158 PHILIP p. CALVERT. 



dian impressed line recalling the primitively bifid condition. The 

 form of the lateral lobes varies in the different groups, and is pro- 

 portionally largest in the Libellulidte. The terminal joint (tp) of 

 the lateral lobe of the Agrionidse, in either of the above views, is 

 the terminal joint of the labial palp ; it is also present in the Aesch- 

 nidse, but is wanting in the Libellulidte. 



A hypopharynx is present as a median organ of moderate size, 

 hairy, and broader at its free end. 



Endo-skeleton of the head. Near the centre of the hind surface of 

 the head is a foramen — the posterior cephalic — by which the cavities 

 of head and thorax communicate. The foramen is divided by a 

 transverse chitinous rod («) into an upper and a lower half. Through 

 the upper division passes the alimentary canal, vessels, tracheie, etc., 

 through the lower the ventral nerve cord. From either end of this 

 rod (a), where it joins the margins of the posterior foramen, another 

 chitinous rod (h) extends forwards and upwards inside the skull to 

 the roof of the head, where frons and vertex meet. Near its lower 

 extremity, rod (6) is connected with a chitinous shelf (c) which sepa- 

 rates the cavity of the frons above from that of the clypeus below. 

 A foramen, whose plane slants from behind downwards and for- 

 wards, is thus formed in the centre of the head ; its margins are rod 

 (a) behind, rod (h) on either side, and shelf (c) in front. Through 

 this foramen the alimentary canal passes downwards to the mouth, 

 and in so doing, is surrounded by the circum oesophageal nerve-ring. 

 In those groups where the eyes do not meet on the top of the head, 

 the epicranial cavity extends backwards between the right and left 

 orbital cavities, being separated from them by membrane. Where 

 the eyes meet dorsally, the cavity of the epicranium is confined to 

 the front of the head and is made up for the most part by the frons, 

 whose cavity is shut off from those of the orbits by a membrane 

 stretched from one side of the frons to the other and strengthened 

 by the two rods (6), and the orbital cavities lie side by side, although 

 separated by an incomplete, superior, chitinous partition. 



Of the three segments of the thoraX (PI. H, fig. 13) the proth or ax 

 is much the smallest and least specialized, and is much less inti- 

 mately I'elated to the mesothorax than the latter is to the metathorax. 

 Its dorsum is transversely divided into three successive areas, called 

 by Rambur, the anterior, median and posterior lobes. The median 

 lobe is usually the largest, and is marked by a longitudinal impres- 

 sion ; the front and hind lobes are usually not impressed. The shape 



