The Structure and Special Physiology of Insects 19 



on it not merely to take up oxgyen from the outer air and give up the 



waste carbon dioxide of the 

 body, but also to convey these 

 gases to and from all the tis- 

 sues of the body. The blood 

 is not red, but pale yellowish 

 or greenish, and is really more 

 like the lymph of the ver- 

 tebrate body than like its 

 blood 



Insects do not breathe 

 through the mouth or any 

 openings on the head, but have 

 a varying number (usually 

 from two to ten pairs) of 

 small paired openings on the 

 Fig. 37. Fig. 38. sides of the thorax and abdo- 



FlG. 37. — Diagram showing respiratorj- system of pupa men. These openings, called 

 of mealy-winged fly, Aleyradesjp.; only two pairs jrades, or stigmata, are ar- 

 of spiracles are present, (.'\fter Bemis; much r ' o ' 



enlarged.) ranged segmentally and in 



Fig. 38.— Diagram of trachea- in head of cockroach. ^^^^ insects are to be found 

 Note branches to all mouth-parts, and the an- 

 tenna;, t., trachea;, or air-tubes. (.-Vfter Miall on two of the thoracic seg- 

 and Denny.) ments and on all the abdomi- 



nal segments except the last two or three. The openings are guarded by fine 



hairs or even little valvular lids to prevent 



the ingress of dust, and are the entrances to 



an extended system of delicate air-tubes or 



tracheae which branch and subdivide until 



the whole of the internal body is reached 



and ramified by fine capillary vessels bring- 

 ing fresh air to all the tissues and carrying 



off the waste carbon dioxide made by the 



metabolism of these tissues. The usual 



general arrangement of this elaborate re- 

 spiratory system is shown in Figs. 34, 35, 



and 36. Short broad trunks lead from 



each spiracle to a main longitudinal trunk 



on each side of the body, from which 



numerous branches arise, these going to 



particular regions of the bodv (Fig. 38) Fig. 39.— Piece of trachea (air-tube), 



and there branching repeatedlv until f/'^'^^J ^agnifiecl, showing spiral 

 ° ^ - thread (tsnidia). (Photomicro- 



even individual cells get special tiny graph by George O. Mitchell.) 



