129 



basal element. Each mandible contains a groove, forming practi- 

 cally a closed canal, which becomes continuous with the exit duct of 

 the salivary pump. 



The rostrum (labial palpi) forms a protection and guide to the 

 mandibles and epipharynx. Its first portion is unpaired and articu- 

 lates within the head, with its basal element. At the apex of its 

 first portion it bifurcates, forming a paired organ, which is divided 

 into a varying number of pseudojoints, depending on the species of 

 the flea. As it is a chitinous structure, these pseudojoints, areas in 

 which there is little chitin, enable it to double up as the mandibles 

 and epipharynx are inserted into the skin. At the apex of the ros- 

 trum are some tactile hairs. 



The maxillae are triangular chitinous plates situated on either side 

 of that portion of the head where the biting organs emerge. These 

 structures serve to protect the origin of the epipharynx and mandibles, 

 rest upon the cutaneous surface in the act of biting, thereby steadying 

 the head and serving as a fulcrum when the flea withdraws its biting 

 apparatus when through feeding. The maxilla have their palpi, 

 which are four-jointed, paired organs coming out at the anterior lower 

 angle of the head. Their function is sensory. 



INSIDE THE HEAD. 



The hypopharynx is a chitinous plate forming part of the floor of 

 the aspiratory canal. To its under surface are attached the muscles 

 which operate the salivary pump. Its lower portion is connected to 

 the mandibles, while its upper portion is connected to the posterior 

 portion of the floor of the aspiratory pharynx by a membranous 

 ligament. 



The aspiratory pharynx extends from the connection of the hypo- 

 pharynx with the mandibles to the oesophageal commissure. In a 

 general way it first passes upward and then turns, passing backward. 

 Its roof is formed by the continuation of the ventral surface of the 

 epipharynx, while its floor is formed by the hypopharynx below and 

 above by the chitinous layer which is continuous with the oesophagus. 

 The anterior end of this particular portion curves strongly downward, 

 where it is attached to the upper portion of the hypopharynx by a 

 membranous ligament. In a general way it may be divided into a 

 vertical and longitudinal portion. The longitudinal portion expands 

 laterally, so that its capacity is greatly increased when dilated. Into 

 the floor of this longitudinal portion empties the vertical part of the 

 aspiratory pharynx, and at the junction of the two there seems to be 

 a valvular arrangement," preventing blood from escaping after it has 

 entered the upper part of the aspiratory canal. The oesophagus 

 starts at the oesophageal commissure and ends in the gizzard. It is 

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