440 



THE INVERTEBRATA 



the brain. These small compact glands are probably hormonic in 

 function though the precise nature of them (as of oenocytes) awaits 

 elucidation. 1 Photogenic organs, found in the glow-worm larva and 

 the female beetle Lampyris, possess a rich supply of tracheae and 

 produce light by the oxidation of luciferin by the enzyme luciferase. 

 In the insects the tracheal system characteristic of terrestrial Arthro- 

 poda attains its most complete development. The ectodermal tubes 

 of the system form a network of which every part is in communication 



a.s. 



Fig. 308. Tracheal system of the locust, Dissosteira Carolina. Modified from 

 Vinal. A, Side view. B, Dorsal view, the lower half to show air sacs, the 

 upper half to show tracheal supply to the alimentary canal, a.c. alimentary 

 canal; a.s. air sacs; l.t. longitudinal trunk; sp. spiracles. 



with every other part. Typically it communicates with the exterior 

 by two pairs of openings called stigmata or spiracles on the thorax and 

 eight pairs on the abdomen (Fig. 308). The main branches leading 

 from the stigmata not only divide into finer capillaries leading to the 

 adjacent organs but communicate by means of lateral trunks with each 

 other. The capillaries or tracheoles never end blindly in the blood but 

 always in the cells of the body, whether muscular or glandular or 

 connective tissue, so that normally the oxygen is conveyed directly 



^ It has recently been shown in Rhodnius that they act as ductless glands 

 controlling metamorphosis. 



