VI APPENDAGES OF INSECTS 229 



remaining straight, pass onwards into the skin guided between two 

 finger-hke lobes (labella) at the end of the labium. 



In the male mosquito the mandibles have disappeared and the 

 hyp9pharynx has become fused with the labium. 



In the House-fly (Mused) and its allieS; which belong to the same 

 order (Diptera) as the mosquitos, the mouth-parts are'adapted entirely 

 for sucking. Mandibles and first maxillae (except the palp) have entirely 

 disappeared, while the labium forms a large conspicuous proboscis, the 

 surface of its broadened-out end (labella) traversed by a system of deep 

 channels by which fluid is drawn in to the mouth when the proboscis is 

 moved, after the manner of a vacuum cleaner, over a surface on which 

 soluble food matter is present. 



In the Butterfly or Moth the mouth-appendages are practically 

 reduced to the labial palps — conspicuous hairy-looking organs (Fig. 

 97, C, l.f) — and the first maxillae {mx.l). The latter are greatly elon- 

 gated and are deeply grooved along their mesial surface, so that when 

 fitted together they form a long tubular siphon. When at rest this is 

 coiled up in a spiral under the insect's head, as shown in Fig. 97, C, but 

 it can be uncoiled and lowered into the recesses of a deep tubular flower 

 corolla. 



In the Insects there is only a single pair of antennae. 



Wings 



In the Insects, and in them alone among the Arthropoda, there 

 occur normally in the adult stage wings. Of these there are usually 

 two pairs, attached to the hinder two of the three segments which make 

 up the thorax. These wings have nothing to do with the series of true 

 appendages : they are flaps of skin rendered stiff by the two layers of 

 stiff cuticle on their upper and lower surfaces, except at their base 

 where the cuticle remains flexible so as to admit of free up and down 

 movements. The two layers of the wing are in close contact except 

 along a branched system of nervures along which blood-spaces and 

 tracheal tubes (see below, p. 232) are interposed between the two layers. 

 The arrangement of the nervures of the wing is very constant within 

 the limits of particular groups, e.g. famiHes or genera, while on the 

 other hand characteristic differences occur between different groups. 

 Consequently the neuration of the wings is made much use of by 

 entomologists in classifying insects. 



The wings show characteristic differences in their general appearance 



