DIPTERA 



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and is often reduced to a maggot, without feet of any kind, 

 and with only a vestige of a head. Some Dipterous larvae, 

 however, belonging to less specialised families, have one, two 

 or several pairs of pseudopods (false feet) and a fully formed 

 head, with functional eyes, antennae, and mouth-parts. Tran- 

 sitional forms between these and the maggot are frequent. 

 It is a general rule that the higher the organisation of the fly, 

 the more degraded is the larva. The pupa is free-limbed, and 

 may be more or less active in aquatic Diptera of the lower 

 division, though it never feeds. In higher Diptera the resting 

 larva undergoes very complicated changes without casting the 

 larval skin; almost all the larval organs may be reduced to 

 a cellular pulp ; certain rudiments, however, survive, and from 

 them the organs of the fly are developed. The pupa -of such 

 flies is lodged within the 

 hard, dry, barrel-shaped 

 larval skin. 



A few Diptera are vivi- 

 parous. The Sarcopha- 

 gidae are often hatched 

 within the oviduct of the 

 parent, while in the Pupi- 

 para nearly the whole of 

 the larval stage is passed 

 in the oviduct. 



Mode of life, etc. — Dipterous larvae feed upon almost every 

 kind of animal and vegetable substance, dead or alive. Many 

 mine leaves, or form galls on trees ; others (Tachinids) devour 

 the viscera of living larvae, or (bot-fly larvae) inhabit the bodies 

 of quadrupeds. The flies often do not feed at all 5 some suck 

 ripe fruits or the blood of many kinds of animals; others 

 (Empidae, etc.) are raptorial, capturing smaller insects and 

 sucking their fluids ; many, especially the Syrphidae, feed upon 

 the honey of flowers. The carcasses of animals, sewage, rotting 

 fungi, and other offensive organic matter are largely converted 

 into the bodies of living flies, and thus scattered over the face 

 of the country. It is a set-off to this service to public health 

 that certain blood-sucking Diptera spread malarial and other 

 diseases. 



Some Diptera (Syrphids, Bombylius, etc.) resemble wasps 

 and various bees in form, colour, and mode of flight ; this re- 



Fig. 103. — Diagram of mouth-parts of gad-fly. 



