534 



INSECTA. 



than two pairs present. They are never present on the head or on 

 the last abdominal segment. They are least numerous in the aquatic 

 larvae of beetles and Diptera, which have but two stigmata placed at 

 the hind end of the abdomen on a simple or forked tube. There 

 are, however, often two openings on the thorax in addition. Some 

 water-bugs (e.y., JVepa Jfanatra, etc.) have at the end of the 

 abdomen two long grooved filaments which lead at their ba.se into 

 two air cavities. Such water-bugs can by this arrangement take up 

 air like the Dipteran larva?, by protruding the respiratory tube on 

 the surface of the water. 



The trachea (fig. 443), which are kept open by the spiral thickening 

 of the chitinous membrane lining them, are always more or less 



perfectly filled with air, and on 

 that account have usually a silvery 

 shining appearance. Their internal 

 chitinous membrane is produced by 

 an outer delicate and nucleated cell 

 layer, and is thrown off with the 

 external cuticle and renewed at 

 each moult during larval life. The 

 dilatations which are not unfre- 

 quently present in the course of 

 the trachea?, and which, in strong 

 flying insects, as Hijmert<>j>ti'r<t, 

 Diptera, etc., are enlarged to form 

 air sacs of very considerable size, 

 may with justice be compared to 



Fig. 443. Tracheal branch with finer , i i- i i mi 



twigs (after Leydig). Z, Cellular tll(? au< SRCS f llll>ds - The . y possess a 



external wall ; Sj>, cuticuiar lining delicate chitinous membrane, which 

 (spiral fibre). . ., . . 



exhibits no trace or the spiral fibre. 



They 'therefore collapse with great ease, and require for their filling 

 special respiratory movements. These are especially noticeable in 

 the relatively clumsy Lamelliconis before their flight. The arrange- 

 ment and distribution of the trachea! system may easily be described 

 by starting with the origin of the principal trunks from the stigmata. 

 Bach stigma leads into one (or more) trachea! trunk, which sends out 

 connecting branches to the neighbouring trunks and gives oft' a tuft 

 of much branched tubes to the viscera. As a rule, there are formed 

 in this way two independent lateral trunks, which communicate by 

 transverse tubes and give off numerous secondary trunks to the 

 internal organs. The finer branches of the secondary tubes are not 



