270 OAK GALLS AND GALL INSECTS 



reproduction has become parthenogenetic, owing to a gradual 

 loss of males, there being only one generation a year. It is 

 probable that the process has been equally slow in other forms, 

 but is so far advanced that only females are known. In Rhodites 

 rosae we may conclude that males will soon be quite as extinct 

 as they at present are in the agamic forms of oak gall-makers. 

 Again, the receptaculum seminis is present both in the agamic 

 and sexual generations, though atrophied in the former. Unless 

 this can be explained by its being inherited in the agamic 

 generation from the sexual, but being functionless has gradually 

 become atrophied from want of use and may eventually dis- 

 appear, it seems to point to the fact that males were present at 

 some remote period. iJetore copulation can take place the 

 ovipositor of the female must be extruded. Copulation takes 

 place soon after the insects leave the galls, and the females sit 

 with the ovipositor extruded awaiting fecundation. But this 

 extrusion is carried out by the females of the agamic generations 

 also, though they have no males to fecundate them and repro- 

 duce parthenogeneticalh'. Does not this apparently prove that 

 these agamic generations at one time had males, or could this act 

 come under the head of inherited instincts ? 



Some galls, especially bark galls such as Andricus radicis, 

 require two years in which to complete their generation cycle. 

 The rudiment of the gall is formed in the first year and further 

 development is arrested till the following spring, when the gall 

 formation is renewed with a fresh period of vegetative activity. 

 Maturity is reached in the autumn, the flies emerging in the 

 spring of the next year. These lay eggs in the buds, which give 

 rise to the sexual generation [A . tvilineatiis Htg.) which in turn 

 oviposit to form the A. radicis gall. Galls formed by the brood 

 appealing in the spring take a much shorter time to reach 

 maturity than those formed by the sexual generation. 



Insects of the agamic generation appear in early spring 

 often in very cold weather, some forms, such as Biorhiza 

 terminalis, irequently ovipositing during the winter, when the 

 snow is on the ground and the temperature at freezing point ; 

 consequently it is a good thing for them to be able to lay their 

 eggs at once, without previous connection with a male. The 

 sexual generation on the other hand appears in the summer 

 when the weather is warm, live longer, and so have more time 

 to spare for the process of continuing their species. 



