DIPTERA. 



67 



feeding upon the meat, rapidly grow until they reach maturity and pass into the 

 pupa stage. Many persons believe that blue-bottles arc full-grown examples 

 of the house-fly, and when informed that such is not the case, and that these 

 insects after reaching the winged stage are incapable of growth, point out that 

 blue-bottles vary greatly in size, and ask what may be the explanation of the 

 difference. The answer is, that the size of the blue-bottle in its final stage depends 

 upon the size of the maggot before pupating, and the size of the maggot upon 

 the amount of nourishment it is able to obtain before its supply of food was 

 exhausted. In any given case, when the supply is limited, the maggots that 

 are the first to hatch will get more food than those that appear later, and 

 in consequence, when the whole of it is exhausted, will have attained a greater 

 length and fatness than the others, and thus become converted into larger flies. Or, 



GROUP OF FLIES AND THEIH GKUBS. 



1, Blow-fly ; 2, Eggs ; 3, Larvae ; 4, Pupa ; 5, Newly-born larva of grey flesh-fly ; 6, Grey flesh-fly : 7, Adult-larva 

 of the same ; 8, House-fly and larva ; 9, Sharp-mouthed fly ; 10, Head of house-fly ; 11, Foot of grey flesh- 

 fly ; 12, Carcase of house-fly killed by fungus growth. (10, 11, enlarged ; the other nat. size.) 



again, if three or four hundred eggs be laid in a dead mouse and the same number 

 in a dead rabbit, it is clear that in the former case the supply of food will be 

 smaller for each larva, and will sooner come to an end than in the latter. 



The grey flesh-fly (Sarcophaga carnaria) is a handsome species, measuring in 

 the female half an inch in length. Seldom entering houses, it is not uncommon 

 in the open country, where it may be seen basking in the hot sun upon stones 

 or walls. Its prevailing colour is pale slate-grey, variegated on the thorax with 

 black bands, and the abdomen with square black spots, set corner to corner like 

 the squares of a chess-board. A noteworthy fact connected with this species is 

 that the eggs hatch within the parent before being laid, so that the young are 

 born alive; they feed upon decaying animal and vegetable matter. The blow- 

 flies belonging to the genera Calliphora and Lucilia, respectively known as 

 the blue-bottle and green-bottle flies, as a general rule deposit their eggs upon 

 dead animal matter. This, however, is by no means always the case, there being 

 many instances on record of the laying and hatching of the eggs upon living 



