480 COMPARATIVE ANATOMY CHAP. 



belong to the trunk segments 1, 3, 5, 8, 10, 12 (and 14 when there are 15 trunk 

 segments in all). 



The tracheal system of Scutigera (Fig. 338) is distinguished by many peculi- 

 arities. The stigmata here are unpaired and lie in the dorsal middle line. Each 

 stigma leads into an air sac, into which on each side about 300 closely packed, 

 radially placed, branched tracheal tubes enter. This form of trachea is specially 

 interesting, as perhaps helping to explain the origin of the so-called book-leaf trachea? 

 of the Arachnoidea, which will be described later. 



The respiratory system of the Symphyla, which in other respects may be reckoned 

 as very primitive Myriapodan forms, is much reduced. There are only 2 stigmata, 

 and these are on the under side of the head under the feelers. This is the only case 

 in the whole division of the Antennata in which a pair of stigmata has been retained 

 in the head. The branched trachea? do not extend either into the legs and feelers, 

 or into the posterior region of the body at all. 



In the Pauropoda the tracheal system seems to be entirely degenerated. 



Hexapoda. 



Apterygota. The tracheal system in a few Thysanura exhibits the original 

 condition in so far that the longitudinal and transverse anastomoses are wanting (in 

 a few species of Machilis, and in the Campodea, Fig. 339). In other Thysanura, 

 however, they are present (lapyx, Nicoletia, Lepisma, and a few species of Machilis}. 

 The most frequent number of pairs of stigmata seems to be 10 (Nicoletia, Lepisma, 

 Lepismina] ; in this case two occur on the thorax and 8 on the abdomen. Machilis 

 maritima has only 9 pairs of stigmata, 2 thoracic, and 7 abdominal. lapyx is said 

 to possess 11 pairs, viz. 7 on the abdomen, and an arrangement standing quite 

 alone among the Antennata 4 on the thorax. In Campodea the number is reduced 

 to 3 pairs belonging to the thorax (the last pair possibly belonging to the first 

 abdominal segment). 



Pterygota (Fig. 340). Transverse and longitudinal anastomoses appear to be 

 everywhere present. Among the longitudinal anastomoses on each side one develops 

 more than the rest, and at first sight it appears as if these tracheal trunks formed 

 the central portion of the tracheal system, from which, besides numerous lateral 

 branches penetrating the body, branches run to the stigmata, and there open out- 

 wardly. In the Diptera, Hymenoptera, and among the Coleoptera in the Hydro- 

 philidce and Lamellicornea, the trachea? may widen out in some places into large 

 tracheal sacs without spiral crenulations, known as air sacs (Fig. 320, p. 462). 

 The number and arrangement of the stigmata vary within wide limits. In the 

 same insect, at the same time, there are never more than 10 pairs of stigmata. Of 

 these 10 pairs there is one pair on the mesothorax, one on the meta thorax, and one 

 on each of the first 8 abdominal segments. 



The imagines of by far the greater number of insects are holopneustic, i.e. they 

 possess many pairs of open stigmata, though the number of stigmata may be reduced, 

 especially in the abdomen. The Aphaniptera alone have more than 2 pairs of 

 stigmata in the thorax ; in them there is one prothoracic pair, not met with in other 

 adult Insecta. The tracheal system of the larvae of insects exhibits interesting 

 peculiarities, which are of great morphological importance. 



1. The most primitive condition is found in those insects which undergo gradual 

 metamorphosis, and whose larva? live during all stages, like the imagines, on land 

 (Orthoptera}. These larva? are holopneustic, and their tracheal system simply passes 

 into that of the imago. Holopneustic larvae may also occur in insects with complete 

 metamorphosis, as in many Coleoptera (Malacodermata], which thus appear to be a 

 primitive group. 



