:240 



Arthropoda. 



The respiratory organs are represented by a system of 

 tubes containing air^ tbe tracbege, wbich branch over the whole 

 body, winding about among the organs and communicating with the 

 •exterior by the stigmata^ which, like the whole system, are 

 symmetrically disposed. There are at most ten pairs of stigmata, one 

 pair on the mesothorax, one on the metathorax, and one on each of the 

 eight anterior abdominal segments, where they lie between the sterna 

 and terga ; there are no stigmata on the head or prothorax.* The 

 stigmata are usually slit-like apertures, which are frequently provided 

 with marginal setee (Fig. 203 s), overlying the opening and pre- 

 venting the entrance of foreign bodies ; the same end may be 

 .attained in other ways. A short transverse stem usually runs in- 



B 



Fig. 203. Apparatus for 

 closing the trachea of a Beetle 

 (diagrammatic). A the appa- 

 ratus by itself, opened. B The 

 trachea with the apparatus 

 closed. The apparatus consists 

 of three chitinous pieces which 

 surround the trachea like y, 

 ring ; the piece (6) is as long as 

 the two others together ; one 

 of these (a) sends out a process 

 for the attachment of a muscle 

 (m), which takes its origin from 

 the third piece (c). When the 



-muscle contracts, a and c are pushed against h, and the trachea is clamped between 



the three pieces, s stigma, t trachea. — Orig. 



mMMm 



wards from the stigma to open into one of the main tracheal trunks, 

 .a varying number of which traverse the whole length of the animal, 

 connected with each other by several transverse vessels and giving 

 ■ofE numerous branches which anastomose over the whole body. 

 Occasionally the longitudinal trunks are absent, and the trachea 

 arising from each stigma breaks up directly into a number of branches 

 which are entirely independent of the rest. Some of the tracheae 

 may be dilated to form vesicles, which vary in size, but are some- 

 times quite large. These vesicles have no actual respiratory signifi- 

 cance, but serve to decrease the specific gravity of the body, and are 

 thus of importance in flight ; in other words the tracheal system is 

 not only respiratory, but in many forms is also aerostatic. All 

 the tracheae are covered by a thin chitinous cuticle which, in the 

 coarser tubes (but not in the vesicular dilations), is supported by a 

 delicate spiral thickening. Respiration is effected by move- 

 ments of the abdomen ; by its contraction part of the air is forced out 

 from the tracheae, and when it expands again a fresh supply enters. 



* The position of some of the stigmata may often be changed, those belonging to 

 the mesothorax may lie between the prothorax and mesothorax (Fig. 201) or even, 

 .as in caterpillars, on the prothorax. 



