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Gall Flies on the Oak. 



[JULY, 



larva lives and feeds after the hatching of the egg. Guest 

 flies or inquilines may lay their eggs in the gall itself, and 

 their larvae on hatching use the gall as a place of habitation 

 without harm to the insect actually causing the gall. 



An interesting, feature in the life-history of these gall flies 

 is the occurrence in many of them of an alternation of genera- 

 tions, a generation of virgin females alternating with a 

 generation containing both males and females, the gall from 

 which the one generation emerges being different from the 

 gall to which that generation gives rise. 



Neuroterus lenticularis , — In the case of this species of 

 gall fly, the galls occur in large numbers on the under side 

 of oak leaves (see Fig. i), showing after midsummer and fall- 

 ing away in the autumn. Each gall holds a single larva. The 

 winter is passed in the larval condition, and the adult flies — 

 only females — issue in the next spring or early summer. These 

 females do not puncture expanded oak leaves, but prick the 

 buds and give rise to small round galls — the Spathegaster 

 baccarum galls — 3 to 5 mm. in diameter, green in colour, 

 spotted with red. These galls are found on the leaves and 

 on the axis of the male catkins (Fig. 2). The adult flies — males 

 and females — issue from these galls about the middle of June. 

 The females after pairing pass to the leaves of oak trees, 

 which they puncture, the result being the galls of Neuroterus 

 lenticularis . 



Oak Apples.— -The oak apple galls result from the puncture 

 of a bud — usually a terminal bud — by a virgin female that has 

 issued from a root gall. These females, which are wingless, 

 prick a bud repeatedly, and as a number of eggs are laid in 

 the same bud the gall which results (Fig. 3) is a many-cham- 

 bered one. At first the gall is soft, but when "ripe " in June or 

 the beginning of July, the central portion is woody. The 

 male and female flies (Teras terminalis) issue in July. The 

 females are without functional wings. The fertilised females 

 pass to the roots of the oak (some, however, prick buds and 

 leaf-stalks, causing galls on these), where eggs are laid, with 

 the result that a gall appears which breaks its way through the 

 cortex. The ripe galls on the root are hard and of a brown 

 colour. The adult flies from these root galls (Biorhiza 

 aptera) pass to the buds as stated above. 



