NO. 8 ANATOMICAL LIFE OF THE MOSQUITO SNODGRASS 73 



have no features peculiar to the mosquito, and even the unusual char- 

 acters of the reproductive organs are common to other Diptera. The 

 simple nervous system is that of the larva with an elaboration of the 

 brain and the optic lobes in the head, a transposition of the first 

 abdominal ganglion to the thorax, and the union of the eighth ab- 

 dominal ganglion with the ganglion of the seventh segment. In the 

 abdomen of the adult, therefore, the first ganglion is in the second 

 segment (fig. 30 C, Gngll), and the last is a composite ganglion 

 (Gng VII+VIII) in the seventh segment. The tracheal system has 

 lost the large dorsal trunks of the larva, and the lateral trunks along 

 the spiracles have been enlarged. 



The circulatory organs. — In the adult mosquito, as described by 

 Jones ( 1954) in Anopheles, the dorsal blood vessel has in general the 

 same structure as that of the larva. The part in the abdomen, how- 

 ever, is more distinctly "chambered" because of segmental swellings 

 before the ostia. An aortic sinus is said by Jones ( 1952) to be present 

 in the adult as in the larva and pupa of Anopheles, Culex, and Aedes. 

 The sinus is a dilatation of the aorta in the dorsal part of the thorax, 

 with the corpora allata-cardiaca attached to it laterally. Anteriorly 

 the sinus is continued into the cephalic aorta. The adult heart, ac- 

 cording to Jones, beats predominantly forward, but periodically re- 

 verses the direction of the beat. The heart has no innervation from 

 any source and therefore its pulsations are myogenic, that is, engen- 

 dered by the muscles themselves of the heart wall. Lateral alary 

 muscles support the heart, but they do not vibrate, and when cut the 

 heart keeps on beating. 



A vibratile muscular membrane across the cavity of the mesothoracic 

 scutellum appears to be an accessory pulsatile organ, as in some other 

 insects. A frontal bulblike organ between the bases of the antennae 

 has been described by Day (1955) as a sense organ, and by Clements 

 (1956) as a pulsating organ for driving blood into the antennae. 

 If it is a sense organ, it is a newly discovered one as Day claims; 

 if it is a pulsating organ it is not unique since a pulsatile organ in the 

 same place is present in various other insects. 



The alimentary canal. — The alimentary canal of the adult mosquito 

 (fig. 29 A) in its general form is quite different from that of the larva. 

 From the pharyngeal pump in the head (PhP) a short, narrow 

 oesophagus (Oe) extends into the front of the thorax, where it joins 

 a wider tube, which is the beginning of the stomach, or ventriculus 

 (Vent). Shortly before its junction with the stomach the oesophagus 

 gives off three pouches, known as the oesophageal diverticula, two 



