W. H. FURLONGE ON THE PULEX IRRITANS. 199 



observed, I have devoted much attention to them, and I think I am 

 justified in expressing the opinion, that they probably serve a very 

 important and hitherto unsuspected purpose, in the respiratory 

 system of the animal, and further, if I am right in my conjecture, 

 that similar organs will probably be found to exist in many 

 other insects. 



I think it possible, then, that these contractile sacs serve the 

 purpose of pumps or syringes, by means of which air is drawn 

 through the external orifices or spiracles, and propelled through 

 the minute capillary vessels of the tracheal system. I am well 

 aware that the suggestion sounds somewhat fanciful, but, if this be 

 not their use, I am unable to conjecture what other purpose they 

 can serve, and certain considerations, connected with the air circu- 

 lation of insects in general, and of the flea in particular, to which 

 I shall advert in another place, seem to point to the necessity for 

 the existence of some such contrivance. I may add that, hitherto, 

 I have not been able to discover any external orifice in direct com- 

 munication with the contractile sac, though I fancy that such an 

 orifice may exist. I would, however, invite the co-operation of my 

 fellow members of the Club, in further observations upon these 

 singular organs.* 



We now pass to the consideration of the third division of the 

 body of the animal, viz. : — 



The Abdomen. — The abdomen is divided, vertically, into eight 



* In the course of some remarks made by Mr. Lowne, after the reading of this 

 paper at the Club, a full resume of which will be found in the last number of the 

 Journal, he referred to certain experiments he had made, with the object of 

 ascertaining the true nature of these so-called contractile sacs, to which I had 

 drawn his attention, as really existent, a few days before the reading of my paper. 

 The result of these experiments and observations, it appears, led him to a some- 

 what different conclusion as to the structure of these organs, if, according to 

 his views, they can be so termed. Mr. Lowne regards these sacs as being simply 

 expansions of the main tracheae supplying the limb, and the apparent muscular 

 structure to which I attribute the rythmical contraction of the sac, he regards 

 as a peculiarly marked development of the ordinary spiral fibre; and he looks 

 upon the dilation and contraction of the sacs as being due to the disturbance of 

 respiration by the action of chloroform, or even by pressure. Mr. Lowne s 

 experiments and suggestions are certainly well deserving of attention, but as yet 

 I have not had an opportunity of testing them. Meanwhile it is worthy of note 

 that although Mr. Lowne entirely repudiates the idea that the contractions of 

 the sacs is the cause, but is rather the effect of the respiratory process, he yet 

 so far adopts my theory as to suppose that the muscular movements of the 

 limbs compress these tracheal sacs, and that thus air is forced through the 

 minute capillary trachea. For my own part, though I have directed the atten- 

 tion of the Club to the existence of these sacs, I do not feel competent to offer 

 any opinion at present as to their real structure. In fact, I imagine that much 

 careful work will be required before the true nature and office of these singular 

 organs can be decidedly affirmed. 



W. H. F. 



