378 TEXT-BOOK OF ZOOLOGY 



the nauseous mass and feed to their hearts' content. Their food being 

 on the spot, they are not obliged to move from their place, and are con- 

 sequently legless creatures or maggots (see larva of bee) ; moreover, as 

 they live in the dark, they are eyeless and of white colour (see larva of 

 cockchafer). 



(c) About a fortnight later the larva enters the pupa stage. In its 

 passage to this condition it does not, however, cast its larval skin ; this 

 merely shrinks somewhat, hardens, and forms a protecting envelope for 

 the pupa, forming a small cask, as it were, in which the pupa rests. 



F. Importance in the Economy of Nature. 



From the manner in which it feeds, the maggot of the house fly (in 

 common with those of other fly species) rapidly disposes of a quantity of 

 putrefying substances, which would otherwise pollute the air. The fact, 

 too, that the fly itself serves as food for many other animals (name 

 some) teaches us to regard even this despised creature as a by no means 

 unimportant link in the machinery of Nature. 



Other Species of Fly. 



A host of flies buzz around us on each warm summer day. 



Among them, recognisable by its deep humming note and the brilliant 

 blue abdomen, we may note the Blue-Bottle Fly (Musca vomitoria), 

 which is fond of depositing its eggs (" fly-blows ") upon meat and cheese. 



The jumping maggots found in cheese are, however, the progeny of 

 another species, viz., the Cheese Fly (Piophila casei). 



The Grey Flesh Fly (Sarcopliaya carnaria) deposits its maggots, 

 which escape from the eggs before they are laid, on meat and dead 

 animals. This ugly creature reaches a length of above | inch, and is 

 easily recognised by its reddish head and the grey and black chequered 

 abdomen. 



Where dung is deposited, we soon meet with the brilliantly green 

 coloured Gold Fly (Musca ccesar). 



The usually gay-coloured flies often seen hovering over flowers with 

 rapidly vibrating wings belong to the genus Syrphu-s. Their larvae are 

 mostly green-coloured, and live on leaves. In external shape they are 

 not unlike a leech, and their food consists of green flies or aphides, 

 whose juices they suck. 



On flowers also we find the Drone Fly (Eristalis tenax), which has a 

 deceptive resemblance to a honey-bee. Its larvae frequent drains, cess- 

 pools, puddles, etc., and are known as " rat-tail grubs." 



