THE EABLY STAGES OF DIPTERA. 27 



discovered, by which the segments can be numbered, and in many 

 cases the segments cannot be satisfactorily delimited in the present 

 state of knowledge." (Dr. D. Sharp, in Verrall's " British Plies," 

 vol. v, p. 32.) 



Many Dipterous larvae have no distinct head. These are known 

 as ACEPHALA, as distinct from the EUCEPHALA, or those furnished 

 with a distinct head. The ACEPHALA include flesh-eating maggots. 

 The term " hemicephalous " has been used by Dufour and others 

 for those larvae in which the head end is of an intermediate form, 

 that is to say, not possessing a sufficiently distinct head to be 

 included in the EUCEPHALA ; and the majority of the families 

 appear to fall into this intermediate division. In these cases the 

 head part is withdrawable within the body, after the manner of a 

 tortoise. 



"Dipterous larvae have the last pair of spiracles largely de- 

 veloped, and they are frequently placed at the actual tip of the 

 body ; when not at the tip, they are usually placed dorsal! y rather 

 than laterally. These are points of distinction as compared with 

 other orders of insects. When the posterior spiracles are the 

 only pair that exists, the larva is said to be metapneustic ; when 

 in addition to these, there is an anterior pair placed a little behind 

 the head, the larva is ampliipneustic ; when there are also inter- 

 mediate spiracles the larva is peripneustic. The frequency of the 

 metapneustic and amphipneustic systems is characteristic of 

 Diptera, the peripneustic system being the usual one in other 

 orders." (Dr. D. Sharp.) 



The Pupa. 



The pupa in the ORTHOBRHAPHA is " either a free, so-called 

 mummy pupa, or it remains enclosed in the larval skin, which it 

 bursts open at its emergence in the form of a T-shaped fissure 

 on the back, or by an irregular sort of lid at the end of the head." 

 (Brauer.) 



In the CYCLORBHAPHA " the pupation always takes place in the 

 larval skin, which hardens and becomes like a barrel. Previous 

 to this, the larva possesses on the 4th or 5th front segments, an 

 arched seam, produced horizontally, and extending above the 

 mouth, which seam encloses enough of the surface of the barrel 

 for the emerging flies to burst off as a lid by means of the frontal 

 bladder." (Brauer.) 



The Habitats. 



The habitats of the Diptera in their earlier stages are most 

 diverse. A large proportion are aquatic (CULICID.E, CHIBONOMID^, 

 STRATIOMYID^, TABANIDJG, EPHYDRIN^E, etc.), some like Eristalis, 

 in the SYHPHID^E, preferring rank pools or drains ; quite a large 

 number live in the earth, as do many TIPULID^E, some THEREVID.E 

 and EMPID^E ; others in rank or rotting vegetable matter, 



