1913.] 209 



SEXUAL DIMOKPHISM IN A SPKCIES OF SCIARA. 



BY F. W. EDWARDS, B.A., F.E.S. 



(Published by pcnnissio7i of the Trustees of the British Museum.) 



At the end of February of the present year, I received a number 

 of Sciarid larvae which had been found by Miss F. Collins in her 

 garden at Clapham, near Worthing, Sussex. The larvae were practi- 

 cally full grown when received ; they were about 8 mm. long, yellowish 

 in colour, and with a double row of oval lemon-yellow granules along 

 the dorsal surface arranged in groups of six on each of the last six or 

 seven abdominal segments. The larvae pupated soon afterwards, and 

 spent nearly three weeks in the pupal state, at the end of which time 

 I was fortunate enough to hatch out some Hies, 4 ^^ and 8 $ . It was 

 at once noticeable that all the males were small spidery creatures with 

 reduced wings and quite incapable of flight, but as there was a 

 possibility that out of so small a number, all the males might have 

 happened to be cripples, I decided to wait until further evidence was 

 forthcoming before deciding that this was a peculiar case of sexual 

 dimorphism. 



The additional evidence was available much sooner than was to 

 be expected, for to my great surprise and pleasure, Mr. W. L. Distant 

 brought me a lot of exactly similar larvae on April 23rd. These were 

 dug up by Mr. Distant in his garden at Norwood, where they were 

 feeding in immense numbers on decaying woody roots ; Mr. Distant 

 told me that he could easily have (collected a gallon of them ! These 

 larvae again were practically full grown. Before pupating they fonned 

 into a procession and explored the boundaries of their tin, but finding 

 themselves unable to migrate, or perhaps thinking they had done so, 

 they all pupated in a mass, forming themselves a common covering of 

 silk and minvite particles of rotten wood. After a fortnight spent in 

 the pupal stage, the flies emerged about May 20th, to the number of 

 about 50 J* and 150 ? . As was the case with the original specimens 

 from Sussex, the males were all much smaller than the females and 

 with reduced wings ; moreover, the wings were not only reduced in 

 size, but the neuration was degraded in an almost constant manner. 

 The males were aljsolutely iucapajjle of flight ; the females, tliough 

 they could fly when compelled to do so by l)eing dropped, never 

 attempted of their own acco'-d to use their wings, the disinclination 

 probably being due to the size and weight of the abdomen. The 

 wings of the male, in life, were about two-thirds as long, those of 

 the female about the same length, as the abdomen ; after death the 



