THE SUCKING APPARATUS: TABANUS 27 



cavity, and with the oesophagus, which opens out of the posterior end of 

 the pharynx. The muscles which dilate these chambers arise from the 

 walls of the head capsule. Those of the buccal cavity are inserted into 

 the anterior surface of the dorsal plate, and arise from the epicranium 

 below the antennae. Those which dilate the pharynx arise from the 

 region of the vertex, and pass down to the dorsal surface of the pharynx; 

 there are some subsidiary muscles attached to the turned down angles of 

 the plate, arising in the lateral regions. 



The blood is pumped up, not by a single dilatation of the chambers, 

 but by rapidly repeated ones, and the mechanism by which the plates are 

 brought again into contact with one another in readiness for the next con- 

 traction has now to be considered. It has been repeatedly stated, with 

 reference to the similar structure in the mosquito, that the replacement of 

 the plates is brought about by the natural elasticity of the chitin, but the 

 process is more complex than this. The true explanation of the collapse 

 of the chambers is to be found in the presence of certain large and tough 

 air sacs, which surround the brain and take up a large proportion of the 

 space within the head capsule ; these are in communication with the rest 

 of the respiratory system of the fly. When the sucking pumps are 

 dilated, room must be made in some manner for the increased contents 

 of the head, and air is either forced out of these sacs into the thorax, or 

 else the air in the sacs is compressed. When the cavity is to be 

 emptied again, the air is either returned from the body to the sacs by 

 means of the contractions of the muscles of the body wall, acting as in 

 respiration, or else the positive pressure in the air sacs becomes active as 

 soon as the contractions of the muscle pass off. With the collapse of the 

 chamber the blood in the cavity passes backwards to the pharynx, or from 

 the pharynx to the oesophagus. 



The mechanism of the mouth parts and sucking apparatus in Tabantis 

 may be summed up as follows. The fly settles down on the skin of the 

 host, and by means of the tactile hairs on the end of the proboscis selects 

 a suitable place. A firm hold is then taken on the skin by means of the 

 claws on the feet, the labella are retracted to expose the piercing stylets, 

 and are also diverged from one another so that their oral surfaces are 

 applied to the skin. The mandibles and maxillae are then put in action, 

 the former acting like a saw and the latter like a file, and by rapidly 

 repeated movements cut a hole in the skin. Meanwhile the epipharynx 

 and hypopharynx are drawn slightly upwards, by means of a pair of 

 muscles which pass from the cornua of the buccal cavity to the epicranial 

 wall in the region of the vertex. When the level of blood is reached 



