20 MEDICAL ENTOMOLOGY 



Of these, the mandibles may be entirely absent, and the first maxillae 

 may be represented only by their palps. In the higher Diptera a portion 

 of the wall of the head capsule may be evaginated to form a part of the 

 proboscis, using the term in its widest sense. 



The Dipterous mouth, highly specialized as it is, is but a modification 

 of the arthropod mouth for a special mode of life, and to understand its 

 structure and mechanism it is necessary to compare it with less differen- 

 tiated forms. The mouth in the Arthropoda is typically a biting one, 

 and is furnished with cutting jaws, the appendages of the head segments, 

 with which the animal seizes and tears up its food. The jaws in the 

 simplest forms are typical jointed appendages, and the mouth is adapted 

 for the reception of solid particles. It will simplify the conception of 

 the structure and mechanism of the Dipterous mouth if it is remembered 

 that the organs which are used by even the most highly specialized 

 forms are homologous with the cutting and clasping appendages used by 

 the primitive insects to obtain their solid food. 



The common cockroach, Periplaneta orientalis, may be taken as an 



example of an insect with simple mouth parts; reference to the admirable 



account of this insect by Miall and Denny has already 



the cockroach been made. The figures on Plate IV are taken from their 

 book. In the cockroach all three pairs of appendages 

 can be readily identified ; they all function as cutting or clasping 

 weapons, and the second and third are typical jointed arthropod 

 appendages. The mandibles, like those of the Diptera, are the least 

 typical, and have only one joint, that by which they are attached 

 to the head ; the point of attachment is termed the ginglymus. By 

 means of its muscles the blade of the mandible can be moved in a 

 transverse plane, till it comes in contact with its fellow of the opposite 

 side. The maxillae are more complex, and consist of a number of joints. 

 These are as follows: two basal joints, the cardo and the stipes, and three 

 distal joints, arising about the same level from the stipes, and termed 

 from within outwards the lacinia, the galea, and the palp. The shape 

 and disposition of these parts is sufficiently indicated in the figures. 

 As will be seen later, the maxilla in the Diptera retains the original form 

 to a remarkable degree. The palp, for instance, is jointed in all but the 

 highest forms, and retains its sensory character throughout. 



The second maxillae lie behind the first, and form the posterior 

 boundary of the mouth. They are fused together except at their distal 

 ends, and are continuous posteriorly with a membrane which occupies the 

 inferior surface of the head, and merges with that of the neck. It is in 



