266 ARTHROPODS. » 



maximum total is ten. Each trachea is kept tense through- 

 out the greater part of its course by internal chitinous 

 thickenings, which apparently have a spiral course. The 

 branches of the trachea penetrate into all the organs of the 

 body, carrying oxygen to every part. The very efficient 

 respiration of insects must be kept in mind in an appreciation 

 of the general activity of their life. 



As the conditions of larval life are often different from 

 those of the adult insects, the mode of respiration may also 

 differ in details. Some of these differences may be sum- 

 marised. 



In insects without tnarked metamorphosis, and even in 

 some beetles in which the metamorphosis is complete, the 

 young insect and the adult both breathe by tracheae with 

 open stigmata. Both are said to be " holopneustic." 



When the larvse live in water, the tracheal system is closed, 

 otherwise the creatures would drown. This closed condition 

 is termed " apneustic." These larvse (of dragon-ffies, may- 

 flies, and some others) breathe by " tracheal gills " — little 

 wing-like outgrowths from the sides of the abdomen, rich in 

 trachese — or by tracheal folds within the rectum, in and out 

 of which water flows. In either case an interchange of gases 

 between the tracheae and the water takes place. In adult 

 aerial life, the tracheae of the body acquire stigmata, and the 

 insect becomes " holopneustic." 



In most insects with complete metamorphosis, the larva 

 (e.g., caterpillar or grub) has closed stigmata on the last two 

 segments of the thorax (those which will bear wings), but 

 there is a pair of open stigmata on the prothorax. In the 

 adult the, re verse is true. 



There are some other modifications, witness what obtains 

 in the parasitic larvse of some flies, e.g., gad-flies. In these 

 the stigmata are open only at the ends of the body. In all 

 cases, however, the stigmata of the adult are already present 

 as rudiments in the larva, though they may not open till 

 adolescence is over. 



Circulatory System. — As the respiratory system is very 

 efficient, estabhshing the possibility of gaseous interchange 

 between the inmost recesses of the body and the external 

 medium, it is natural that the blood-vascular system should 

 not be highly developed. Within a dorsal part of the body- 



