II] 



EXTERNAL FEATURES AND SKELETON 



15 



foramen. It slants upwards and backwards. Hence the oesopha- 

 gus, in running forward towards the mouth, passes downwards 

 along this foramen. The brain lies above the oesophagus, between 

 the supporting endosternites of the tentorium. From it the two 

 circum-oesophageal commissures pass downw r ards, encircling the 

 oesophagus, to join the sub-oesophageal ganglion, which lies below 

 the tentorial foramen. 



The Appendages of the 

 The Antennae. These are placed 

 epicranium (fig. 1, ant; fig. 2). Each 

 has its base inserted into a small pit or 

 hollow, the torulus. The antenna itself 

 is always short, and is formed of from 

 three to seven joints. The latter is 

 the usual number. In the Gomphinae 

 the number of joints does not exceed 

 four. The first or basal joint is called 

 the scape; it is short and thick. The 

 second j oint or pedicel is longer, and only 

 slightly thickened. The remaining joints 

 or distalia are very slender. The com- 

 parative lengths of the joints vary in 

 different genera. In Calopteryx and 

 Synlestes, the pedicel becomes hyper- 

 trophied, so as to equal or even exceed 

 in length all the other joints put together. 

 Fig. 2 shews a number of different types 

 of Odonate antennae. The sensory 

 function of these organs is very poorly 

 developed in all Dragonflies (see chap, 

 vn). 



The Mandibles (fig. 3). These are 

 very strong, thick, unjointed appendages, 

 hinged to the head-capsule by a true 

 ginglymus articulation. Viewed extern- 

 ally they are somewhat pear-shaped, 

 the pointed distal end representing the 



Fig. 2. Various forms of an- 

 tennae. A. Aeschna brevi- 

 styla Ramb., imago. B. 

 A ustrogomphus ochraceus 

 Selys, imago, c. The same, 

 larva. D. Hemigomphus 

 heteroclitus Selys, larva. 



E. Nepsticta canescens Till- 

 yard, at metamorphosis. 



F. Synlestes weyersi Selys, 

 imago. G. The same, larva. 

 d x to d 6 distalia ; ped pedi- 

 cel; sc scape. (All figures 

 x 11.) Original. 



stalk of the pear. The 



