14 THE IMAGO [CH. 



the labrum above, and by its appendages below and at the sides. 

 These appendages are the mandibles (md), the first maxillae (mxj) 

 and the second maxillae or labium (Im) ; the first two placed 

 laterally, the last ventrally. Hence the mouth is capable of great 

 extension, the gape being as wide as the face itself. The labrum 

 and labium act as upper and lower lips respectively, while the 

 mandibles and first maxillae constitute two pairs of laterally- 

 working jaws. 



The Gula. The only remaining portion of the head-skeleton 

 proper is a small membranous piece called the gula. Referring 

 again to our simile of the box, this would occupy the lower part of 

 the back of the box, below the occiput. In the higher Anisoptera 

 the large orbits unite in a median line which runs downwards 

 behind the head. Below this line, or, in the other forms, below 

 the lower portion of the occiput, the head joins the neck. If, now, 

 the head be removed, the rounded opening of the posterior fora- 

 men will be seen, through which the cavities of the head and 

 thorax are in communication with one another. The small gula 

 closes this foramen from below. Although so insignificant in size, 

 it is of interest in being generally considered to represent the 

 ventral plate or sternum of the sixth or last head-segment. It 

 forms a base of attachment for the labium. 



The Tentorium. We have already seen that the frons is really 

 the most anterior portion of the epicranium. The anterior edge 

 of the epicranium, bordering the clypeus, is turned inwards to 

 form an internal chitinous shelf, separating off an upper frontal 

 cavity from a lower clypeal cavity. Again, the two ends of the 

 frontal furrow also send a pair of ingrowths into the head-cavity. 

 These run downwards and backwards close together, and meet the 

 above-mentioned shelf at its posterior end. They then run on 

 a little further to end up on either side of the posterior foramen. 

 These two rods are in their turn connected by a short transverse 

 rod which divides the posterior foramen into upper and lower 

 halves. The lower passage carries the nerve-cord, the upper the 

 oesophagus, salivary ducts and tracheae. Thus we see that the 

 head has a definite endoskeleton, formed of four separate pieces 

 or endosternites. The whole structure is called the tentorium. The 

 passage between the parts of the tentorium is called the tentorial 



