514 MEDICAL ENTOMOLOGY 



of the Diptera, an oesophagus, a mid-gut, which makes up by far 



the greater part of the tract, a hind-gut with a 

 The alimentary canal . , , , , . , . . 



rectum, and the Malpighian tubes. 



The sucking apparatus, or pharynx, consists of two plates superimposed 



upon one another in the transverse plane, the dorsal plate having a series 



of muscles by means of which it can be withdrawn 



The pharynx or frQm ^ ventra j one j t ^ j n t ^ e ventra i portion of 



sucking pump 



the head, about the level of the bases of the antennae. 



Each plate is heart-shaped, with its narrow end directed forwards and 

 rouncled off (Plate LXIV, fig. 7). The muscles which withdraw the 

 dorsal plate from the ventral one, and so create the necessary negative 

 pressure, arise from the dorsal and lateral walls of the head, and pass 

 obliquely inwards to the upper surface ; they are inserted into the lateral 

 areas of the plate, not into the middle line as is so frequently the case. 

 There is no sphincter muscle, the flow of blood being apparently deter- 

 mined by a peristaltic method of contraction. 



The pharynx does not extend quite to the point at which the mandibles 

 converge to form the food channel. The interval between the two is 

 occupied by a shallow chamber with membraneous walls, which may 

 be termed the buccal cavity ; this is supported at the sides by a chitinous 

 arch produced forwards from the plates of the pharynx. It is not 

 provided with a musculature. A considerable portion of the ventral 

 surface in this region is occupied by the salivary pump, the distal end 

 of which is split up into a number of fine chitinous processes, which 

 appear to guard the opening into the buccal cavity on each side. The 

 exact means by which the food canal in the proboscis is connected with 

 the pharynx has not been made out. They have no chitinous continuity. 



The oesophagus is an extremely thin and delicate tube, commencing 

 at the posterior end of the pharynx, and passing through the brain and 

 neck to join the mid-gut at the posterior end of the thorax. Its wall 

 contains a good deal of muscular tissue, and can be seen to be in active 

 peristalsis during the act of feeding, if young bugs with a transparent 

 integument are examined. 



The mid-gut (Plate LXIV, fig. 1) comprises almost the whole of the 



rest of the tract. It may be divided into two portions, which probably 



The mid A t correspond to the cardia and the proximal intestine in 



the higher Diptera. The cardia, or anterior part, is 



the widest, and runs straight backwards to the posterior end of the 



body ; it is constricted in seven or eight places in the circular direction, 



the constrictions giving it a saccular Appearance, and is separated from 



