FEEDING AND BREATHINC? 



17 



possession of a digestive cavity v^ithin which the swallov^ed 

 food-material undergoes the changes necessary to fit it for 

 furnishing nourishment to the living tissues. This cavity 

 may be regarded as a modified tube running from a forwardly 

 situated opening, the mouth, to a hinder opening, the vent 

 or anus, through which the useless remnants of the food 

 are ejected. Insects, like many of the highly organised 

 animals of various groups, have a definite head formed by 

 the union of a number of primitive body segments, and in 

 front of or beneath this head the mouth is situated. 



An insect's mouth is furnished with jaws for seizing, 

 masticating, piercing, sucking, or otherwise dealing with 

 suitable food-substances. The jaws are arranged in pairs, 

 and it was long ago shown (Savign)^, 181 6) that they are 

 modified appendages belonging to the series of paired 

 jointed limbs characteristic of Arthropods generally. These 

 jaw- limbs differ greatly in form in different groups of insects 

 according as they are used for biting solid food material, 

 or for piercing and sucking or licking up various fluids. 

 We may conveniently introduce this study of insect jaws by 

 examining those of a somewhat primitive type of biting 

 insect such as a common earwig (Fig. 6). 



Below the face region (clypeus) of the earwig's head is 

 hinged a median flap with straight lower edge and rounded 

 margins. This is the upper Hp (labrum) which bounds the 

 mouth in front ; it is to be regarded as part of the insect's 

 head skeleton (Fig. 6, A). Just behind it and only in part 

 hidden by it, lie the front jaws or mandibles (Fig. 6, B), 

 stout, strong organs each consisting of a single finely 

 moulded piece, the broad base articulating with the head 

 skeleton by means of a knob-like condyle behind and a 

 concave surface (ginglymus) in front, the outer edge evenly 

 rounded, trending to the sharp, inwardly directed apical 

 teeth, the inner edge approximately straight with a ridged 

 grinding or molar area towards its base. The two man- 

 dibles are arranged facing each other, they can be draw^n 

 together by the action of adductor or apart by abductor 

 muscles. When the mandibles are drawn together the 



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