THE MOUTH PARTS 437 



The groove in the cat flea is open at the lower end, but in many forms 

 it becomes closed by the extension backwards of the ventral border of the 

 frons. (Plate LVI, fig. 5.) It is closed at the upper end, so that the 

 grooves of the two sides are not continuous with one another. The 

 amount of upward extension of the groove varies in the different genera. 

 In some cases it extends only a short distance upwards, and is continued 

 towards the dorsal border by an internal thickening or ' incrassation ', 

 (Plate LIV, fig. 1) believed to be due to the coalescence of the two 

 sides of the groove at its upper end. The eye is situated anterior to the 

 middle of the groove. It is non-facetted, and is conspicuous on account 

 of its dense pigmentation. In many fleas the eye is absent or vestigial 

 (Plate LVI, figs. 2 and 5). 



The antenna lies deeply in the antennal groove, and is thus pro- 

 tected from injury when the insect moves about among the hairs, 



exactly as is the case in the Pupipara, which have 



-111- r i T TTT Antennae 



a similar habit. It consists or three joints (Plate LIU, 



fig. 6), of which the distal one, known as the ' club', is much the largest, 

 and is specially modified. The club is a large oval body, divided along 

 its posterior border into a number of pseudo-joints, the intersections 

 between which do not extend far into the substance of the organ. The 

 amount of segmentation in the distal joint of the antenna differs in 

 different genera, and is a point of taxonoinic importance. In many 

 forms, as for instance Ceratophyllus (Plate LIV, fig. 12), the antenna 

 is distinctly segmented all the way round, instead of only on the posterior 

 border. The middle joint of the antenna is short and ring-like, and 

 bears four delicate and faintly pigmented hairs of considerable length, 

 but extremely fine, and generally only recognizable when the antenna 

 is dissected out of its pit. The proximal joint is narrow but longer, and 

 bears a row of five hairs, shorter than those of the middle segment, but 

 of equal tenuity. 



The indentation of the wall of the head capsule by the antennary 

 grooves on the two sides divides the head cavity into two portions, the 

 upper one of which contains the pharynx. 



The mouth parts of the flea are adapted for sucking and piercing, 

 like those of the Diptera, and though the differences between the two 

 types are considerable, the homology can be followed closely enough to 

 be of service in explaining the anatomy and mechanism. 



The parts present are the same as those in the Nematocera, the least 

 specialized Diptera, namely, labrum and epipharynx, fused to form a 

 compound organ, and the hypopharynx ; the mandibles, first maxillae, 



