VI] 



THE NERVOUS SYSTEM 



125 



nerve-cells. The pons is a more slender commissure passing in 

 the form of a flattened arch above the central body, and separated 

 from it by an area carrying numerous tracheae. 



The large and deeply pigmented cerebral trachea (fig. 56 c, tr) 

 ramifies in a fan-like manner upon and into the cortex of the brain, 

 penetrating chiefly along the plane which appears to mark the 

 original boundary between proto- and tritocerebrum. 



The most conspicuous formation in the procerebral lobes of 

 the Dragonfly is the much discussed pair of bodies known as the 

 stalked bodies, mushroom bodies, or corpora fungiformia. Although 

 these do not shew in the Dragonfly the very peculiar and specialized 

 development which they attain in some other insects (e.g. the 

 social Hymenoptera), yet they are very strongly developed, 



Fig. 57. T.S. through right mushroom body of Aeschna brevistyla Ramb. ( x 60). 

 cal calyx; cl cauliculus; gc giant nerve-cells. Original. 



particularly in the large Dragonflies, such as Aeschna (fig. 57). 

 Each mushroom body consists of three parts, (i) a stalk or cauli- 

 culus (cl), (ii) a calyx (cal) and (iii) a large mass of giant nerve- 

 cells (gc). The stalk is a single short cylindrical mass of nerve-fibres 

 arising vertically upwards from the medulla of the procerebral lobe, 

 and terminating in the calyx. The calyx is a large undivided mass 

 of small ganglion cells taking a very deep stain throughout. The 

 cells of the calyx are arranged in more or less radiating lines, all 

 closely packed together. The calyx forms a kind of external 

 support or "raised back" for the giant cells, and is not folded into 

 the "cup" shape more usually seen in the higher insects. The 

 giant cells of the mushroom body are collected into a large mass 

 placed internally to the calyx. These cells are flask-shaped, with 



