II FLIES WITH AQUATIC LARVAE 213 



Some other two-winged Flies, such as the Blow-fly, 

 which form a pupal cocoon out of the larval skin, 

 emerge by pushing off the end of the cocoon. For 

 this purpose the head of the fly is provided with a 

 vesicle capable of dilation or contraction. Reaumur 

 says that he took it for granted that the fly of 

 Eristalis would emerge in the same way. Finding a 

 pupa in which the part of the cocoon adjacent to the 

 respiratory horns was burst open, he looked into 

 the cavity and saw part of the body of the fly 

 alternately elongating and contracting itself. This 

 he naturally supposed was the head, but on closer 

 examination it proved to be the tail-end of the body. 

 Shortly before the fly emerges it must turn itself 

 completely round in its cocoon, a manoeuvre of some 

 difficulty, seeing how nearly the body fills the cavity 

 in which it lies. 1 The flies appear in early spring, 

 and again in autumn. 



Several species of Eristalis or allied Flics came 

 under Reaumur's notice. Though agreeing in essen- 

 tial points of structure and mode of life, they differed 

 in size, coloration (especially in the winged state) and 

 to some extent in the situations selected for the 

 deposit of the eggs. 



More than one species every year laid eggs in 

 buckets partly filled with water, which were kept 

 in Reaumur's garden, and he was able to observe 

 the operation. The winged female flew round the 

 inside of the bucket, as if to explore its surface, and 



1 Mr. J. J. Wilkinson finds that the supposed reversal of the 

 body in the larval skin is a fable. The fly comes out head 

 first (1903). 



