352 APHANIPTERA. 



In addition to the presence of eyes and combs, I have found 

 the following very useful distinctions in defining species : — 



I. — The relative length of the joints of the tarsi. 



2. — The number of joints in the labial palpi. 



3. — The structure of the mandibles — in some a large, flat, 

 double-toothed saw ; in others a long delicate bristle. 



4. — Their very various antennae. 



Van Heurck, in his work on The Microscope^ published two 

 years ago, gives an illustration of the pygidium of a flea, and 

 states that it contains from thirty-two to thirty-eight areolae. As 

 the pygidium of P. goniocephalus contains only twenty-eight, 1 

 think it probable that this also may be of use as a specific distinc- 

 tion. 



I now come to the antennae. The older naturalists quite over- 

 looked these, which, as a rule, lie hidden in grooves close behind 

 the eyes. Kirby and Spence, for instance, have a very good 

 drawing of the parts of a flea's mouth ; but, like many previous 

 observers, they do not appear to have noticed the real antennae, 

 but mistook the maxillary palpi for them, and consequently the 

 names they gave to the other parts of the mouth are inaccurate. 

 Latreille, also, although he overlooked the real antennas, appears 

 to have been in doubt, for he speaks of these maxillary palpi as 

 ^^Antennce, potiiis palpi!' Louis Figuier, in one of his popular 

 works, published not more than twenty years ago, still states that 

 these palpi are the flea's antennae.* 



Regarding the true antennae, Westwood writes : — " The 

 antennae are minute articulated organs, varying in form in the 

 different species, composed apparently of four joints, the third of 

 which is very minute, and forms the cup-shaped base of the ter- 

 minal joint, which in some species is furnished with numerous 

 transverse incisions, which have been considered by Curtis as so 

 many distinct articulations." There is, I think, no doubt that 

 Curtis was quite right, for in this photo they are plainly separate 

 joints. I do not think, however, that the so-called third cup- 

 shaped joint is a joint at all, but consider it as merely a thickening 

 of the top of the second joint, which forms a support for the base 

 of the third. To show how greatly these organs vary both in size 



* Les Insecles, p. 30. 



