76 MOSQUITOES OF NORTH AMERICA 



the supra and infra-oesophageal ganglia and their commissures. At first it is 

 narrow, but posteriorly becomes a large chamber (the pumping organ). 



" The pumping organ occupies with its muscles a large portion of the head 

 behind the level of the cerebral ganglia. In the state of rest its lumen is tri- 

 radiate in transverse section. The walls are formed of three large and thick 

 chitinous plates, one placed on either side, and one superiorly. Into each of 

 these plates powerful muscles are inserted. The plates are connected by thin 

 non-chitinous membrane, and their edges are rolled so that they form a spring 

 capable of returning to their original position so soon as the separating force of 

 the muscles ceases. 



" Posteriorly, where the pharynx becomes very narrow, a sharp bend occurs 

 and a valvular action is produced. The whole forms a very powerful suctorial 

 apparatus. 



" The CEsophagus. — Immediately beyond the pumping organ the cliitinous 

 layer ceases, and the rest of the fore-gut is formed of excessively thin membrane. 

 At the junction of the two portions a sharp bend occurs, and the floor projects 

 so as to form a valvular flap. 



" The thin- walled oesophagus is a large dilated sac, whose walls are supported 

 by surrounding structures. Into the posterior wall of the dilated and thin- 

 walled oesophagus projects the papilla-like anterior portion of the mid-gut. 



" The Diverticula of the (Esophagus. — From the oesophagus two or three 

 diverticula, similar in nature to the oesophagus, extend backwards. Of these, 

 one is of great size, and usually contains gas bubbles. This usually extends into 

 the abdomen, and is a prominent object in dissections and sections. 



'"'' The Homologue of the Proventriculus. — There is no true proventriculus as 

 in many insects. There is, however, an interesting fold of the fore-gut into the 

 mid-gut which represents this organ. The muscular bundles are here increased, 

 and the whole forms a valvular muscular organ. 



'"'' The Mechanism of Feeding. — The powerful pumping action which must 

 result from a drawing asunder of the three large chitinous plates of the pumping 

 organ is very evident. These plates, also, when drawn apart must, by reason of 

 their spring-like shape, revert to their original positions close together, without 

 any muscular aid. Posteriorly the valve-like arrangement mentioned before 

 prevents regurgitation. Further, when the blood reaches the junction of the 

 oesophagus and mid-gut the invaginated portion is withdrawn, and is distended 

 by the entering blood into a distinct ' crop,' the valvular function is suspended, 

 and the blood flows onward. 



" The Mid-gut. — The mid-gut extends from the proventriculus to the origin 

 of the malpighian tubes. 



" The anterior narrow portion of the mid-gut lies in the thorax, and does not 

 become distended with blood. The posterior portion when fully dilated fills the 

 greater portion of the abdomen, the viscera being pushed into the last few 

 segments. 



" The Hind-gut. — The hind-gut is short and passes in one or two bends from 

 the pylorus to the anus. Immediately beyond the pylorus there is a considerable 

 dilatation which is poorly supplied with muscular fibres : into this open the five 

 malpighian tubules. For a short distance beyond this the lumen is narrow 

 (ileum), but becomes gradually larger (colon) . At the termination of the colon 

 there is a slight constriction, after which the canal dilates again to form the 

 rectum. 



" Into the rectum project six solid growths, the so-called rectal glands, which 

 are, however, papillae. Posteriorly the rectum ends in the anus close above the 

 genital canal. 



