W. H. FURL0NGE ON THE PULEX IRRITANS. 203 



numerous tracheal vessels which, as just described, proceed from 

 the under surface of the organ and are united to the main trachea 

 of the animal — I did strongly hold the opinion that the office of 

 the pygidium was respiratory , — in fact, that it was a collection of 

 spiracular orifices. Certain facts, however, which have very 

 recently been brought under my notice by our distinguished member 

 Mr. Lowne (who, I believe, inclines to the view that the pygidium 

 is in some way connected with the reproductive organs), seem to 

 militate greatly against the respiratory theory, which I, therefore, 

 feel compelled provisionally to abandon. The recent discovery of a 

 similar organ in the lace fly, however, will probably attract in- 

 creased attention to the subject, and we may hope that, at no 

 distant date, the true interpretation of this extraordinary structure 

 will be discovered. 



Having now completed my description of the external structure 

 of the flea, with the exception perhaps of the male organs of 

 generation, which will be more conveniently described in another 

 place, I come to the consideration of the structure of the internal 

 organs of the animal, which present features of great interest. 



I had hoped to have been able to complete my whole subject on 

 the present occasion, but I think it will be better to reserve the 

 second division of my paper for another communication, which I 

 hope at an early opportunity to have the honour of bringing before 

 the Club the more especially as I desire on that occasion to append 

 some contributions to the life-history, and development of the 

 insect, which, I trust, will not be wholly devoid of interest. 



Plate XV— Bed Flea (male) X 250. 



Plate XVI. — Fig. 1, superior terminal plate, showing pygidium and pair of 

 spiracles ; 2, side view of pygidium, showing the tracheal tubes ; 3, enlarged 

 figure of a single orifice in the pygidium ; 4, ideal section of the same j 5, the 

 labium or ligula ; G, the labial palpi. 



