ENEMIES OF APHIDES. 37 



air by removing the cover each day for a few moments, and drop- 

 ping upon it a single drop of water from a glass rod. The larva 

 will then extend itself, and if viewed under a magnifier will be 

 seen to draw in air through the piercing organ, bubble after bubble 

 passing down the oesophagus and accumulating in the stomach. 

 In this way only can the pharyngeal and oesophageal passages be 

 traced in the living larva. 



From each of the main tracheal trunks large branches are 

 thrown off to every segment, and especially to the surface of the 

 flask-shaped posterior end of the circulatory vessel ; to the head, 

 numerous loops and curves being here interposed, in order to 

 allow of the invagination of the parts without stopping the flow of 

 air; and lasdy, to the nervous ganglion (/, Fig. i, PI. III.), situated 

 on the ventral side behind the head. 



To ensure the continual supply of oxygen to this important 

 organ, each main branch from the tracheal trunks sends off at a 

 point not far from its commencement a small tube which passes 

 direct, generally following the course of a nerve thread, and without 

 branching, until it reaches the surface of the ganglion, where it 

 breaks up into numerous fine vessels. These tubes are shown at 

 k, k, Fig. I. About the head also a complex and duplicate system 

 of looped and connected tubes (/, m, Fig. i) provide against any 

 check or hindrance to the supply of oxygen from the movements 

 of the larva. 



The Organs of Special Sense. 



So far as my observations go, these are to be found in the pair 

 of double palpi situate above the mouth, and, in a pair of minute 

 organs, in the edges of the upper part of the mouth. 



The palpi have already been mentioned as placed on two pro- 

 tuberances above the upper end of the opening of the mouth. 

 Two are found on each side, placed obliquely side by side, and 

 although the structure of the lower part of the organs appears to 

 be identical, the organisation of the corresponding apices on each 

 side is so different as to suggest considerable difference of function. 



The outer papillae on either side is rounded at its extremity, 

 and provided with five minute circular openings, each encircled 

 by a chitinous ring, and 1 think provided with a transverse mem- 



