392 PHYLUM ARTHROPODA 



The tracheae seem to arise as tubular ingrowths of skin, 

 and, primitively, each segment probably contained a dis- 

 tinct pair ; but their number has been reduced, and they 

 are often in part connected into a system. With the 

 doubtful exception of one of the primitive CoUembola, and 

 the certain exception of caterpillars, no insects have any 

 tracheal openings in the head region. There are rarely 

 more than two pairs in the thorax ; there are often six to 

 eight pairs in the abdomen ; the maximum total is ten 

 pairs. Each trachea is kept tense throughout the greater 

 part of its course by internal chitinous thickenings, which 

 apparently have a spiral course. The branches of the 

 tracheae 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. 



In insects without marked 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 larvae live in water, the tracheal system is closed, other- 

 wise the creatures would drown. This closed condition is termed 

 " apneustic." These larvae (of dragon-flies, may-flies, and some others) 

 breathe by " tracheal gills " (see Fig. 219) — little wing-like outgrowths 

 from the sides of the abdomen, rich in tracheae — or by tracheal folds 

 within the rectum, in and out of which water flows. In cither case, 

 an interchange of gases between the tracheae and the water takes place. 

 In adult aerial life the trachea of the body acquire stigmata, and the 

 insect becomes " holopneustic." 



In most insects with complete metamorphosis, the larva {e.g. cater- 

 pillar 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 reverse is the case. 



There are some other modifications — for instance, what obtains in the 

 parasitic larvae of some flies, e.g. gadflies. In these the stigmata are 

 open only at the end 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, air passing into the inmost recesses of the body, 

 it is natural that the blood-vascular system should not be 

 highly developed. Within a dorsal part of the body cavity, 

 known as the pericardium, the heart lies, swayed by special 



