II] THORAX AND ABDOMEN 15 



labium, or lower lip, is more or less membranous or fleshy, 

 and usually functions as a shea'th for the protection of the other 

 mouth-parts. The mandibles and maxillce are each represented 

 by a pair of long pointed processes adapted for piercing. In 

 addition, a pair of well-developed maxillary palps is always 

 present, situated one on each side of the base of the labium. 

 The palps are extraordinarily well developed in the Culicidae 

 (Fig. 4), in the genus Megarhinus being nearly as long as the 

 whole body of the insect. 



The thorax is composed almost entirely of mesothorax, 

 the prothorax and metathorax being very small and fused 

 with it. A small part of the metathorax projects backwards 

 over the base of the abdomen and is known as the scutellum. 



The abdomen is composed of a variable number of segments, 

 more or less closely fused together. The number of segments 

 externally visible may vary from nine to five, or rarely four. 

 In certain cases the first two segments are fused together and 

 the first one is often very much shortened. In the male the 

 terminal segments are curled under the body, forming what is 

 termed the hypopygium, which serves to protect the copulatory 

 appendages. 



The three pairs of legs are attached to the prothorax, 

 mesothorax and metathorax, respectively. The legs are in- 

 variably composed of five parts known as the coxa, trochanter, 

 femur, tibia and tarsus. The tarsus is generally five-jointed ; 

 its terminal joint bears a well-developed pair of claws, and 

 underneath each of these is often a free pad or membrane, 

 the pulvillus. This structure is often absent amongst the 

 Orthorrhapha and is usually better developed in the male 

 than in the female. Between each pair of claws is situated 

 a median structure known as the empodium, which may have 

 the form of a pad, or a bristle. Occasionally the pulvilli are 

 absent and the empodium takes on their function. 



The wings are of very great importance from a classificatory 

 point of view. As in all insects they consist of simple folds 

 supported by veins or nervures, the arrangement of which is 

 known as the venation. The wings are membranous but may 

 be covered with hairs or scales, and the shape and disposition 



