246 ARTHROPOD A [CH. 



order the mouth-parts when not in use are bent under the body 

 and lie along the under surface of the thorax. The labium is 

 jointed and its edges are curved so as to form an incomplete tube, 

 only the base of which is partly covered by the labrum. Within 

 this groove four sharply pointed styles the mandibles arid first 

 maxillae work. There are no palps, (iv) BITINO AND SUCKING. 

 The Hymenoptera or Bees and Wasps have mandibles not unlike 

 those of a Cockroach and use them for biting and moulding their 

 food (pollen) and the wax they secrete from their bodies. The 

 laciniae of the maxillae form blade-like structures and their palps 

 have much diminished in size. The labial palps however are large, 

 and the conjoined median outgrowths from the labium (corresponding 

 to the ligula of the Cockroach, 3, Fig. 98, C) form a kind of grooved 

 tongue along which the nectar in the flowers is sucked up. 



It thus appears that although the mouth-parts of Insects are 

 highly modified in connection with the kind of food they live on and 

 the modes in which they obtain it, nevertheless the various mouth- 

 parts have a common ground plan, and although the authorities differ 

 as to details, a fundamental similarity runs through these appendages 

 in the different orders. 



Wings of Insects 



The wings ot Insects are folds ot the integument, flattened 

 so that the two sides are in contact on their inner surfaces. At 

 certain places however slits are left and through these tracheae 

 pass taking air to the wings. The cuticle of both upper and 

 under layers is also thickened along certain lines and the presence 

 of these thickenings divides the wing up into a number of areas. 

 These lines are usually called wing-veins or,, wing-nerves, but as 

 they are neither veins nor nerves it is better to call them nervures. 

 The presence and disposition of the nervures is of the highest 

 importance in classification. Wings may be thin, membranous, and 

 transparent, as in the Grasshopper (Fig. 100) and Dragon-fly (Fig. 74), 

 where there are an enormous number of nervures, or in the Flies 

 and Bees, where there are few nervures, or they may be thickened 

 and strongly chitinised as in the front wings of Beetles. In this 

 last group the anterior wings are called elytra (Gr. ZXvrpov, a cover) 

 and they always 'meet together in a median longitudinal line, 

 so that when they are closed the insect appears to be wingless 



