302 



The Two-winged Flies 



called balancers, or halteres, whose use seems to be chiefly that of orienting 

 or directing the fly in its flight. The possession of these balancers 

 is a certain diagnostic character in distinguishing Diptera from all 

 other insects. The wings are membranous and usually clear, and 

 supported by a few strong veins. No flies can bite in the sense 

 of the chewing or crushing biting common to beetles, grasshoppers, and 

 other insects with jaw-like mandibles, but some have mandibles elongate, 

 slender, and sharp-pointed, so that they act as needles or stylets to make 

 punctures in the flesh of animals or tissues of plants. The great majority 

 of flies, however, have no mandibles at all and no piercing beak, but lap up 



liquid food with a curious folding fleshy proboscis, 

 which is the highly modified labium or under-lip. 

 They feed on flower-nectar, or any exposed sweet- 

 ish liquid, or the juices of decaying animal or 

 plant substance. To take solid food as the 

 house-fly does from a lump of sugar, the solid 

 has to be rasped off as small particles which are 

 either dissolved or mixed in a salivary fluid 



FIG. 411. Head, antennae, 

 and beak of mosquito, lat- 

 eral aspect. 



that issues from the fleshy tip of the proboscis. 



All the Diptera have a complete metamorphosis, the young hatching 

 from the egg as footless and often headless larvae (maggots, grubs), usually 

 soft and white, and in many cases ob- 

 taining food osmotically through the 

 skin. The life-history is usually rapid, 

 so that generation after generation suc- 

 ceed one another quickly. Thus it may 

 be true, as an old proverb says, that 

 a single pair of flesh-flies (and their 



progeny) will consume the carcass of 

 an ox more rapidly than a lion. The 

 pupae of the more specialized flies are 

 concealed in the thickened and darkened 

 last larval moult, the whole puparium 

 looking much like a large elliptical brown 

 seed. 



The Diptera include the familiar 

 house-flies, flesh-flies, and bluebottles 



of the dwelling and stables; the horse-flies and greenheads, that make 

 summer life sometimes a burden for horses and their drivers; the buzzing 

 flower- and bee-flies of the gardens; the beautiful little pomace-flies with 

 their brilliant colors and mottled wings that swarm like midges about 

 the cider-press and fallen and fermenting fruit; the bot-flies, those disgust- 



FIG. 412. The blow-fly, C^llipt'.ora ery- 

 throcephala. Larva, pupa, and adult. 



