520 



IN VERTEBRA TE M0RPH0L06 T. 



metathoracic pair being usually represented by a pair of club- 

 shaped bodies on the sides of the segment, termed halteres or 

 balancers. The wings are always trans- 

 parent and the veins by no means 

 abundant. In a few forms, such as the 

 Sheep-tick {Melophagus) and the Fleas 

 (Pulex), the wings are entirely wanting 

 in harmony with the parasitic habits 

 which these forms possess, but they 

 form exceptions to the general rule. 



The mouth-parts are adapted for 

 sucking and also for piercing ; the 

 labrum (Fig. 245, Ir) and labium (la) 

 are prolonged into grooved processes, 

 forming together a tube within which 

 lie, in the female -Mosquitoes {CuLex) 

 and Gadflies {Tahanus), two pairs of 

 elongated needlelike rods which repre- 

 sent the mandibles (md) and maxillae 

 (mx), to which a fifth unpaired stylet 

 may be added which arises as a growth 

 from the lower wall of the pharynx 

 Qiy). In other forms the maxillae only 

 OP A Gnat, Oulex, the have the acicular form, the mandibles 

 Labbum turned to fusing with the labrum, and in all cases 

 the maxillary palps are present, while 

 the labial palps are undeveloped. In 

 the ordinary House-fly (llitsca) the ex- 

 tremity of the sucking-tube is expanded 

 into a disklike structure, and in all 

 forms the salivary glands open near the 

 extremity of the tube, 

 usually maggotlike (Fig. 244), entirely 

 destitute of feet, and in some forms the head even is indis- 

 tinguishable. The metamorphosis is complete, the pupa being 

 in the Mosquitoes active, swimming about in water, though 

 more usually it is incapable of motion, and enclosed within 

 the last larval skin, thus belonging to the coarctata variety. 



Fig. 245.— Modth-p arts 



ONE SIDE (from Hebt- 

 wia). 

 7iy = hypopharynx, a 

 process of labium. 

 la = labium. 

 Ir = labrum. 

 md = mandible. 

 mx = maxillae. 

 p = maxillary palp. 

 The larvae are 



