70 



in pinned specimens, but if the thoracic wall is cleared in potash and 

 dissected one finds at the anterior end two small elevations over- 

 hanging the neck and separated from one another in front by a nar- 

 row interval. These represent the two lateral halves of the tergite 

 of the prothorax, and are termed the pronotitm, just as the tergite of 

 the middle segment is known as the mesonotum. The tergite of the 

 metathorax, or the inetanotum, is represented by a narrow strip of 

 chitin which arches from side to side under cover of the scutellum. 

 On the ventral surface of the thorax all the segments are represented, 

 and in addition there are some pieces which belong properly to the 

 neck, but have become displaced backwards. Starting from the middle 

 line in front, the area where the thorax joins the neck is occupied by 

 a loose membrane which permits of free movement of the neck on the 

 body, and also permits of a considerable amount of distension when blood 

 is flowing through the oesophagus. In this membrane there are three 

 pairs of sclerites, none of which are united in the middle line. The 

 first pair are quite free in the membrane, the second and third pairs 

 attached laterally to the side walls of the thorax. These are the jugular 

 sclerites, belonging to the neck. They are all small and oval in shape, 

 and are not conspicuous on the external surface of the thorax, being con- 

 cealed by the head. Behind them there are several sclerites closely 

 pressed together and united in the middle line. These are differentiated 

 portions of the sternite of the prothorax, and are termed the praesterna, 

 sternum, and sternellum respectively. The praesterna are united in 

 the middle line by a thick ridge, and together form a broad trans- 

 verse plate with a thickened anterior border, which limits the thorax 

 in front, and to which the membraneous neck is attached. The ster- 

 num is semi-circular in shape, and is wedged in behind and between 

 the praesterna. Behind this there is the sternellum, a heart-shaped plate, 

 closely fitted between the two sterna of the mesothorax. Arising from 

 these divisions of the prosternite there is a complex arrangement of 

 fibrous bands which serve to strengthen the walls of the thorax and 

 to provide attachments for muscles ; these will be referred to later. 



The sternite of the mesothorax is divided only in the middle line, 

 and consists of two large quadrilateral plates, which occupy about one- 

 third of the total lateral area. The two plates join one another in 

 the middle line at an acute angle, and so give the thorax a keeled 

 appearance. Behind them there is a small oval plate representing the 

 sternite of the metathorax, and behind this the first sternite of the 

 abdominal wall. 



