506 THE INVERTEBRATA 



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. 348). 



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

 Tahanus 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 eptpharynx, 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, the 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. 



The mouth parts of the female mosquito (Fig. 348 A) in principle 

 diff"er from those described above only in the absence of a pseudo- 

 tracheal membrane on the labellar lobes and the more slender and 

 elongated labium. Mandibles are absent in the males, maxillae being 

 represented only by palps in this sex. The housefly Musca (Fig. 348 D, 

 E, F) has lost all piercing mechanism, mandibles being absent, maxillae 

 only being represented by the palps, and the mouth parts consist of a 

 folding labium with highly developed pseudotracheal membrane on 

 the labellar lobes and prominent epi-hypopharyngeal tube. Musca 

 feeds largely on fluid matter but in the presence of soluble solid food, 

 e.g. sugar, solution is effected by regurgitating alimentary fluid on it. 

 By means of small chitinous teeth situated round the point to which 

 the pseudotracheae converge, surfaces of solids can be scraped so 

 enabling enzymes in the regurgitated fluids to act rapidly. 



The tsetse fly, Glossina (Fig. 348 B), also possesses no mandibles 

 and only the palps of the maxillae. It does, however, feed on mam- 

 malian blood after piercing the skin. In this form the whole labium 

 is rigidly chitinized; the labellar lobes, from which all traces of 

 pseudotracheae have disappeared, are small and provided with 



