egg- laying process very closely, and found that 

 the insect did not de- 

 posit the egg immed- 

 iately after making the 

 incision, but acted on 

 the principle of " one 

 thing at a time," inas- 

 much as she carefully 

 pricked the buds and 

 then returned to deposit 

 the eggs in previously 

 prepared channels. In 

 this way he found that 

 on January 27, 1878, the fly deposited 582 eggs during 87 hours. 

 Towards the end of May these galls were fully developed oak-apples. 



Fig. 152. Galls of Diyo- 

 phanta divisa. 



Fig. 153. Galls of Biorhiza aptera. 



