390 W. H. HARRIS ON THE DEXTITION OF THE DIPTERA. 



series of short articles to that periodical upon the structure 

 of the teeth of some fifteen species of Diptera.* 



As the seasons have come round I have continued the 

 investigations, with the result that a tolerably large collection 

 of interesting objects has now been accumulated, and a certain 

 modicum of knowledge obtained. 



Preliminary to a critical examination of the dental organs, 

 it is both interesting and instructive to observe the action of 

 a fly during the act of feeding. The following method has 

 been attended with tolerable success. Dissolve a small piece of 

 clear gum arabic in a saturated solution of sugar and water, 

 apply a small quantity of this mixture to the iuside surface 

 of the glass cover of a live box, and set aside to thoroughly 

 harden. 



The blow-fly may be usefully employed for the ex23eriment, 

 its mouth being typical of the majority of species. Enclose 

 the fly within the live-box, and it will quickly be found that 

 the prepared repast meets with approval, for the margins of the 

 lobes of the labium will be closely applied to the food, the central 

 part of the mouth will assume a concave form, and very quickly 

 a drop of saliva will be emitted, so as to occupy the central 

 part of the concavity. A rapid vibratory motion of the lobes, 

 in a direction at right angles to the median line of the mouth, 

 will be the predominant feature — this action being very pro- 

 nounced in the posterior part of the mouth — and a steady 

 stream of fluid will flow towards the oral cavity ; this may be 

 observed by small particles of the dislodged food floating upon 

 the surface of the cui-rent. 



At fiequent intervals the lobes will be fully expanded for 

 three or four times in rapid succession. It is assumed the teeth 

 are then brought into operation ; but as the movements are so 

 quickly performed it is impossible to speak with absolute 

 certainty from observation of the actual contact of the organs 

 with the food. 



In all books on Entomology to which I have hitherto had 

 access it has been stated that the pseudo-tracheae are channels 

 for the conveyance of liquid food, and this statement is generally 

 accepted as fully proved. It may therefore be considered rank 

 presumption on my part to state that since I have paid some 



* Science Gossij), xx., 1884, xxi., 1885, xxii., 1886, and xxiii., 1887. 



