392 W. H. HARRIS ON THE DENTITION OF THE DIPTERA. 



points, thus giving the organs so furnished a pectinated appear- 

 ance; or one denticle may be considerably enlarged, while the 

 corresponding one is dwarfed. 



Among certain species of Diptera, however, as ah-eady men- 

 tioned, the teeth are contained in a single row in each lobe of the 

 proboscis, and it is among such that the variety of forms of these 

 oi'gans reach their fullest development. From the simple 

 bidentate strap-like form found in the blow-fly, they attain by 

 slight modifications the highest specialised type, and in such 

 approach in form, and probably usefulness, similar organs to be 

 found in some of the lower vertebrates. 



It is probable that the compound groups of teeth render the 

 insects an additional service by guarding the entrance to the 

 mouth from the intrusion of solid particles too large for the use 

 or comfort of the insects. By the approximation of the two 

 groups of organs a perfect screen is obtained through which the 

 liquid food can be filtered before absorption, and particles of 

 sohd material firmly lodged between the teeth are of frequent 

 occurrence. 



Another fact of some interest may be mentioned. The pseudo- 

 tracheae are very largely developed both in size and number in 

 the mouths of all flies hitherto met with whose teeth are of the 

 compound type. With the development of the dental organs 

 there is a decline in size and number of the pseudo-tracheae until 

 they finally disappear from the mouths of those insects whose 

 teeth attain the highest state of development. Their absence, 

 however, is compensated for by the introduction of certain 

 modifications and additional organs, doubtless of equal value to 

 the insects under the altered habits which their dental organs 

 appear to indicate. 



As some of our members are already interested in the subject 

 of this paper, and I desire, if possible, to enlist recruits, I 

 may be permitted to give a brief outline of the method of 

 dissection and preparation of the objects which has given the 

 best results. 



Having killed the fly, sever the lobes from the trunk of the 

 proboscis just at the junction of these parts. Spread the lobes 

 apart upon an ordinary glass slip, taking particular care to have 

 that side uppermost which we were observing in the living animal ; 

 place a thin cover-glass upon the object, and apply suflicient 



