J. Rennie and C. H. Sutherland 
203 
in size in parasites occurring together are due to variations in the rate of 
growth than that the same host should have been parasitised on two or more 
separate occasions. 
Duration of Larval Period. 
First Generation. In the season 1918, Bucentes larvae were obtained as 
early as February 20th. These larvae were on an average about 4 or 5 mm. in 
length at this date. In 1919, these larvae were first obtained on February 
28th; they were from 3 to 5 mm. in length. This suggests that the parasitic 
larvae found in February are the product of eggs laid the previous autumn, 
the larvae having hibernated in the host. 
Second Generation. From June 21st onwards, large percentages of the 
collected host larvae were found to be infected with Bucentes, whose maggots 
were of such a size, viz. 1-3 mm., as to lead to the suspicion that the Tipula 
larvae had been recently infected. From these Bucentes maggots a second 
generation of flies was bred which began to emerge on July 25th. An earlier 
generation of flies was bred out in May and the early part of June and bv 
June 13th all these flies had died. Moreover, from May 16th to June 16th 
Tipula larvae were collected on four different occasions, to the number of 
355, and of these none were found to be infected by Bucentes. This evidently 
indicates that all the parasites had pupated, involving the death of the 
host larvae, so that the Tipula larvae collected between these dates were those 
which had escaped infection by Bucentes during the previous autumn. In 
view of the foregoing, viz. that there is a period of over four weeks during 
which no infected Tipula larvae can be found in the field, that within this 
period adult flies appear to die off and that subsequently infected larvae 
again appear, the conclusion seems obvious that a second generation of 
B. geniculata commences about this time. 
Bucentes maggots of the second generation were observed on June 16th, 
when they measured about 3 mm. The earliest date on which mating was 
observed was June 2nd, and no imagines survived in the observation cage 
after June 13th. Opposition, and the infection of Tipula larvae in the field, 
may therefore be assumed to take place approximately within a corresponding 
number of days. Pupation of the second generation of larvae was observed 
to begin in the experimental cage about July 8th. That is to say, the maximum 
extent of the larval period possible was in this case from June 3rd to July 
8th, viz. five weeks. This estimate is only an approximate one, as oviposition 
has not been observed and the period which elapses between oviposition and 
the hatching of the larva is unknown. 
We have been able to find in the literature dealing with this subject only 
the scantiest references to this species. Nielsen (1918), in a table showing the 
life cycle of a number of Tachinidae, includes Bucentes geniculata in the form 
reproduced below, wherein P. signifies Pupa, L. = Larva, I. = Imago. 
April—May June July Aug. Sept. Oct. Nov.—April 
Bucentes geniculata P.I L.P.I. I. I.L. I.L. P. P. 
