160 Knapioeed Gall-fly 



Effects of larvae on production of seeds in galled 

 floiver-heads of Ceiitaurea nigra L. 



In order to obtain some definite information concerning the effects 

 of the Urophora larvae on the production of seeds, a number of plants 

 were collected and a series of counts made. Perfect accuracy is not 

 claimed for the results obtained, as, owing to various circumstances, 

 the conditions were such that a high degree of accuracy could not be 

 expected. The plants were collected on Oct. Ist. 1913, that is to say, 

 late in the season, when many of the seeds had certainly escaped, in 

 addition to those probably taken by seed-eating birds. As the plants, 

 however, were collected within a space of about twenty square yards, 

 it may be assumed that these factors affected all the plants in this area 

 in a more or less similar manner. Fifty plants, single shoots cut off 

 at the ground-level, were selected, and these bore 147 flower-heads ; 

 twenty-four of these, however, were immature and therefore disregarded. 

 Of the remaining 123 heads, seventy-four or 60-1 per cent, contained 

 galls, whilst forty-nine or 39-8 per cent, were ungalled. Nine of the 

 galled heads contained no seeds, and these for the purpose of calculation 

 were neglected ; from the remaining sixty-five galled heads 1077 seeds 

 were obtained, averaging 16-5 seeds per head. 



Twenty-five of the forty-nine ungalled heads were rejected for 

 various reasons ; ten contained no seeds whatever, whilst the seeds 

 in the remaining fifteen were more or less eaten by seed-eating lepido- 

 pterous and free living dipterous larvae. 



From the twenty-four ungalled heads that were considered countable, 

 755 seeds were obtained, averaging 31-4 seeds per head. Thus showing 

 a reduction of nearly 50 per cent. (31-4 - 16-5) in the number of seeds 

 produced in the galled heads, compared with the number produced in 

 ungalled heads. The number of seeds (31-4) per head obtained from 

 these particular heads is very low, and would be misleading if one were 

 to assume that this was the average number obtainable from normal 

 well-grown flower-heads of Centanrea nigra. 



It may be explained that the soil on which the above plants were 

 grown was very poor and stony, and the specimens were gathered late 

 in the season when some of the seeds had escaped, as stated above ; 

 these considerations may help to explain the small average number of 

 seeds per head. 



By way of contrast may be quoted the number of seeds obtained 

 from some flower-heads of the knapweed gathered by Prof. Hickson, 



