MECHANISM OF MOUTH PARTS OF FLEA 441 



basally to a median piece. The whole is often termed the rostrum ; the 

 number of joints of which it consists is a point of importance in classifica- 

 tion. The nomenclature is a little confusing, for it is 



, ,, ,, ,., , , ,. The labium 



by no means certain that the bilateral portions are 



really palps in the ordinary sense. The term rostrum, moreover, is used 

 in connection with the Muscid flies to indicate the part of the head which 

 is protruded to become a part of the proboscis. Its literal meaning 

 renders it hardly applicable in the case of the flea. 



The paired portions of the labium are slender jointed organs, each 

 consisting of four segments. Of these the proximal one is the longest, 

 being equal to half the length of the whole ; the distal one is a little 

 longer than the second and third. All the joints are flattened laterally 

 and strongly concave on their internal surfaces, the two palps forming 

 a sheath in which the mandibles and labrum-epipharynx are concealed in 

 the natural resting condition of the parts. The outer surface bears a 

 pair of hairs at the distal end of each joint. The distal end of the fourth 

 segment is obliquely truncated. 



The proximal portion of the labium consists of an oblong plate of thin 

 chitin, to the distal end of which the paired portions are articulated. 

 The plate is concave on its upper or dorsal surface, and extends from 

 about the level of the middle of the maxillary flap to the peri-oral ring, 

 and is thus concealed for a considerable portion of its extent by the 

 genal comb. At its proximal end there is a smaller plate of chitin in 

 the middle line. 



The nomenclature of the parts which would bring the flea into 

 line with other insects would be to regard the large basal plate as 

 the mentum, and that proximal to it, and only visible in dissections, 

 as the submentum. The paired portions may then be termed the labella, 

 in which, in the case of the flea, four joints can be recognized. The 

 whole labium is to be regarded as originally paired, and formed of the ap- 

 pendages of the sixth segment of the head, though there is little to indicate 

 how much is appendage proper and how much is part of the head wall. 

 It is hardly correct to speak of the labium as ' bifid '. 



The mechanism of the mouth parts will be readily understood from 

 what has been said with regard to that of the dipterous mouth. The 



wound is made by the protraction and retraction of 



A , ,.,, , . ,, cf . Mechanism of 



the mandibles, retraction being the more enective mou th parti 



movement of the two, as is shown by the direction 

 of the teeth. The labrum-epipharynx cannot act as a 'pricker', as it 

 has been termed, for it is blunt at the tip and has no cutting teeth. 

 56 



