436 THE INVERTEBRATA 



of the sawflies (Fig. 322 D) and the legless form of bees, wasps and 

 ants (Fig. 320 A). The sawfly larva has a superficial resemblance to 

 the lepidopterous caterpillar, but is distinguished by its single pair of 

 ocelli and the absence of crotchets or spines on the abdominal legs. The 

 prolegs of the abdomen occur on different segments in the two 

 forms under consideration as reference to Fig. 322 clearly shows. 



Order DIPTERA (Flies) 



Insects with a single pair of functional wings, the hind pair repre- 

 sented by stumps (halteres) (Fig. 324); mouth parts suctorial and 

 sometimes piercing or biting, usually elongated to form a proboscis ; 

 prothorax and metathorax small and fused with the large mesothorax ; 

 metamorphosis complete, larvae cruciform and always apodous, the 

 head frequently being reduced and retracted; pupa either free or 

 enclosed in the hardened larval skin (puparium). 



This is a very large and highly specialized order of insects. The 

 imagines are mostly diurnal species, feeding on the nectar of flowers, 

 but a number are predaceous, living on other insects (e.g. the robber 

 flies). A further development which takes place in several families is 

 the acquisition of blood-sucking habits. The representatives of this 

 oecological class are of great importance because they harbour and 

 transmit pathogenic organisms, causing such diseases as malaria, 

 sleeping sickness, elephantiasis, yellow fever and some cattle fevers. 



The several kinds of mouth parts which have been developed in 

 the Diptera have departed widely from the primitive biting type. 

 There is always a proboscis formed principally by the elongated 

 labium, ending in a pair of lobes, the labella. This labium serves as 

 a support and guide to the remaining mouth parts which are enclosed 

 within it (Fig. 325). 



The most complete system is to be found in the gadflies, e.g. 

 Tabanus and Chrysops, Within the groove of the labium are to be 

 found a pair of mandibles and a pair of maxillae, sword-like piercing 

 organs, by means of which the wound through the skin of mammals 

 is made. Into the wound so formed is inserted a tube composed of 

 the epipharynx, an elongated chitinization of the roof of the mouth to 

 which the lab rum is fused, and the hypopharynx, a corresponding 

 elongation of the mouth floor. The blood passes into this tube, being 

 drawn up by the pharyngeal pump within the head. The hypopharynx 

 carries a duct down which the salivary fluid is passed. Besides this, 

 the proboscis of a gadfly can be used for taking up fluids exposed at 

 surfaces. Such exposed fluid is drawn into small channels, Xho^ pseudo- 

 tracheae, which converge to a central point on the underside of the 

 labellar lobes. There it meets the distal end of the epi-hypopharyngeal 

 tube, up which it passes. 



