DIPTERA. 741 



or reservoirs apparently belonging to the fore-gut, in which the 

 juices are stored before digestion (Fig. 381). The usual number of 

 malpighian tubules is four, but five occur in some families. 

 Certain curious projections into the lumen of the rectum, termed 

 anal papillae, are conspicuous in this order. These eminences 

 are abundantly supplied with tracheae. There are a pair of pro- 

 thoracic and a pair of metathoracic stigmata of complex design, 

 and a varying number of pairs of abdominal stigmata much more 

 simple in structure. The nervous system shows a great tendency 

 to concentration, but in some families, e.g. the Culicidae, there 

 are five or six abdominal ganglia. 



Most Diptera are oviparous. The larva which leaves the egg 

 is usually an cruciform maggot (Fig. 474, etc.), without true legs. 

 " Pseudopods " and other aids to locomotion may exist. The head 

 is usually small and in the true maggots may be much reduced, 

 practically invisible, and tucked into the body. The larvae 

 may be peripneustic, i.e. with stigmata arranged along the sides 

 of the body ; or amphipneustic, with only two pairs of stigmata, 

 one at the posterior end of the body, the other near the anterior 

 end ; or metapneustic, that is, with only one pair of stigmata, at 

 the posterior end. Many dipterous larvae are aquatic, and 

 then they are often metapneustic, and penetrate the surface film 

 with the part of the body bearing the stigmata, thereby exposing 

 the latter to the air. Others are found in carrion, amongst 

 plants, and in decaying organic matter generally, while many 

 dipterous larvae are parasitic in the bodies of other insects. 

 Either the last larval skin is cast away, and the pupa is an 

 obtected pupa, lying exposed, but protected by a hardened 

 chitinous exudation, which may fasten the appendages more or 

 less to the body ; or the creature changes to a delicate pupa with 

 free appendages, within the last larval skin, which remains as a 

 tough enclosing shell, frequently called the puparium. This 

 latter pupa is spoken of as a coarctate pupa (p. 651). During the 

 pupa-stage there is a considerable amount of histolysis, the 

 tissues breaking down and reconstituting themselves usually 

 from certain centres of growth termed imaginal discs (p. 651). In 

 some forms such as Chironomus and the Culicidae many organs 

 of the imago are already formed in the last larval stage. 



For purposes of identification the Diptera may be divided 

 into five groups : (i) Nemocera. (ii) Brachycera, (iii) Aschiza, (iv) 



